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A station needs to be connected to an effective antenna, be this a horizontal dipole, or some sort of vertical. These are ideally resonant antennas. While some newer radios include basic SWR metering, external instruments may be required when buildng antennas.
However, any antenna is better than none, so electrically short antennas may be used, usually with an antenna tuning unit.
A range of wire antennas have been designed over the past century, such as the Zepp (used on the Zeppelins), various doublets, the G5RV, and loops. One of the simplest is the resonant dipole, with the benefit that no antenna tuner is required. These have an impedance of arounds 50 ohms, so can be fed with coax.
While assembled and kit antennas are available, it is quite easy to build antennas using a few hand-tools and easily available materials, perhaps even recycled ones.
Lets say we want to operate on 40 metres, at 7.100 MHz, and we have a decent sized yard, and trees, so decide on a dipole. This is a frequency at which Technicians and Novices are permitted to operate using Morse (but not FT8). This happens to calculate to needing to be 20.056 metres overall, or 10.025 metres per leg*. Some times the length can be somewhat different to a fraction of the nominal wavelength. If we want to operate on a local net, low is better, for DX, high is good.
length = 299.79 ÷ 7.1 × 0.25 × 0.95 = 10.0252 metres
* 32.89 feet or 32' 10&frac58";. That 10 metres is about 33 feet is useful for at least one question in the pools, although I think it is the overall size of a 20 metre band dipole.
Insulated electricians' or "building" wire is fine. This wire typically has seven copper strands making up each conductor, with 2.5 mm² being a suitable size. It can be split out of twin and earth or similar cable, often termed "TPS" for the thermo-plastic sheath. If this isn't common in your country, LAPP, Helukabel, or similar brands of coursely stranded or "Class 2" wire can likely be ordered. Class 5 is also suitable, but may need some form of knotting or clamping.
I went down a rabbit hole trying to describe this, and discovered a standard, IEC 60228, previously VDE 0295. This specifies both the lay-up and maximum resistance of the conductors. The classes are below, with the characteristics lised applying to sizes of wire we'd typically use:
Class 1 is a single, round copper or tinned copper wire. This is for fixed use only, and may be used used in the UK and US, and historically in Australia. Romex is a US brand.
Class 2 is stranded wire with at least 7 round strands, or occasionally 6 shaped ones. In Australia cables made from this are used inside walls. Such cables, or insulated wires of this design can be used inside plastic or other conduits. This style is what I suggested above.
Class 5 is finely stranded copper or tinned copper wire. This is used in appliance power leads and extension cables, termed "flex" by Brits. In both these cases the minimum strand diameter is specified for each size.
Class 6 features even finer strands of copper or tinned copper. Similar to Class 5, these also may included things like wire used in multimeter leads.
For larger sizes in classes 1 & 2 aluminium versions are available, in some cases formed into sectors, etc. It is often used in overhead power distribution.
See: Prysmian Group notes
Remember that "wire" refers to a conductor with or without a single layer of insulation. Cable typically is several conductors within a plastic sheath, although a single wire may be sheathed, or in some cases joined into a figure 8 without a sheath. In some countries mains cable for installation may have plain copper for the earth. Note that the stranded wire refered to is not Litz wire, used in some inductors.
Copper-clad steel (CCS) such as "Copperweld" is used by some hams.
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| This is the central feed-point of a dipole antenna. The red and green is silicone tape, which has either crumbled due to UV, or been attacked by birds. Under the yellow tap there is a PL-259 plug connecting to the SO-239 on the centre. It is a Budwig HQ-1. The hole in the top allows a variation of the dipole to be made, the Inverted-V, useful for when you have a single tree, or single pole or chimney in the middle of your property, and can use lower objects, such as the top of a fence as the ends. There are also centre units which connect to coax without the connector, or to ladder-line. More advanced versions contain a balun. Why 4 wires? The blue wires are for the 40m band, the heavier green and yellow for 20m. Provided there is some spacing, you can have several dipoles one one feeder. |
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| This is on of the end insulators of the dipole. It is a high-strain plastic insulator designed for electric fence use. Just twisting the wire is often sufficient, if it is reasonably stiff. It would also be used on an inverted-L, Zepp, Carolina Windom, or other wire antenna. Small - medium sized ceramic versions are also suitable, while the slightly softer black plastic ones also work. |
We have connected it to some 50 ohm coax hung it up, and now we need to know if it is resonant. There are a few ways to do this, using the transmitter and an SWR meter; or using an antenna analyser, a box with a tuneable oscillator and fancy SWR analysing circuitry inside; or a NanoVNA.
If an antenna is not the correct electrical length, then energy will be reflected back to the transmitter, which is not an ideal situation, and if this mismatch is bad enough, the transmitter may be damaged, especially if it lacks the SWR protection some modern units have.
The ideal SWR = standing wave ratio is 1:1, with an increase in the number on the left meaning a poorer match. This should be below 2:1. If using a radio without a linear amplifier, below around 1.5:1 may be a case of "She'll be right". If using an analyser and the SWR is not good, it will tell you where the SWR is the lowest, either on a graph, or by tuning across the band; with a meter you will have to do a series of test to find whether it is above or below the desired frequency. If it is at a lower frequency you need to trim the antenna, or adjust the element to make it shorter. If it is too high, you may need to add wire, or adjust it to be longer. Some vertical antenna bases have hex screw clamps for the wire element, which allow a degree of adjustment. If cutting elements, be a little conservative, and trim in small increments.
A typical SWR meter starts around A$150, depending on its parameters. It uses a section of feedline and parallel wires to form a "directional coupler", such that energy going forwards (to the antenna) and back from the antenna can each be sampled. The typical CBer's or Ham's unit might be 200 m long, 100 deep, and a little less high. This might cover 1.5 to 200 MHz. Full sized, and pocket sized version are available for VHF and UHF. There are also units which separate the directional coupler and display, and for vision impaired, talking ones. Another variation has power meter movements for forward and reverse on one meter face, arranged so that the SWR can be read using the needle crossing point on lines for various values of SWR.
Unless meter illumination or features such as peak hold, or digital readout are needed, no battery or external supply is required.
One listing is here: Strictly Ham SWR meter list
Once connected, and range switches are set, and FWD (forward) is selected. The transmitter is then keyed up, in AM or FM modes, and the Calibration know adjusted so that the needle points to the CAL mark, and the maximum point on the scale. The radio is de-keyed, REV (reverse) selected, and the radio keyed again. The needle will typically deflect, indicating a mismatch. If the mismatch is higher than desired, the antenna can be adjusted, and the reading taken again, until you can find a low SWR point.
You can see the inside of one SWR meter here: Wikipedia: SWR meter
The longer of the antennas on my Wavelength page is a dedicated amateur products, but the shorter was from a kit containing a long whip, designed to be cut down to suit any frequency between around 70 and 500 MHz. Thus cutting in accordance to a scale on the included card was needed, then tuning to ensure low SWR on 2 m.
Modern HF and multi-mode VHF radios have an SWR circuit, there the power output is folded back. This is often integrated with power control circuitry. (Apparently some 100 watt HF radios put out a pulse at something like 150 watts, before folding back to the selected power, even if this is 5 watts). Many also include a rudimentary SWR display, using information from this circuitry, meaning you may not need to buy a stand-alone meter. A few of the larger radios also include a basic antenna tuner.
An antenna may be resonant, but not 50 ohms. An example is a full-wave delta loop, which has an impedance of 102 ohms. The dodgy method is to just use 50 ohm coax, and an antenna tuner; the right way is to use a quarter-wave length of 75 ohm coax (RG-59, etc) to match the antenna to 50 ohm coax. For some a "balun" or other transformer is needed. Balun means Balanced to Unbalanced (and tiny versions are used to connect balanced ribbon to the unbalanced coax input on TVs). Some are 1:1, others 1:3, 1:4 1:6, or 1:9, depending on the impedance of the dipole. Un-Uns also exist, for interfacing coax to things like a half-wave long wire, which has high impedance.
 | A 6 metre Delta, named for the Greek upper-case delta Δ. The timber crossbar is about 3 metres long. As the feedpoint is in the bottom the antenna is horizontally polarised. A bit hard to see, maybe, but at the point cable is taped to the pole, there is a joint between 75 and 50 Ω coax. This is soldered, and a bit bodgy; I should have used an F connector on the 75 ohm cable, an adaptor, and a TNC or FME connector on the RG-58. RG-6 is used for pay-TV, satellite TV, and ordinary TV antennas. I think the 75 Ω is RG-59. (RG-6 is a little thicker, and RG-11 and RG-216 are larger again). |
If the SWR suddenly increases on a frequency it was good, this means either the coax has water in it, or more likely, a wire or element has broken.
The alternative way to measure antenna mismatch is using a Directional Watt-meter. This allows us to measure the power coming from the transmitter to the antenna; and to compare this to that coming back. If we found 400 watts is going out, and 5 watts is coming back, that is quite a good figure (1.25:1). The lower the return, the better.
Additional information: The most famous is the Bird 43 Thruline Directional Wattmeter. You purchase the body which includes the meter, then add elements or "slugs" which plug in to measure specific power levels and frequency ranges of interest. A standard 43 is priced at US$446.
These slugs have a large arrow on them, and can be plugged in in either direction. To make the readings discussed above, for a 2 metre repeater, the 500C and 10C elements are needed. The forward reading is taken with the larger slug, with the arrow pointing from transmitter to (load) antenna. Reversing this slug to check for excessive reflected power (excessive SWR), before fitting the low power slug, being extra careful to ensure the arrow points from load to transmitter.
A value of φ = 400 / 5 = 0.125 (1.25%) is derived from the powers.
SWR is derived thus: ρ = (1 + √0.0125) / (1 - √0.0125) = 1.25175371924, or about 1.25:1. Note that the relationship between the 1.25 figures is coincidental only.
If your club added repeaters on 70 cm (430 or 440 MHz ranges), D suffix elements are needed. C and D elements are US$120 each. Thankfully, either set can be used at 225 MHz. The same meter could also be used for testing a community broadcaster's AM or FM antennas, with just a new slug or two.
You can view the product information, and download the the manual, which includes formulas and tables: Bird 43
This isn't a piece of test equipment, it is a rhythm used in poetry, consisting of five sets of unstressed and stressed syllables. Shakespeare used it extensively, for example, "Shall I compare thee to a summer's day?" in his Sonnet 18.
Did you notice the da-DUM da-DUM da-DUM da-DUM da-DUM rhythm?
The term "iambic" does apply to certain "side-swipe" keyers, where one key sends a dit, the other a dah; and squeezing them, alternative dits and dahs: dah-di-dah-di = C. These require additional electronics to generate the elements, but some radios include this, so just need simple switch contacts.
Penta, meaning 5 is applied to those valves (tubes) with 5 elements - Anode, Cathode, and 3 grids (plus heater); or 5 grids, the pentagrid converter (aka Heptode, for 7 elements). A multi-function meter is a "multi-meter", below.
Note that using an ohmmeter or ohms range on a multi-meter to to measure an antenna will usually give one of two results, either a DC open, for antennas such as dipoles, and quarter-wave and five-eights verticals; or a DC short for folded dipoles, loop antennas, and the transformer on a half-wave. A volt-meter is NOT useful for testing an antenna, beyond checking for shorts in antennas or feedlines which should be open, or opens where there should be continuity.
Low cost "Nano-VNA" devices (under $200) are now available from China, and these are useful for analysing antennas, cables, filters, and various components of repeater systems, etc. Vector Network Analysers (costing A$5k to A$80k) are discussed in the Extra exam.
Antenna analysers are another way to test the characteristics of an antenna across a range of frequencies. Thus they can tell if an antenna is resonant at the desired operating frequency. They can indicate whether an antenna is capacitive or inductive. Often several times the price of a Nano-VNA, they may be be becoming less popular.
Both devices can display the resistance and the reactive component of the antenna's impedance. This greater information compared to an SWR meter makes it easier to know whether the antenna needs to be lengthened or shortened.
That said, making SWR reading at multiple frequencies across a band, and graphing them can potentially show useful trends. For things like vehicle-mounted 80 metre antennas, the low SWR point can be very narrow, and may require small steps in frequency to find.
Access to a range of test equipment is yet another reason to join a club.
An antenna tuner makes the radio see a 50 ohm impedance, but does not itself "tune" the antenna. It may also be called an automatic tuner (if automatic), an ATU (antenna tuning unit), or a "trans-match". Manual tuners typically consist of one or two large variable capacitors, and either a switch tapped inductor, or a roller inductor. These can be motor driven is some systems. Automatic tuners usually consist of a bank of capacitors, along with a bank of coils, selected using banks of relays. Often another relay moves the capacitor bank between input and output, and a final one bypasses the tuner. On occasions, a tuner may not be able to find a solution, so the radio-side impedance may still be poor.
Ideally a fairly low power signal is used while tuning an antenna. This needs to be CW or AM, not SSB, so that the power is constant. The now second-hand only LDG YT-100 tuner commands variosu Yaesu radios to 10 watts AM, and to key, up via the Computer Aided Transceiver (CAT) interface. The newer Z-100 can speak to ICOM radios, or work as a generic tuner. These tuners buzz as the processor seeks a tuning solution. In the case of MFJ "Memory Tuner" prodcust, and many others, briefly keying on a frequency you have previously used will cause the tuner to switch to the memorised settings, and see if they still provide a good match. If the antenna has changed in some way it will seek a better tune.
It is certainly possible to build your own tuner, and a hamfest may be the more affordable source of inductors and variable capacitors, including used ones. Note that for higher powers a decent spacing between plates is required to avoid flash-over. Kits for relay based automatic tuners of various ratings are also available online.
For manual tuners, noting the switch or dial positions for each band can be handy, such as:
| Band | Induc. | Capac. |
|---|---|---|
| 80m | A | 48 |
| 40m | C | 27 |
| 20m | F | 12 |
For longer wavelength bands especially, you may have several values in rows, or better, columns, such as for CW, FT8, Voice, and even specific nets. You might make this on card, and write the positions in pencil, in case the wire stretches a little, or you place metal items under the antenna, such as concrete with reinforcing mesh. Often you may need to vary the capacitor a little anyway.
While many tuners are placed within the shack, with coal or balanced line to the antenna, "remote" tuners are placed outside, and have the wire and ground connected directly to them. SGC is a well-know brand. These require a power connection, usually around 12 volts DC. These can also be used with a vertical element.
Not specifically on the exam, some hams just use whatever "random wire" fits in their yard, along with an indoor or outdoor tuner. If taking this approach, be aware that a wire of half a wavelength on a particular band will exhibit high impedance at that frequency, so you may wish to adjust the length to avoid this, especially if using a coax connection. There are however deliberate end-fed half-wave (EFHW) antennas, which a transformer (un-un) to match to this high impedance. Some vertical antennas also do this, and include a transformer in the base. These have the benefit of providing good performance without radials.
Off topic, but useful: A common practice among those building small diameter loop antennas is to use a vacuum variable capacitor to match the loop. These use a metal bellows, which is designed for infrequent (set-and-forget) adjustment in fixed-frequency systems, such as broadcast antennas. These bellows will fatigue over time if constantly adjusted, and tear!
For the past 70+ years, the most convenient way to connect a radio device to its antenna is via coaxial cable; the (US) RG cable numbering system starting during WWII. It stands for "Radio General". "Co-axial" means that the two conductors are around a common centre, or axis. This consists of an inner conductor made from stranded or solid wire, copper clad aluminium or steel wire, or silver plated copper; or from a thin metal tube, sometimes corrugated. This is insulated from an outer metallic shield; this consisting of braided wire, foil, or solid metal, sometimes corrugated for a degree of flexibility. The "dielectric" between these can be solid or foam plastic, or Teflon spacers in the case of solid cable, meaning the dielectric is air, or in broadcast systems, dry nitrogen. The ratio of the diameter of the outside of the inner, and the inside of the outer, in combination with the type of dielectric determines the characteristic impedance, such as 50 ohms (sometimes 52 Ω) for most transmitting systems, and 75 ohms (sometimes 72 Ω) for many receive only ones.
The story goes, that after WWII, in re-establishing TV in the UK, shut down for fear it would be used for bombers to navigate, engineers found that two sizes of water pipe gave a 48 ohm impedance, meaning transmitters and antennas were built to this specification. When transmission re-started the day before the VE day anniversary, they provided an introduction and welcome, replayed the Mickey Mouse cartoon which was the last item broadcast, and carried on as normal.
The 50 ohms use in radio communications systems is a compromise between 30 ohms, which would provide maximum power transfer at high power (for transmitters), and 75 ohms, which has lower loss for reception.
Hams often use RG-58C/U for lower power systems; and RG-213 or RG-8/U for higher power systems, and for VHF, UHF. Various LMR cables have also become popular. Heliax, made by Andrew, and related products are used for long runs, especially at VHF and UHF, and in repeater systems. H155 and H500 are European types. 5D-2V and 10D-2V are examples of Japanese items, with 5D-FB being a foamed dielectric version. Note that RG-8X is similar to RG-58.
Off this exam there are significantly higher impedance cables, with the inner wires very thin compared to the overall diameter; RG-62/U at 92 ohms and RG-63 at 125 ohms. These, or similar cables with an extremely fine centre wire, are used to prevent loss between the antennas used for a car radio, which is electrically very short on MW & LW (AM), and the radio itself. The antenna has high impedance, and the capacitance of the cable would shunt the received signal.
Plastics, such as used in coaxial cable consist on long double chains of hydrocarbons, such as Polyvinyl chloride, and these long chains give the plastic strength. However, ionising radiation, such as ultraviolet light, smash into the plastic, and break the chains, thus making the plastic powdery and crumbly. Adding carbon to the plastic, making it black, can help reduce this activity. This is also important with cable-ties / zip-ties, when used outside, as black ones last much longer than natural (translucent white) plastic. Stainless steel is best. While white products for marine use exist, light coloured cables should typically only be used indoors, or in temporary (field day) set-ups.
It is important that the coax not be deformed, as this affects the impedance, and can cause reflections. To avoid this, lightweight cables should not be crushed by cable ties, with electrical tape being a better option, and as above, black tape lasts longer. Minimum bend radiuses must be observed for all cables.
Velocity factor is the speed of signals in the cable, expressed as either a percentage, or decimal fraction of the speed of light. If you could get this to to exceed 1, you need to know Stockholm is in Sweden, as you will have to go there to collect your Nobel Prize, for getting signals to exceed the speed of light! Back to reality, and different dielectrics in coax affects the speed of signals in coax; a foam dielectric might had a VF of 0.85 (85%), a solid one around 0.66 (66%). If you are just connecting an antenna to your radio this does not matter.
Coax also has an interesting property, shared with other feedlines: They can perform impedance transformation. If an impedance (say) greater than that of the cable is placed across one end, then a signal applied at the other end will see a lesser impedance, by the same difference. Thus you can make a matching section to interface a 102 ohm delta loop to a 50 ohm radio, by making initial link from the loop using 75 ohm coax, a quarter-wave long. This has to take the velocity factor into account. Thus, for 50.150 MHz, you will calculate a free-space quarter-wave is 1.493 m. If VF = 0.85, then you need a 1.269 m; and for 0.66, just 985 mm. It works both ways, so the loop will see the radio as a 100 ohm load.
Another use for coax is as a filter. A quarter-wave stub which is open-circuit at the far end appears as a short at the frequency in question, so it will suppress that frequency, perhaps useful if you live near a repeater, or a pager transmitter. If closed, it will pass only the relevant frequency, as it appears open at the connection point. In both cases VF must be taken into account.
Note that while a coax has a specific impedance, if you measure it with your multimeter, you will find the end-to end resistance to be near zero, as you find with any wire. You should find an open circuit between the inner and outer. It will have some capacitance, and were you to apply an insulation tester to a long length you may notice it charging, and if you don't discharge it, then touch the inner and outer, you may get a shock.
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| These are a selection of Coaxial connectors terminated onto cable, and inter-series adaptors. These are a PL-259 (UHF) plug to BNC socket; a BNC plug on RG-58C/U; a BNC plug to RP-SMA socket; an SMA plug on RG-223/U; an SO-239 (UHF) socket to SMA Socket; and N socket on the other end of the RG-223/U. Across the bottom is RG-213 from the folded dipole. |
Returning to my comment re convenience, coax can be taped to masts, passed through walls, including metal ones, or run in proximity to other cables, thanks to being shielded. Depending on the specification, they can also be either directly buried in the earth. or run within conduit.
An alternative is "ladder line" or "window line" a balanced cable consisting of two conductors in parallel, spaced by plastic spacers, or a plastic ribbon with cut-outs. Window-line, often 450 ohms, is about 30 mm wide, ladder-line, often 600 ohms, up to maybe 100 mm, more on high-power transmitter sites. These are similar to 300 ohm TV ribbon cable but bigger.
These lines need to be run away from metal, including other cables. For TV ribbon stand-offs were available when this was a popular option for TV. For window and ladder line, an arrangement such as allowing the wire to curve down from the dipole or doublet centre to a feed-through point in a wooden window-frame may be the best option.
Way off the exams, waveguides are used in the microwave bands, and provide good efficiency.
When testing a transmitter, so as not to cause interference to other Hams, connect the radio to a "dummy load", also called an "artificial antenna". This consists of a non-inductive resistor mounted on a heatsink, and shielded. Alternatively, the resistor can be placed inside a cylindrical heatsink which also provides shielding, so the signal does not radiate. "Non-inductive" means that the resistor does not consist of a spiral of resistive material, so that it does not have a large inductive component.
Devices are marked with impedance (usually 50 Ω, power ratings (continuous and/or with some form of duty cycle), and information about frequencies at which the SWR is low. This is because some are fine only at HF, others go into the GHz range.
On later exams, dummy loads are installed in some two-way systems, such that a device called a circulator dumps RF energy into a dummy load if the antenna fails, or develops high SWR. These loads are also called a terminator.
The Amateur will need to measure various electrical parameters, such as battery or power supply voltage, mains voltage, current used by equipment, and resistance of components, or of connections.
Note that these are usually done using a hand-held or bench multimeter. These can have manually set ranges, or automatic ones. In the former case you select, say, the relevant voltage or current range, in the latter you just select the mode, such as voltage, resistance, or current.
A voltage meter, (voltmeter), or multimeter in voltage mode is connected across the two points, say the positive and negative terminals of a battery, or a the radio's power connector. For this a DC range (say 20 volts if around 12 volts is expected), or DC voltage mode, must be selected. For the low voltage output of a transformer, an appropriate AC voltage range is selected, or the AC volts mode. For mains a quality meter, such as Category III must be used. It may be wise to initially set a range above 240 volts, even if 120 volts is expected, in case it is wired wrongly. Other terms for voltage are electromotive force (EMF), electrical potential, and potential difference (PD).
Off the exam, but if you live in a country with large transformers serving multiple houses (like Australia), and are back from the street, so have a long power lead-in, and several large loads are on, the common earth-neutral connection can be raised above surrounding earth, meaning that if your radio earth is not bonded to the power earth, there can be an electrical potential between the two, enough for a mild shock, say if touching PC and radio! Hot water, oven, and heaters together might draw 50 amps, and say 0.5 ohms resistance = 25 volts AC. Alternatively, a portion of the current will attempt to flow between the PC and Radio via the shield of the audio connection, or via a serial or USB connection - not good for either device! This demonstrates the importance of linking your radio ground to ground rods, metal water pipes, etc.
For current measurements the circuit must be cut at the point at which the measurement is to be taken, and the meter inserted in series. Points such as fuse sockets, or power switches can be handy. An appropriate current mode and range must be selected. This is also called an ammeter.
Measuring resistance uses the internal battery of the meter to push current through the circuit under test. The circuit being tested must be un-powered, and large capacitors discharged. To get an accurate reading on a component it may be necessary to disconnect one connection. This is termed an "ohmmeter". If you test a large capacitor it will appear as a low resistance device, with this increasing as the capacitor charges. One fault in capacitors is a high DC leakage. A very large inductor will appear high resistance, with this dropping over time. Note that testing large inductors, including transformer windings can be dangerous, as when you remove the probes, the magnetic field you have generated collapses, and a high voltage pulse is generated, potentially enough to knock you over. The test, however deals with capacitors.
Precision mains powered equipment is sometimes left on, or needs to be turned on for around half an hour to come up to temperature, to have the best accuracy, but not doing this will not damage anything.
Original multimeters used an electromagnetic meter movement, where a needle moved across a scale. These only used batteries for the ohms range. The king of these meters is the AVO-8, make in Great Britain, and only just discontinued, but early models are available on ebay for as little as £20, a bit like being offered a vintage Rolls Royce for the price of a Kia.
Then came the VTVM, or Vacuum Tube Volt Meter, which was usually mains powered, and used a valve amplifier to provide a high impedance input. These were replaced by FET (field effect transistor) based analogue meters. oftem battery powered.
Digital multimeters (DMMs) have become the standard since the 1990s. Most use a dial to select the mode and ranges, but in the late 1980s side-mounted, mechanically interlocked buttons were a trendy method to select mode and range on some more expensive models. |
| An analogue meter, indicating 405 volts AC. As the meter is set to 500 volts, we read from the main black scale, below the mirror, and read on the 0 to 5 range, multiplying by 100. The offset red scales are for lower AC voltage, and take into account voltage drop in the rectifier. |
Many meters are what are called 3.5 digits (3½ digits), as they display up to 2000 counts, with scales such as up to 19.99 volts, 1.999 mA, 199.9 ohms, marked as the 20 v, 2 mA, or 200 &2Omega; scales. Higher end hand-held meters are 4.5 digit, going up to 19999. Intermediate one are often called 3.75 or 3¾ digit meaning 4000 count or 5000 count; 3.5 digit also refers to 3000 and 3200 count units.
One thing which annoys me is that many cheap meters have a 200 mA range, then a 10 amp range, so reading at 1 amp is only 1.00, whereas a with a 2 Amp or similar range, it can read 1.000 amps, handy if setting up a current source for low-ohms testing.
Category marking is important for safety, as some power sources, including the mains is capable of delivering a LOT of energy, should a voltage spike cause an arc in the meter: CAT IV is outdoor power connections and switchboards; CAT III is industrial power, and domestic power within switchboards; CAT II is wiring beyond the switchboard and mains withing equipment; and CAT I is electronic equipment, after the transformer, including perhaps lower power valve power supplies, but not high-power linear amplifier supplies, or microwave ovens. CAT 0, often not marked as such, is for low power electronics. Using a higher than necessary category meter is perfectly fine.
Additional features include may continuity beeper with diode junction voltage; audio frequencies (with limited accuracy); and capacitance; hFE (transistor gain). Some include a thermocouple to allow contact temperature measurement. A few do inductance. NCV is Non-Contact Voltage, where a lamp lights if the top of the meter is placed near mains voltage wiring. Battery test places a resistance (that is, a small load) in parallel with the meter to allow a better estimate of teh battery being tested.
Auto-ranging can be convenient as you don't have to change range as you test various points of a circuit for voltage. The downside is that it takes a moment for the meter to select the range, which it does in every reading. Some such meters allow push-button selection of a suitable ranges, if you needed to test a large number of points for a voltage in a known range, or were selecting from many resistors for ones with a certain value. Another benefit is that a meter can have a many of ranges, as this is not limited by the number of physical positions the selector switch can have. The MP 730007 has this benefit over the MP 730008 manual unit.
Perhaps more popular in the US is the automatic mode version, where if no voltage is found between two points a resistance will be displayed, otherwise the voltage will be shown. A mode button accesses several extra modes, while for any current measurement a different socket is used.
A few years ago I bought a couple of meters, as sometimes it is handy to be able to read, say voltage and current simultaneously; and they have different ratings and ranges. The TENMA has 4, 40, and 250 volt AC ranges, the RS one only 200 and 600 VAC ranges, so the former is better for reading lower AC voltages. Both feature white LED back-lighting.
I've just bought a multicompPRO MP 730425, the first item in the list below. It is quite compact, and rated for mains use, with a non-contact mains voltage detector. Its down-sides are no current and no 400 mV range, although the latter perhaps does not matter for most users. It has a high contrast backlit white-on-black EBTN display. UNI-T also sell these, with a red base.
The meters below are available from Element 14 in Australia, and the Asia-Pacific; Farnell in Europe (inc UK), Middle East & Africa; Newark in the Americas; and also CPC the UK. E14 is a reference to Silicon, the 14th element on the periodic table.
TENMA and multicompPRO are house brands. Unless noted they are "handheld" types, with a main unit and two removable leads.
Except for something like a secondary meter, perhaps stashed in your car, removable leads are far more convenient, and can be replaced as needed. This arrangement also allows (typically) 4 mm banana plugs, sleeved where appropriate, to be used to connect the meter to equipment under development, etc. Various leads with crocodile clips, or with small hooks which can attached to IC pins are also available.
Below are a few useful options, but before you checkout, there are a few tools below which may be useful.
| Rating | Count | Price | Model / Link | Comments |
|---|---|---|---|---|
| CAT III 600V | 4000 | A$31 | multicompPRO MP 730425 | No Current, Cool EBTN display. No diode VF. Affordable. |
| CAT III 600V | 6000 | $A45 | multicompPRO MP730008 | DC current only, 600V only on AC. Battery test. |
| CAT III 600V | 6000 | $A38 | multicompPRO MP730007 | Auto ranging. Many ranges inc 6A. Excellent value! |
| CAT III 600V | 4000 | A$47 | TENMA 72-14500 | 400 mA max. Frequency & Duty Cycle |
| CAT II 1000V | 20000 | A$182 | TENMA 72-14630 | Affordable bench-top unit. Resistance to 200 MΩ |
| CAT II 250V | 2000 | A$44 | TENMA 72-13430 | Auto ranging. 2A range. Entry level. Similar to TENMA discussed. Not the best value. |
| CATIII 300V | 3000 | A$46 | multicompPRO MP730001 | Pen type, no current. |
| CAT III 600V | 4000 | A$42 | multicompPRO MP730835 | True RMS, EBTN display, 40 MΩ max R, No 400 mV range, Single probe Active wire identification. Auto mode! |
| CAT IV 600V & CAT III 1000V | 6000 | A$54 | multicompPRO MP730026 EU-UK | True RMS, 60 MΩ max resistance, DC voltage ranges as low as 60 mV. Backlit. Great option. |
| CAT III 600V & CAT II 1000V | 2000 | A$108 | TENMA 72-13500 | Includes inductor ranges. Includes error preventing Indication of lead connection. |
| CAT III 600V | 20000 | A$100 | TENMA 72-2600 | A good standard meter, with 0.05% DC accuracy. Capacitance ranges. |
| CAT II 600V | 2000 | A$15 | DURATOOL D04143 | Decent meter at entry level price. |
| CAT IV 600V & CAT III 1000V | 2000 | A$96 | CHAUVIN ARNOUX P01191739Z | Auto-ranging, fixed leads, no current. |
| CAT IV 600V & CAT III 1000V | 2000 | A$119 | CHAUVIN ARNOUX P01191740Z | Auto-ranging, fixed leads, with current up to 200 mA. |
Should the link above for a product you are interested in fail, try searching on the product number. You can also search by feature, or seach by parameters such as price, count, etc.
The other one is from RS Components (originally Radio Spares), the RS-PRO RS-12, with a CAT III-600 V & CAT II-1000 V ratings, and a battery test mode (a loaded voltmeter, meaning it places a resistor in parallel to provide a small load to the battery). It is 2000 count manual range unit, is now A$87, several times what I paid. See: RS Part# 1231939
Its auto-ranging cousin is the RS-14 at A$88. See: RS Part# 1231938
Prices include tax (GST), are approximate, and subject to variations in exchange rates, etc.
RS is "Allied" in North America.
Additional brands include Amprobe, Keysight, Metrix, TPI, Hioki (especially for pen type) and Keithley. The AVO name lived on until recently, as part of Megger AVO, with digital products, including the AVO835 and AVO310. FLIR has also moved into the meter space. For those buying on a corporate expenses account, Fluke is an option. Duratool is a E14 house brand, with some decent looking products, as long as you avoid the CAT I sub $10 items. Some of the multicompPRO are re-branded UNI-T products.
 | On the 600 volt AC range, this meter is reading 427 VAC. The two power-boards are connected to different phases, so instead of 240 volts, there is a nominal 415 volts between phases. The phase to neutral voltage for this reading calculates to 246.5 volts, a fairly usual voltage in Australia. especially close to a transformer, as was the case here. This is perhaps not something you should try at home, unless you know what you are doing. Power was switched off while making the connections, and this is a CAT II situation using a CAT III-600 V meter, so well within its ratings. |
Instrument Choice in northern Adelaide has a wide range of meters, including a several under A$100. They do however spam your facebook if you visit using the same browser you use it in. Copy the link, and use it is an different one, if you wish.
Direct link: Instrument Choice DMMs
It is best to filter by price, between $10 and around $100, etc.
Expensive hand-held meters may be 4¾ digit, 50,000 count; and bench-top include 5½, 6½, or 7½ digit options, with increasing prices (some exceeding $10,000). These may include features such as 4 wire resistance testing, various logging functions, and the ability to connect to computers and other instruments.
Lower cost meters, including the ones I bought actually measure AC volts as an average, and display a calculated RMS figure, based on the assumption that the signal is a sine wave. More expensive units read "root mean squared" or "True RMS", and display that. A figure of 240 RMS means that applying this to a heating element has the same effect as applying 240 volts DC. I have added a couple of affordable ones above. Instrument choice also have low cost ones.
A 240 volt RMS sine-wave has a peak voltage of 339.4 volts, this being 1.4142 times the RMS. For audio and related signals we often discuss the peak to peak voltage, say "2v p-p", as this is easily read off a CRO screen. This is partly because these signals are complex, with significant harmonic content.
Additional modes are frequency, usually, audio frequencies, but may not have good mains frequency measuring accuracy or resolution, some were pretty rough, as I think they did an analogue frequency to voltage conversion, before digitising this; temperature, using an external thermistor, usually more for high temperatures, than an accurate room temperature reading; transistor gain (HFE), and capacitance. A few invert the resistance reading to milliSiemens. Many have a diode-test, where a small, current limited, voltage is applied to the probes, and this can be used to determine the forward voltage drop of a diode or other semiconductor junction, although not usually high enough for an LED; and if shorted in this mode, the meter beeps, allowing you to concentrate on placing the probes whiel continuity testing. There are specialist units for automotive use, displaying RPM and Dwell angle (essentially, duty cycle) from the 12-volt* side of the ignition circuit.
* Note that high voltage pulses, from back-EMF from the coil, do appear on the low voltage side, and can give a nasty shock.
An additional rating is that for the voltage which the Common terminal can be above ground by. This is important when testing three phase systems, and for voltage drop measurements, where both probes are connected to high potential points. This is typically similar to the voltage between terminals rating, with CAT classes also applying. This is indicated by a line from the COM terminal to a ground / earth symbol, with a voltage written beside it. It is below the display on the RS meter above. The roughly 250 volts each input is above earth is well within the 600 volt rating. Ditto on the CAT II situation being fine with a CAT III device.
If testing between phases on a 600 volt system each probe will be 347 volts above ground; on some 277 / 480 volts they will be 277 volts above ground, and 480 volts apart; on corner grounded 480 vols systems it is possible to have both probes 480 volts above ground. If measuring for 240 volts on a "wild-leg" system one probe may be 120 volts from ground, the other at 208 volts. Between phases on a European 690 volts system the probes will be 400 volts above earth.
A range of accessory leads are available. These should be marked with voltage and current ratings, along with a CAT number.
Proper frequency counters are usually a desktop format, and many read hundreds or thousands of MHz with 1 Hz resolution, or better. Many can be connected to caesium standards (atomic clocks) or the clock signal of a special GPS receiver, for frequency accuracy.
An insulation tester is a variation of the ohmmeter, which generates a high voltage in order to test the insulation in mains and other electrical installations, and in motors. The most famous is Megger, named for the megaohm. A typical unit for domestic and commercial electricians outputs up to 1000 volts, and can test up to 2 gigaohms. 15 kV, 40 TΩ units for substations, etc are also available, at about $10,000!
Dedicated "L-C-R meters" are also available, which measure inductance, capacitance, and resistance over a wide range, and with good accuracy. In the old days several "bridge" methods were used to compare the unknown component with known ones, to measure them with high accuracy. The noise bridge, however is NOT an example.
While digital meter modules can be used when building power supplies, analogue meters are still used in some, and in many linear amplifiers to display parameters such as plate voltage and current; and in SWR meters and some antenna analysers. LED bargraphs were also used used on some amplifiers.
Also outside the exams, test probes with neon lamps or LEDs can be used for mains testing, with US versions having 120, 240, 277, 480, and sometimes 600 volt lamps. Older European ones have 50, 127, 220, and 380 volts. My newest European "Kewtech" one has +12, -12, 24, 50, 120, 230, 400, and 690 volts, plus phase rotation and continuity.
 |
| This Sperry brand tester shows that the voltage is between 277 and 480 volts. This probe is designed for US use so has the more popular US industrial three phase voltages show, in addition to the domestic voltages of 120 and 240 volts. My similar Amprobe lights the 480 volt neon on 415 volts mains. |
An alternative way to measure larger currents is using a clamp meter, where the magnetic field generated by a current in a wire is read. These have a jaw which opens to go around a cable or bus-bar, so the cable need not be disconnected and reconnected. There are a few types. One type uses a current transformer, and reads AC only. Hall effect versions read both AC and DC. Many have clamp and LCD display integrated. External current transformers are also available, to plug into a meter. The vintage "TONG TEST" has the magnetic energy act directly on the meter movement, reading 0 to 100 Amps, AC or DC. Clicking the jaws like a castanet may sound fun, but this can damage them! Also, the conductors must be separated, otherwise the forwards and backwards currents in the cable will balance out to zero.
One of several methods of fitting connectors to cables is soldering. It is also used to mount components on printed circuit boards, and often to fix wires to panel mounted parts. This uses a low temperature melting point alloy, such as tin and lead, in the form of a wire with a rosin core. When heated, this rosin flux becomes liquid, and cleans the metal surface to be soldered, then the solder melts onto these surfaces, forming a metallurgic bond. A good joint is reasonably shiny; a bad joint dull or grainy, or it forms a ball on the wire only, or bonds to the pad only.
However, as a "heavy metal", lead (Pb) is toxic, and an environmental pollutant. It causes damage to the brain and nervous system, increasing violent behaviour, as leaded gasoline did. It can cause birth defects.
Thus, there are rosin-cored lead-free solders, which are mostly tin (Sn), and requires higher tip temperatures. Lead-free solder can be pure tin, or with a small quantity of other metals, with the examiner listing tin-copper as acceptable, in a question seeking the unacceptable acid cored option.
Pure tin has several challenges, termed "tin pest", including a self-catalysing oxidisation process. Tension in the crystal structure of the metal can cause whiskers to grow, causing shorts.
Copper (Cu) may be used at 0.5% to 0.7%. Silver (Ag), if used, is done so at a content of 3%. Nickel (Ni) can used at 0.05%. Zinc (Zn) and Manganese (Mn) are other options. Indium (In) is costly, and only used in special applications, as is Gold (Au).
SAC, indicating the Sn, Ag, and Cu symbols for Tin, Silver, and Copper is a common in commercial production, although the silver content adds to the cost.
Weller has a Sn 99.3%, Cu 0.6%, Ni 0.05% product, which without silver is affordable. There is the option to buy as a sample pack with three wire sizes. See: WSW Solder Wire Sampler, 21 grams. Thin solder is ideal for printed circuit board assembly, such as WSW 0.5 mm, 100 grams. For heavy tags and terminals a 1 mm or heavier is best, such as WSW 1.0 mm, 100 grams. As long as the PCBs are not excessively fine, WSW Solder Wire 0.8 mm, 100 grams is a compromise suitable for most jobs. At under $30 for the these rolls, they are not significantly more expensive per unit mass than small hobby packs, and should last a hobbyist a few years. For completeness, there is also a WSW 0.3 mm, 100 grams reel, for very fine hand soldering SMDs.
There are many online videos about soldering, but the basic method is to put the hot iron on the joint, melt a tiny bit of solder onto the iron to improve heat transfer from the iron to the joint, wait a second or so, then apply the solder to the joint, and when enough solder has melted, withdraw both the solder and the iron. The joint must not be disturbed for a few moments while it cools, or the joint can fracture. Note that it is OK to keep soldering other contacts on a PCB.
Australians and Brits pronounce the L, Americans do not. The word comes from the early 1300s English word "soudur", via 1200s French "solduree" and "soulder", from the Latin "solidare", meaning "to make solid". The verb form comes from mid-1300s English, "sawd", meaning to "mend by soldering".
Acid-core solder is only used for non-electronic use, as the aggressive flux would cause the leads to corrode; and (lead-based) solder for aluminium has acrid smelling, very aggressive flux, although the term can also refer to an aluminium brazing rod, with a high melting point. Silver solder, used for jewellery and silver-smithing, contains a large amount of silver, and also has a high melting point. All these are usually used with a gas torch.
Once heated lead-tin solders soften over about a second, and likewise harden over a few seconds. Pure tin snap between states. Note that both remain at a temperature which will cause burns for some time, depending the mass of the joint. Solders with small amounts of silver or/or copper are available, with improved performance.
Always wash your hands after soldering. A small PC-style fan or filtered fan to keep the flux fumes away from your face is a good idea too.
Note that for soldering higher current terminals will typically require a higher powered iron (around 80 w) with a large tip, giving greater thermal mass. A simple small iron may hava a rating of 20 watts, but temperature controlled irons may have a greater rating, allowing fast heat up and recover, without overheating.
The Jaycar / Duratech TS-1620 keeps me happy. It is the Aussie plug version of E14's Duratool product, but contains only the base and iron, without extra tools. E14's include a tube of Sn-Cu solder, cutters, small needle-nose pliers for lead forming, etc: Soldering Kit. The only downside is that it has a UK, plug. This means the plug has to be reloaced with an Australia / NZ one if used here, or plugged into a quality adapter, ideally complying with BS 8546, meaning it has shutters on the socket side. The Active and Neutral pins should be insulated for the first 8 to 9 mm from the plug face, and it must have an earth pin.
While wiping an iron on a damp sponge has been a popular way to clean an iron, a brass wool tip cleaner similar to a stainless pot scourer is a better option. Cleaner in holder (~A$4) and cleaner only (~A$3).
E-14 also have a range of potentially useful drivers, some discussed on my Safety page.
Various medical, space, and safety critical systems have an exemption from ROHS and similar regulations, allowed lead-based solder to be used. This is because pure tin can grow tine crystaline whiskers, which can cause short circuits.
An alternative for cable termination, not on the exam, is crimping connections. This requires a range of specific crimp tools, or a tool, and interchangeable jaws. These deform the rear of the pin or terminal, so they also compress the conductor, and form an air-tight metal-metal interface. Crimping applies to the inner and shield of coax of versions of RF connectors designed for such termination. Examples include BNC, TNC, SMA, N, and "UHF", FME, and other styles. It also applies to various power connectors, such as 0.25" push-one spade terminals, bullet connectors, and eyelets, the latter used for grounding connectors, where a high fault current would cause a soldered joint to melt.
Also crimped are the terminals in Anderson Powerpole connectors, adopted as a standard 12 volt connector by various emergency support groups such as RACES / ARES and WICEN; and many clubs. Ask first, but this often means if you want to bring a 23 cm radio (and antenna) along to a "field day" operating event, you should be able to plug it into a battery or other supply belonging to the group, or another member. RJ connectors, used for telephones, and Ethernet; and for microphones and remote head connections on many radios are crimped. Note that the 6P6C RJ12 connections on the Yaesu FT-857D, which allow the control head to be separated from the body of the radio are wired in reverse to the telephone Commander "key system" terminations. A standard blue cable and coupler can be used to extend the microphone lead. These are also "insulation displacing", as the conductors is not stripped prior to crimping.
A TNC connector is a threaded, or screw-on connector, the name standing for "Threaded Neill–Concelman", after the inventors; it is more stable, and thus superior to the popular bayonet "BNC". The threaded N connector, and the large (and rare) C connector are also named for the gentlemen above. The latter uses a bayonet fitting.
TNC also stands for "Terminal Node Controller", the modem and packet assembler (smart interface) between the PC and Radio in packet radio systems. Some were quite fancy, including mail-boxes, saving having the PC on all the time. The TNC to PC connection was usually a RS-232C serial cable. I think the question writer hopes you will conflate a coaxial connector type with the cable type used to connect a device with the same name.
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| A TNC plug on thinner Heliax, with heat-shrink. This shows the pin, and shield contacts. This is a 50 ohm version, and maintains this impedance through the connection, meaning little power reflection. |
Quadrature means at 90 degrees, such as a phase shift in data systems, or pilot tones for AM Stereo.
"In phase" means two signal rise and fall at the same time, something which also applies to generators, so if you join the Navy, your ship will probably have a generators connected to the main engines, and smaller units running on truck-sized engines, and when switching one into the system, you need to make sure they are in-phase when you close the breaker, or you could black-out the whole ship!
Connecting a voltmeter in quadrature, or in phase are both silly concepts, they are connected in parallel. A synchroscope will help you decide when the close the breaker, although there may well be automatic systems.
Another distracter is "Relaxation Oscillator", which are simple oscillators. An example is a neon lamp flasher, using the Pearson–Anson effect, consisting of a high resistor feeding a capacitor from a DC source of maybe 150 volts or more, with a neon across the capacitor. Once the cap reaches around 90 volts the neon with "strike", and discharge the capacitor to below the minimum voltage at which it can maintain current flow, so it extinguishes, allowing charging to recommence. These are quite cool, as no transistors, etc, are required. Using small compact fluorescent lamps, in double or triple switched arrangements, where there are long lengths on cable can cause a current to flow into the lamp through capacitive coupling between the wires, charging the internal capacitor until the lamp attempts to start, at which point the capacitor discharges, and the lamp shuts down, thus causing it to flash every few seconds - an unintentional relaxation oscillator. See: Relaxation_oscillatorPearson–Anson effect
Once again, these are actual exam questions, from the published NCVEC Technician pool.
T7C01
What is the primary purpose of a dummy load?
A. To prevent transmitting signals over the air when making tests
B. To prevent over-modulation of a transmitter
C. To improve the efficiency of an antenna
D. To improve the signal to noise ratio of a receiver
A dummy load absorbs energy from the radio without radiating, or transmitting it "over-the-air", answer A.
The RF energy becomes becomes heat.
T7C02
Which of the following is used to determine if an antenna is resonant at the desired operating frequency?
A. A VTVM
B. An antenna analyzer
C. A Q meter
D. A frequency counter
To analyse an antenna's characteristics, including its resonant frequency, we just might use an antenna analyser, answer B.
T7C03
What does a dummy load consist of?
A. A high-gain amplifier and a TR switch
B. A non-inductive resistor mounted on a heat sink
C. A low-voltage power supply and a DC relay
D. A 50-ohm reactance used to terminate a transmission line
This consists of a non-inductive resistor, which is mounted on or within a heatshink, answer B.
T7C04
What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?
A. 50:50
B. Zero
C. 1:1
D. Full Scale
The perfect match is a 1:1 ratio, C. The number on the left is always 1 or greater, with 1 on the right. It corresponds to no deflection of the meter needle.
T7C05
Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?
A. To protect the output amplifier transistors
B. To comply with FCC rules on spectral purity
C. Because power supplies cannot supply enough current at high SWR
D. To lower the SWR on the transmission line
Many MODERN radios start winding back power with moderately poor SWR, to protect the output transistors, answer A.
The Yaesu FT-857D manual indicates this starts at 1.5:1, and a warning indicator appears at 3:1.
Note that older amateur and ex-commercial radios can be damaged by excessive SWR.
T7C06
What does an SWR reading of 4:1 indicate?
A. Loss of -4dB
B. Good impedance match
C. Gain of +4dB
D. Impedance mismatch
4:1 is a poor match, or "Impedance mismatch", answer D.
At this SWR the loss is a little less than 2dB.
T7C07
What happens to power lost in a feed line?
A. It increases the SWR
B. It is radiated as harmonics
C. It is converted into heat
D. It distorts the signal
In many systems lost power becomes heat, and feedline losses are no exception, answer C.
T7C08
Which instrument can be used to determine SWR?
A. Voltmeter
B. Ohmmeter
C. Iambic pentameter
D. Directional wattmeter
A Directional watt-meter does a similar job to an SWR meter, but gives its readings as absolute power levels, rather then a ratio. The answer is D.
T7C09
Which of the following causes failure of coaxial cables?
A. Moisture contamination
B. Solder flux contamination
C. Rapid fluctuation in transmitter output power
D. Operation at 100% duty cycle for an extended period
Moisture entering the cable, causes corrosion in the shield, and degrading the dielectric, answer A.
T7C10
Why should the outer jacket of coaxial cable be resistant to ultraviolet light?
A. Ultraviolet resistant jackets prevent harmonic radiation
B. Ultraviolet light can increase losses in the cable’s jacket
C. Ultraviolet and RF signals can mix together, causing interference
D. Ultraviolet light can damage the jacket and allow water to enter the cable
UV light will can make plastic brittle or crumbly, allowing moisture to enter, damaging the inside of the cable, answer D.
Black plastics tend to resist UV better than white or coloured ones.
T7C11
What is a disadvantage of air core coaxial cable when compared to foam or solid dielectric types?
A. It has more loss per foot
B. It cannot be used for VHF or UHF antennas
C. It requires special techniques to prevent water absorption
D. It cannot be used at below freezing temperatures
Air cored, or rather, air-dielectric coax is low-loss, and great for VHF & UHF*, but you do need to follow the manufacturer's instructions to keep water out, answer C.
Heliax cables require also specific connectors, supplied by Andrew. The rear of the connector is sealed with the hot-glue lined heat-shrink tubing supplied with the connector. All connectors used outdoors need appropriate protection, often in the form of a mastic or self-amalgamating tape, followed by black electrical tape, then light grey (which discourages the cockatoos, as it looks like the galvanised tower legs, which hurt their beaks when they try to chew them).
* Very large coaxial cable is unsuitable for higher UHF signals, as is starts to behave like waveguide, and the different speeds of signal propagation through the cable are a big problem.
T7D01
Which instrument would you use to measure electric potential?
A. An ammeter
B. A voltmeter
C. A wavemeter
D. An ohmmeter
This is another term for voltage, whichis measured in volts, so a voltmeter, answer B.
T7D02
How is a voltmeter connected to a component to measure applied voltage?
A. In series
B. In parallel
C. In quadrature
D. In phase
The voltmeter is placed in parallel with the component, answer B.
We may be measuring a voltage coming from a battery or supply element (regulator, etc), or we may wish the know the voltage being dropped across a diode or resistor. In either case, we just plce the meter across it.
T7D03
When configured to measure current, how is a multimeter connected to a component?
A. In series
B. In parallel
C. In quadrature
D. In phase
The ammeter is placed in series with the circuit, so that the current passing through the component also flows through the meter, answer A.
Measuring current within a circuit requires the circuit to be broken in some way, such as unsoldering a termination, or cutting a PCB track. It is also possible to measure across an open switch, or a fuse holder, noting that some form of fusing is still required.
T7D04
Which instrument is used to measure electric current?
A. An ohmmeter
B. A electrometer
C. A voltmeter
D. An ammeter
Current in in Amps (Amperes), so it is the ammeter, answer A.
T7D05 has been removed.
T7D06
Which of the following can damage a multimeter?
A. Attempting to measure resistance using the voltage setting
B. Failing to connect one of the probes to ground
C. Attempting to measure voltage when using the resistance setting
D. Not allowing it to warm up properly
Trying to measure voltage while in the resistance mode could mean too much current flows through the meter's electronics, and if an analogue meter, the meter movement's delicate wire winding. Answer C.
You also can't try to test the resistance of a component while a circuit is live.
T7D07
Which of the following measurements are made using a multimeter?
A. Signal strength and noise
B. Impedance and reactance
C. Voltage and resistance
D. All these choices are correct
A multimeter can measure voltage and resistance, answer C.
T7D08
Which of the following types of solder should not be used for radio and electronic applications?
A. Acid-core solder
B. Lead-tin solder
C. Rosin-core solder
D. Tin-copper solder
Acid-cored solder will cause corrosion of component leads and PCB pads, answer A.
T7D09
What is the characteristic appearance of a cold tin-lead solder joint?
A. Dark black spots
B. A bright or shiny surface
C. A rough or lumpy surface
D. Excessive solder
A poorly heated joint is grainy or dull, answer C.
Such joints are at risk of providing an ineffective electrical joint, or failing physically. The old B&W TV which responded to a solid whack on the top or side may have been suffering for dry joint(s), the result of primative wave soldering of the PCB.
T7D10
What reading indicates that an ohmmeter is connected across a large, discharged capacitor?
A. Increasing resistance with time
B. Decreasing resistance with time
C. Steady full-scale reading
D. Alternating between open and short circuitr
The capacitor initially appears short circuit, then charges to the voltage in the meter's test voltage, appearing open. In other words, the resistance increases, answer A.
T7D11
Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?
A. Ensure that the applied voltages are correct
B. Ensure that the circuit is not powered
C. Ensure that the circuit is grounded
D. Ensure that the circuit is operating at the correct frequency
Ohmmeters must only be used on unpowered circuits, answer B.
Something which turns up periodically on "Farcebook" is Americans complaining about broadcasting on the upper frequency parts of 40 metres. This is because it is part of the 41 metre broadcast band outside the Americas.
On to: Antennas and Feedlines
You can find links to lots more on the Learning Material page.
This has taken a fair bit of work to write and update, so if you have found this useful, there is a "tip jar" below.
Written by Julian Sortland, VK2YJS & AG6LE, June 2023.
Tip Jar: A Jefferson (US$2), A$3 or 50 Mexican Pesos. Thanks!