Home - About AR - Learning Material - Exams - Clubs - Posters
This covers some aspects of the correct set-up of station equipment, and equipment used to aid this set-up.
As mentioned in my Technician notes, combinations of capacitors, inductors, and resistors can be combined to form filters. At audio frequencies operational amplifiers can also be used with these components. In some cases these can be tuned for frequency, slope or notch-width, and amount of cut or boost. Notch filters are mentioned in the questions, and these can be used to remove things like annoying carriers, digital signals, and the like, although if there are digital signals it may be worth checking whether you really should be operating voice there!
On this page filtering questions relate to the receiver or reception.
DSP stands for Digital Signal Processing, and often uses a fairly powerful specialised processor.
It is possible to purchase external devices which connect between the radio's audio output and the speaker or headphones. BHi is one example, while Timewave devices are available used. BHi also make a speaker with inbuilt DSP.
Many modern radios contain DSP ICs, which on intermediate grade radios is applied to the audio frequencies, and in more expensive radios, in the IF (intermediate frequency) stage, and in many, the conversion to audio is also done by DSP. Radios which do this are sold as "Software Defined Radios".
DSP can work as simple filters, cutting out low and high audio frequencies, automatically "notch out" carriers and other tones; and also reduce noise in the audio passband. The amount of Noise Reduction (NR) can be adjusted, as at its most aggressive it can make the voice sound metallic or robotic. It may also filter out music on Shortwave broadcasts. Some DSP may have mode settings, so it does not affect Morse.
DSP is not all-powerful, so a good quality filter(s), such as the Collins / Inrad mechanical products provide the DSP with the best possible signal to work with.
At one point I played with some PC software (free download) which performed waterfall and DSP filtering.
These devices range from the low cost "Soft-rock" and similar board, which down-convert a section of the band down to a band of audio frequencies, which can then be processed by a PC, using its sound-card as the input; to very expensive radios, either as a box controlled by a PC, or with traditional front panels. It is also possible to simply amplify the RF signal and feed it into a card designed for recording composite video. Low cost USB connected DVB-T TV receiver dongles can be used to receive VHF and UHF radio signals, with suitable software. This is the standard in Europe, Australia, NZ, etc, but should be available for posting to markets with other systems from ebay or AliExpress sellers.
Various radios have a "NB" button. This helps to reduce impulse noise from things like car ignition systems. These may have also helped against the Soviet era "Woodpecker" over-the-horizon radars, which used HF radio frequencies. Similar systems are said to be back in operation, with the fascist Putin's criminal invasion of Ukraine.
They work by reducing receiver gain or sensitivity during these pulses.
While radios form the earlier parts of last century contained only valves / tubes, into the late 1970s or early 1980s the typical HF Ham transmitter or transceiver included at least a valve final stage. Now 100 watts PEP output can be achieved from 160 to 6 metres (or even 4 metres) using one solid-state device (meaning a transistor or FET).
However, valves or tubes remain popular for HF linear amplifiers, and can be quite affordable, especially if using Soviet, Russian, or Chinese devices. Note that voltages up to around 2500 volts are used on the anode, and with a large current supply capability, the supply can be LETHAL! Where the supply and the amplifier are separate, you must ensure there is bonding of the supply and the amplifier cases, in addition to suitably rated and protected connections for the supply lines and ground return wires. A single box may be better.
It is also possible to generate around 600 watts using Motorola transistors mounted on a copper heat spreader, then a heatsink. While voltages are less likely to be dangerous, it is easy enough to stuff-up, and destroy these devices instantly. A removed question asserted that the excessive drive power could cause the instant destruction of the device. Supply voltages are 50 to 70 volts, at up to 50 amps. Thinking out loud, 48 volt batteries are coming to vehicles, as either the traction battery on "mild hybrid" cars, or the accessory battery on certain battery-electric vehicles. Maybe these can be used for amps while either mobile, or stationary for field operations.
LD-MOS is one form of RF transistor; searching for "LD-MOS amplifier" can find amplifiers using these. W6PQL is one supplier.
The questions relating to the operation of tube amplifiers also relate to setting up tube final transceivers, and to tube based ex-broadcast transmitters, although the latter tend to only need periodic adjustment. The correct setting of the TUNE control results in a dip in plate (anode) current, as at this point the amplifier is operating efficiently.
An example of a rig with mostly solid-state electronics, but a tube final is the Kenwood TS-820S, dating from 1979, which features both a mechanical dial, and a digital frequency display. This has a Drive control for the drive valve circuit, and Load and Plate controls for the final valve amplifier circuit. If you wish to see the manual, it is on the Kenwood site: TS-820S Manual (PDF).
The FT-101 series from Yaesu was a competing product line, with a similar configuration.
Correct setting of the ALC of a transceiver driving an amplifier is necessary to prevent "excessive drive". Excessive drive may cause distortion, as included in the previous version of the question on this issue. This could result in splatter (interference) outside the intended signal bandwidth, and potentially in the worse cases, damage to the amplifier.
Worth noting is that while voice has an envelope which results in a fairly low average power output, AFSK signals such as RTTY, or the FT8 family of protocols may result in 100% RF power output during transmit periods. In some cases this can stress the transmitter or amplifier. ALC should be off for these modes.
When working pile-ups for a rare DX station, you must transmit away from his or her frequency, usually around 5 or so kHz up, but this varies. This use of different frequencies that is called "split" operation.
Many radios allow you to select between VFO A, VFO B, or SPLIT. Larger radios may have separate dials for this. On smaller radios a regular or "soft" button (on the FT-857D these are the buttons below the display which have multiple functions, depending on the selection using knob on the lower left) select which VFO the dial is tuning.
Off the exam, dual VFOs are also useful during VHF contests where there are stations on both SSB and FM. The A and B VFOs can be handy for switching between 2 metres SSB around 144.150 MHZ, and 2 metres FM around 146.500 or 146.520 MHz. Likewise on 6 metres, on VFO can be SSB from 50.150 MHz and up; and 52.525 MHz FM; Suitable frequencies can be set on 70 cm too; and other bands, as available.
In some cases contacts between US and overseas stations my require such operations, as different frequencies are authorised in different countries, or even in different regions under US jurisdiction. Likewise, where bands such as 4 metres are permitted in Europe (including Greenland), and 8 metres in Ireland; but not the US, transmission from Europe can be on one of their bands, and from the US on 10 or 6 metres.
Occasionally cross-mode may be required, perhaps on 10m where US Techs can only use SSB, and somewhere else only FM, as was once the case for VK Limited licence holders.
While some radios do have two receivers, in many cases dual-VFOs are emulated in firmware, and using Split mode not a whole lot different to repeater mode. Radios such as the FT-847 can adjust the transmit frequency to compensate for Doppler shift during satellite contacts, as the receiver is adjusted for the same reason.
There are a range of devices used to test the SWR of antennas, and for finding their resonant frequency or frequencies.
The SWR Meter has either a single meter movement and a switch for forward or backwards, or a crossed needle meter with two movements on one dial, allowing more direct reading. Some modern transceivers have a built in SWR metering function.
Directional RF watt-meters, especially the Bird 43, with changeable "slugs" or elements, each for a specific power level and frequency range are probably the most accurate.
Both of these devices above use energy from a transmitter to make the readings. Something like finding the resonant frequency of a helically wound mobile antenna for 80 metres requires multiple readings at many frequencies across, and sometimes beyond, the band.
Working without a transmitter, using an internal oscillator, the Antenna Analyser can perform a range of tests on an antenna to reveal its complex impedance. The basic MFJ products are becoming less popular, and they do not directly indicate whether it is inductive or capacitive (such as 42 + 27j Ω vs 42 - 27j Ω); checking at nearby frequencies can provide a pattern. They can however show resonant frequency. The exam indicates that they can be used to determine the characteristic impedance of coaxial cable. Some will show a frequency vs SWR plot on an LCD screen.
Most recently, the NanoVNA product has become available, at prices below most analysers. VNAs are generally more useful than the basic analysers above. While only operating at 1 to a few GHz, these are based on the very expensive Vector Network Analyser, which go to many GHz.
A bit off topic, a spectrum analyser and tracking oscillator can be used to analyse antenna system components such as filters, including the cavity filters used it repeaters. VNAs and NanoVNAs also perform this task.
If you have are using these on a shared site, but with your own antenna(s), signals received by your antenna from nearby transmitting antennas will affect your readings. In the worst case it could damage the test equipment. The same applies if there are nearby broadcast sites, especially TV, FM, or perhaps DAB+ radio.
With the Bird family you should check using the slug used for forwards power in reverse, before using more sensitive ones. Both the slugs and meter movement can be damaged by RF power. Carefully check the orientation, power rating, and frequency range before keying the transmitter. Maybe even say it out loud. Do not use slugs which are much smaller than the forward power, even if SWR is very good. Reading 5 watts reflected on a 50 watt slug is better than risking destroying a 10 watt one, if your forward power is a few hundred watts.
As an aside, these devices are not really suitable for use on existing commercial multiplexed antenna systems, say adding a 70 cm repeater into a system with other UHF services. Any power reflected from the antenna or antenna array ends up in the terminator (dummy load) port off the circulator.
These are usually quite simple circuits, containing just a short antenna (around 10 cm), a germanium or point-contact diode, a ceramic capacitor, and a meter movement, and perhaps a potentiometer to vary the sensitivity. This is very similar to a microwave oven leakage detector. A few include an op-amp.
Most just have a simple 0 to 10 scale, allowing only for relative signal readings. They can be used to compare field strength, for things like evaluating the pattern of antennas, or the relative forward gain of two devices. Devices which give absolute readings are much more complex, and require careful calibration.
Many SWR meters include a basic FS meter.
They certainly cannot assess the quality of a signal, such as distortion of any kind. The question discussing the use of these has been removed.
Calibrated versions exist, and these can be tuned to the frequency of the station of interest. The latest costs US$15,000, and includes GPS and logging functions: PI 4100 Medium Wave Field Strength Meter
A multimeter combined several functions, most often volts and ohms, plus milliamps and/or amps, sometimes given the name V-O-M meter. Simpson Electric sell their $500+ "Made in USA" analogue meters as "VOM multimeters".
There are two primary types, digital and analogue. Digital meters use LCD, OLED, LED, or fluorescent digits. These give a good resolution, with displays going up to 1999, 2999, 3199, 3999, 5999, or 19999, as examples. As 0 is one of the values which can be displayed, meters are sold as 2000 count, 6000 count, etc. With DC voltage and current they also read negative values.
Analogue meters use the sweep of a needle across a calibrated scale to indicate values. For voltage and current tests, most meters are operated only by the energy of the circuit under test. Top-end analogue meters include a valve amplifier (a VTVM, Vacuum Tune Volt Meter), or a FET (field effect transistor) amplifier. These increase the input impedance of the meter.
An analogue meter on the 200 volt range might have a mark for every 2 volts, and the ability to estimate to the half volt, or maybe a quarter, so 200 values or so. An analogue CRO (below) might have 40 marks, and 80 meaningful values, although modern ones have digital measurement functions.
High input impedance means that the meter puts a lower load on the circuit under test. In many cases this is unimportant, but if the circuit has high impedance, then this avoids excessive loading on the circuit, which drags the voltage down, giving a false reading.
If one were to form a voltage divider from a 560 kΩ resistor, and a 910 kΩ resistor we would have 61.9%* of the supply voltage at the junction. Were we to measure this with a 10 mega-ohm input digital meter, or a VTVM (vacuum-tube voltmeter) the loading will be small, and the ready close to the correct value. That said there would still be a small error. However, a 20 kΩ per volt analogue meter on the 5 volt range would place a load of 100k on this circuit, totally swamping it, and giving a reading of a fraction of what it should be. There are some digital meters with significantly higher input impedance, such as Keithley's "Electrometer" products.
I listed some useful meters on my Technician Operations page, along with further comments, and photos.
* The 61.9% figure above was selected, as using this as the reference ratio for one input of a high input impedance comparator IC (LP311) allows a simple timer to be made, using a simple Resistor-Capacitor combination on the other input, with the period being close to one time constant of this simple circuit element, termed τ, or R × C. The actual value reached in one TC is 63.2120558829%, and there are resistor combinations which will set this voltage ratio more closely.
While analogue meters only read positive DC voltages or currents, digital meters simply display the value with a minus sign (-). A few meters even change mode automatically on voltage readings, displaying AC or DC based on which is the prominent value, although this can be over-ridden. Be careful checking hazardous voltages in the last case, and in all cases it is good practice to check the meter with a known live source, if checking a circuit is "dead".
When adjusting tuned circuits we are often looking for a maximum or minimum in the signal or parameter, not an exact level. It is often easier to spot this when we are watching a needle, rather than looking for trends on a digital display.
Non-contact voltage detectors are also handy for checking for mains voltages. I believe most will NOT detect high DC voltages.
Neon or LED testers can also be handy, but do get an approved one. These have indicators for a number of voltage points. Automotive ones do 6, 12, and 24 volts; mains ones typically do 120, 230, 400, 690 or 120, 208, 240, 277, and 480 volts, perhaps 600 too. Some do 12 to 690, including 48 or 50 volts (690 is the delta voltage is 400 volt windings are placed in star or wye), used in things like tunnel ventilation. Kewtech have some nice ones, and the Klein Tools ET60 looks useful too. None have a point for Canadian 346 or 347 volts (the wye of 600 volts delta), but would indicate for 277 volts. Older units may have 50, 127, 220, and 380, the older European voltages (lighting was 127 volts in some areas, the star (L-N) of 220 volts delta used for power (one L-L connection); or maybe 110, 240 and 415, the UK & Commonwealth ones.
Note that the point at which an indicator lights varies, my Sperry (US brand) 120, 240, 277, 480 volt neon tester did not indicate 480 on Aussie 3 phase power at around 430 volts, but the Amprobe (LED?) one did. See pic on my Tech Ops page.
Some meters incorporate a clamp which can measure current without cutting the wire. There are also external current measuring clamps available. Many use a current transformer. In a CT the lead carrying current is the single turn primary of a transformer, outputting a smaller current. It appears that external clamp meters include a shunt resistor, which converts the current to a voltage, so the output is 1 millivolt per amp; and to prevent hazardous voltages appearing on the terminals. Some use a hall effect device instead. The seond most important thing to note is that the current carrying conductors must be separated, otherwise there will be no differential current flow through the transformer, so no output.
A CT or other sensitive clamp tester can be used with both power wires, or with power and earth wires all going through the clamp, to detect leakage currents to earth, either going back through the ground wire, or through an alternative earth path (telecommunications or radio earth, for example). Looking for an imbalance in the power wires is how GFCI / RCD / ELCBR safety switches work.
Some large 3-phase pole transformers in rural NSW include a CT on each phase, feeding a box with three analogue meters, large enough to be visible from the ground. Presumably these assist in balancing loads across the phases, when a new customer is added, etc. Most develop 5 amps at the rated current. The most important thing to note is that what is effectively a 1 to many turn transformer can develop lethal voltages on its terminals.
The original 'scope was the "Cathode Ray Oscilloscope", consisting of a CRT with a grid in front of the phosphor face. (These are cousins of hospital heart-beat monitors, and similar medical displays.) Typically, even in non-metric countries, this is in centimetres, with a central lines marked with 2mm increments. The vertical knob selects the number of millivolts or volts per centimetre, or per division, 8 cm being the typical total height. If the signal is a sinewave, then trace is set to the centre of the scale, such that positive and negative excursions can be displayed. It can be moved towards the bottom of the screen for digital signals, etc.
The sweep knob is for micro-, milli-, or whole seconds per division, with high nano- on some. A width of 10 cm is typical. BYD made a TV sized demonstration unit for schools. This allows the period of a waveform, or a part of it to be determined. If the period if a signal is 2 μs, then the frequency can be determined to be 500 kHz. The ramping voltage from the sweep oscillator is also amplified to several hundred volts to drive the deflection plates.
The main input is also called the Y-input, as a voltage on it deflects the beam up or down, the Y-axis. Internally there is an amplifier which raises the voltage swing to several hundred volts, to electrostatically deflect the electron beam as travels from the cathode to the screen.
The typical CRO has 1 MΩ input impedance. A 10x probe can be used which divides the reading by 10, but increases the input impedance by 10 as well, and helps to overcome the shunt capacitance in the cable and front end amplifier, etc.
Before digital meters, CROs were sometimes used to measure voltage in high impedance circuits. However, the benefit is that you can view complex waveforms, and assess things such as peak or peak-to-peak voltages, see over-shoots on square waves, "glitches" which cause unexpected behaviour, and at least estimate the linearity of things like sawtooth waves (used to scan CRT monitors and TVs).
Most tube based units are mains powered.
CROs have a second input termed the X input or X-axis. Selecting it disconnects the sweep circuit from the X-axis amplifier. Assuming the centre is the zero / resting point negative voltages deflect the beam to the left, and positive voltages deflect to the right.
The horizontal knob has a position which allows the X-input, which deflects the beam left or right. If signals of the same frequency are presented to the X and Y inputs, a circle will be displayed. If one is double, then the signal will be an 8 or an infinity symbol, ∞. If one is three times, you get the ABC (Australia) logo. Yes, this Lissajous curve was the origin of the logo.
| |
| A traditional CRO displaying a 1:3 Lissajous curve. | The simple copyright-ineligible pattern used as the ABC logo. |
Read more at: Wikipedia: Lissajous curve
TV and older computer monitor screens use magnetic fields to deflect the beams. CROs use electrostatic deflection, which can operate at higher frequencies. The CRO thus includes both horizontal and vertical amplifiers to drive the deflection plates.
Some CROs have two or more (vertical) channels. These can display things like the input and output of a circuit, such as an amplifier; or they can display the sum of, or difference between, the two. They will also display phase shift in filters. Fancy ones can display things like a TV field on one line, and a single, selected line on the other (clearly the timebase used in this case is different). There are also big dollar analogue storage units, which are large, very complex devices, some trolley mounted.
True CROs have mostly been replaced by digital storage oscilloscopes (DSOs) which in most cases use coloured LCD displays. Older units were monochrome. Some are in a vertical or tablet-like handheld portable format. Element 14 have Multicomp PRO branded digital CROs in various styles from under A$300 in multimeter-style format. and bench-top for A$470. Paying a few tens of dollars above the lowest price may provide a unit with significantly higher frequency response. Some are very sophisticated able to perform analysis on filters, if networked with an oscillator.
If an attenuated sample of the modulated signal from a transmitter is put into the X or vertical input, with the normal timebase running, then an envelope will be displayed. For an audio sinewave feeding an AM transmitter this will be a sinewave in the upper of the display, and a mirrored one at the bottom, the space between filled with a dim trace at RF. A well modulated signal will have maybe a 90% reduction in level, and a 90% increase. With good processing a signal may have 95% downward modulation, and 125% upward modulation, this called asymmetrical modulation; although this is normally done in broadcasting, rather than ham radio. That said, the 160 metre broadcast from ARNSW uses asymmetrical AM, as they have a broadcast transmitter.
For a Morse signal the envelope should not be a square-wave, but should have a rounding at the top of the leading edge, and also as it drops away. This avoids key-clicks being heard outside the signal's normal bandwidth.
Even an old Tekronix or Australian made BWD CRO is most useful for understanding the behaviour of many circuits. BWDs use transistors for initial amplifiers and oscillators, and valves / tubes for the output stages driving the plates.
Note that the input signal, along with the timebase, act directly on the beam. Modern "digital storage oscilloscopes" use what can be termed a "raster display", as they are the output of the internal computer. There were expensive analogue storage CROs, quite complex to use. The analogue circular screen vs raster distinction also exists in radars.
You can see images on Wikipedia: Oscilloscope. One image shows what appears to be a sine wave distorted by "flat-topping", where an amplifier has too much gain, or the input is too high, and the output tries to get too close to or exceed the supply rail voltages.
For completeness most include a Z input. Application of a voltage to this input dims or extinguishes the trace. A negative voltage may brighten it. These are used by various accessories, including component testers.
For smaller bench-top test equipment building a shelf so they are at eye level can be useful. Sometimes colleges end programmes or update equipment, and dispose of gear which can still be useful for hobby use, some times hams working in these areas can obtain such gear for club members.
USB connected oscilloscopes exist, ranging from little better than audio frequency hobby units to professional grade, with varying voltage ratings.
To determine the Peak Envelope Power of an SSB transmitter, or an amplifier used with an SSB transmitter, the "two-tone test" in performed. While monitoring the output with a oscilloscope two audio signals which are not harmonically related are fed into the transmitter's microphone input, and the peak voltage of the modulated signal is measured across a 50 ohm dummy load. A spectrum analyser can also be used, especially in the case below.
However, the examiner sees that using this to determine linearity in transmitters. Non-linearity can cause unwanted "products" in the transmitted signal, and potential interference. You can read more on: Wikipedia: Two-tone testing
You first divide it by root 2 (you can also multiply by 0.7071). Note that with these signals the peak voltage is half the peak to peak voltage. You then place the resultant RMS voltage into the power formula. In most cases R is 50 ohms.
VPEAK = VP-P / 2
VRMS = VPEAK / √2
P = VRMS² / R
Say we read the signal at 50 volts per division, and it is 8 divisions (often cm) tall, peak to peak, so VP-P = 400.
VPEAK = 400 / 2 = 200
VRMS = 200 / √2 = 141.42135623731
P = 141.42135623731² / 50 = 20000 / 50 = 400 watts PEP
400 watts PEP is the Australian limit. Just to prove the two 400 values are just me using goofy values, try 7.6 cm. Or try 8 divisions at 100 volts per division. What about 273.86 VRMS? Answers are below.
Formal questions on this are in Electrical Principles 1, in a few pages time.
You can also make various DIY (do-it-yourself, or "home-brew") test jigs and test gear. These can be stand-alone, or be an accessory to a multimeter or CRO. My LED-Tester is an example of the latter. Others include V-I "curve tracers" to characterise diodes and transistors, and simpler zener diode testers. Clearly, ones capable of fully testing higher voltage devices will use potentially hazardous voltages and/or currents, as do valve / tube testers.
Kit multimeters also exist, but many are perhaps a bit "ordinary". ICL7107 based voltmeter kits out of China on eBay might be better value.
These are actual questions from the General exam pool.
G4A01
What is the purpose of the notch filter found on many HF transceivers?
A. To restrict the transmitter voice bandwidth
B. To reduce interference from carriers in the receiver passband
C. To eliminate receiver interference from impulse noise sources
D. To remove interfering splatter generated by signals on adjacent frequencies
If a carrier or other signal is in the receiver passband, it will be heard as a tone, whine, or whistle. A notch filter removes or reduces this tone, answer B.
These can be manually set analogue device, or part of a DSP (digital signal processing) function, internal or external to the radio, in which case the notching may be done automatically.
G4A02
What is the benefit of using the opposite or "reverse" sideband when receiving CW?
A. Interference from impulse noise will be eliminated
B. More stations can be accommodated within a given signal passband
C. It may be possible to reduce or eliminate interference from other signals
D. Accidental out of band operation can be prevented
Radios such as the FT-857D have a range of modes such as AM, FM, DIG, USB, LSB, CW, and CW-R. Selecting CW-R potentially helps reduce interference which would otherwise be heard using the standard CW mode, answer C.
Note that it is possible for the standard position to have less interference, while interference is present on the reverse sideband, so CW-R isn't a magic Interference-Go-Away button, just an option to try. This typically applies to Morse, but may help with something like Feld-Hell, also sent using on-off keying.
G4A03
How does a noise blanker work?
A. By temporarily increasing received bandwidth
B. By redirecting noise pulses into a filter capacitor
C. By reducing receiver gain during a noise pulse
D. By clipping noise peaks
Noise blankers reduce the impact of impulse noise by reducing gain during these pulses, answer C.
G4A04
What is the effect on plate current of the correct setting of a vacuum-tube RF power amplifier's TUNE control?
A. A pronounced peak
B. A pronounced dip
C. No change will be observed
D. A slow, rhythmic oscillation
A dip in the current indicates the correct tuning of this control, and the best efficiency, answer B.
G4A05
Why is automatic level control (ALC) used with an RF power amplifier?
A. To balance the transmitter audio frequency response
B. To reduce harmonic radiation
C. To prevent excessive drive
D. To increase overall efficiency
ALC is used to prevent excessive drive into the amplifier, answer C.
G4A06
What is the purpose of an antenna tuner?
A. Reduce the SWR in the feed line to the antenna
B. Reduce the power dissipation in the feedline to the antenna
C. Increase power transfer from the transmitter to the feed line
D. All these choices are correct
An antenna tuner (aka antenna coupler), can help match a transmitter to an antenna feedline, increasing the transfer of power to that feedline, answer C.
These allow an antenna to be used on a frequency at which it is not resonant; or where it is resonant, but has a different impedance, such as a delta loop. These questions are discussing the situation where the tuner is located at the operating position, and the coax or balanced line runs from there to the antenna. Thus the same SWR occurs on the cable as without the tuner, and dissipation is not reduced. This differs from the practice at MF (AM) broadcast stations, where the tuning is done in a hut at the base of the mast.
G4A07
What happens as a receiver's noise reduction control level is increased?
A. Received signals may become distorted
B. Received frequency may become unstable
C. CW signals may become severely attenuated
D. Received frequency may shift several kHz
Cranking up the DSP can cause the voice to become metallic and distorted, answer A.
G4A08
What is the correct adjustment for the LOAD or COUPLING control of a vacuum tube RF power amplifier?
A. Minimum SWR on the antenna
B. Minimum plate current without exceeding maximum allowable grid current
C. Highest plate voltage while minimizing grid current
D. Maximum power output without exceeding maximum allowable plate current
This knob should be adjusted for maximum power without excessive plate current, answer D.
G4A09
What is the purpose of delaying RF output after activating a transmitter's keying line to an external amplifier?
A. To prevent key clicks on CW
B. To prevent transient overmodulation
C. To allow time for the amplifier to switch the antenna between the transceiver and the amplifier output
D. To allow time for the amplifier power supply to reach operating level
This allows time for the amplifier to switch into line, rather than bypassing the antenna into the transceiver during receive, answer C.
When using transverters, pre-amplifiers, and/or power amplifiers, it is necessary to ensure the various devices are and ready for the transmitted signal, or switched out of the signal path, as required.
G4A10
What is the function of an electronic keyer?
A. Automatic transmit/receive switching
B. Automatic generation of dots and dashes for CW operation
C. To allow time for switching the antenna from the receiver to the transmitter
D. Computer interface for PSK and RTTY operation
These are an electronic circuit which generate Morse dits and dahs using simple switch contacts as the input, rather than a complex mechanical device with a pendulum, answer B.
G4A11
Why should the ALC system be inactive when transmitting AFSK data signals?
A. ALC will invert the modulation of the AFSK mode
B. The ALC action distorts the signal
C. When using digital modes, too much ALC activity can cause the transmitter to overheat
D. All these choices are correct
The ALC can apparently distort AFSK signals, so should be turned off, answer B.
G4A12
Which of the following is a common use for the dual-VFO feature on a transceiver?
A. To allow transmitting on two frequencies at once
B. To permit full duplex operation -- that is transmitting and receiving at the same time
C. To transmit on one frequency and listen on another
D. To improve frequency accuracy by allowing variable frequency output (VFO) operation
This allows you to listen to one frequency, and transmit on another, answer C.
This is useful whether you are a station chasing a DX station, or you are the DX station dealing with a pile-up, or "dog pile".
G4A13
What is the purpose of using a receive attenuator?
A. To prevent receiver overload from strong incoming signals
B. To reduce the transmitter power when driving a linear amplifier
C. To reduce power consumption when operating from batteries
D. To reduce excessive audio level on strong signals
The attenuator can reduce the level of strong, typically local, signals to prevent overload of the receiver, answer A.
G4B01
What item of test equipment contains horizontal and vertical channel amplifiers?
A. An ohmmeter
B. A signal generator
C. An ammeter
D. An oscilloscope
This is the oscilloscope, which displays signals on a screen, answer D.
While for many uses, the internal ramp generator provides the horizontal component, there are tasks, such as frequency comparison, where both X and Y inputs are used.
G4B02
Which of the following is an advantage of an oscilloscope versus a digital voltmeter?
A. An oscilloscope uses less power
B. Complex impedances can be easily measured
C. Greater precision
D. Complex waveforms can be measured
A 'scope allows measurement and visualisation of complex waveforms, and determination of peak or peak-to-peak voltages on these, answer D.
Budget digital meters do not give accurate RMS readings for waveforms other than sine-waves. Peak values of unusual waveforms also cannot be determined, as both the average and RMS values bear little relationship to the peak value in these cases. Also, things like clipping and gross distortion can be observed, and likewise the duration of pulses.
G4B03
Which of the following is the best instrument to use when checking the keying waveform of a CW transmitter?
A. An oscilloscope
B. A field strength meter
C. A sidetone monitor
D. A wavemeter
Again, this is the CRO, or oscilloscope, answer A.
A variation on this is the "station monitor" or similar product, previously sold as an accessory for the flagship valve based home station rigs.
G4B04
What signal source is connected to the vertical input of an oscilloscope when checking the RF envelope pattern of a transmitted signal?
A. The local oscillator of the transmitter
B. An external RF oscillator
C. The transmitter balanced mixer output
D. The attenuated RF output of the transmitter
An ATTENUATED sample of the RF output of the transmitter, answer D, is connected to the vertical input of the "silly-scope".
G4B05
Why do voltmeters have high input impedance?
A. It improves the frequency response
B. It allows for higher voltages to be safely measured
C. It improves the resolution of the readings
D. It decreases the loading on circuits being measured
A high input impedance means that it places less load on the circuit being tested, answer D.
This is important to provide accurate readings in high impedance circuits, and even to not affect the operation of the circuit under test due to loading.
G4B06
What is an advantage of a digital voltmeter as compared to an analog voltmeter?
A. Better for measuring computer circuits
B. Less prone to overload
C. Higher precision
D. Faster response
These generally have greater precision, answer C.
A digital meter can directly display a voltage, such as 13.47 volts, while on the 25 volt range of an analogue meter, you will only have marks for each half volt, and can really only guesstimate to a quarter volt. An example is on my Technician Operations page.
G4B07
What signals are used to conduct a two-tone test?
A. Two audio signals of the same frequency shifted 90 degrees
B. Two non-harmonically related audio signals
C. Two swept frequency tones
D. Two audio frequency range square wave signals of equal amplitude
Two audio signals which are not harmonically related are used, answer B.
G4B08
What transmitter performance parameter does a two-tone test analyze?
A. Linearity
B. Percentage of suppression of the carrier and undesired sideband for SSB
C. Percentage of frequency modulation
D. Percentage of carrier phase shift
This allows linearity do be assessed, answer A.
G4B09
When is an analog multimeter preferred to a digital multimeter?
A. When testing logic circuits
B. When high precision is desired
C. When measuring the frequency of an oscillator
D. When adjusting circuits for maximum or minimum values
In many case you may be adjusting one or more small inductors or trimmer capacitors with an insulated tool, while seeking a minimum or maximum deflection, answer D.
G4B10
Which of the following can be determined with a directional wattmeter?
A. Standing wave ratio
B. Antenna front-to-back ratio
C. RF interference
D. Radio wave propagation
By taking forward and reflected power readings, and performing calculations, the SWR can be determined, answer A.
G4B11
Which of the following must be connected to an antenna analyzer when it is being used for SWR measurements?
A. Receiver
B. Transmitter
C. Antenna and feed line
D. All of these choices are correct
The antenna feedline and antenna are connected to the analyser, answer C.
There is no need for other equipment, such as a transmitter, as the analyser contains an RF oscillator.
G4B12
What effect can strong signals from nearby transmitters have on an antenna analyzer?
A. Desensitization which can cause intermodulation products which interfere with impedance readings
B. Received power that interferes with SWR readings
C. Generation of harmonics which interfere with frequency readings
D. All these choices are correct
Signals coming back down the feedline from nearby transmitters can, at the very least, affect the measurements, answer B.
G4B13
Which of the following can be measured with an antenna analyzer?
A. Front-to-back ratio of an antenna
B. Power output from a transmitter
C. Impedance of coaxial cable
D. Gain of a directional antenna
These analysers can measure impedance, so this can be applied to determining if a cable is 50 ohm, 75 ohm, or something else, answer C.
No adequate answer is provided as to how this is achieved. However, two suggestions are provided on page 23 of this MFJ-259B manual. Certainly, the impedance of low quality or deteriorated coax can vary from that of new quality product.
A VNA can be used as well.
Good quality coax may be available affordably as short ends off reels at hamfests, etc.
The answers to the PEP power calculations above. Note that these are not part of the exam for this level, all you need to know is that two tones are used, and that one is not the harmonic of the other.
VP-P - 7.6 × 50 = 380. VPEAK = 380 / 2 = 190 volts.
VRMS = 190 / √2 = 134.350288425444 volts.
P = 134.350288425444² / 50 = 18050 / 50 = 361 watts PEP.
VP-P - 8 × 100 = 800. VPEAK = 800 / 2 = 400 volts.
VRMS = 400 / √2 = 282.842712474619 volts
P = 282.842712474619² / 50 = 80000 / 50 = 1600 watts PEP. Cheeky!
P = 273.86² / 50 = 74999.2996 / 50 = 1499.985992 watts PEP.
The VP-P for this power is 774.59 volts, so the trace peaks at +3.87 and -3.87 cm, a mere 1.3 millimetre less on the top and bottom than he 1600 watt example above. Given this is little more than the thickness of the trace, super-accurate measurement is not possible.
Given I = √(P/R) = √(1500/50) = √30 = 5.477225575 amps.
This is comparable to a bar heater in a 230 volt country, or a heating panel in commercial accommodation: 277 volts × 5.4 amps = 1495.8 watts.
Note that while voltages at the centre of a directly fed dipole might be reasonable, note that voltages at the ends, where current is low, can be very high.
Not every antenna system is 50 ohms. Some TV broadcast systems are 75 ohms, for example, and a range of high power coax is available, from RG-11 to Heliax, and very large coax made from concentric copper tubes.
Meanwhile, a delta or other loop, consisting of around a wavelength of wire, has an impedance of 102 ohms. A typical HF rig outputs 100 watts. Lets chuck around a few numbers:
VRMS = √(P × R) = √(100 × 102) = √10200 = 100.995049383621 volts.
I = √(100/102) = √0.980392156862745 = 0.990147542976674 amps.
This is comparable to 110 volts applied to a 100 watts lamp, with 0.90909 amps flowing.
Balanced line varies from around 300 to 600 ohms. For "legal limit" power: VRMS = √(1500 × 600) = √900000 = 948.683298050514 volts.
End-Fed Half-wave (EFHW) antennas use transformers or "UNUNs" with a high windings ratio, which can place extremely high voltages on the output, even at more sensible powers.
On to: Amateur Practices 2 - Interference, DSP, Speech processors, and S-meters
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, August 2024.
Tip Jar: a Jefferson (US$2), A$3 or other amount / currency. Thanks!