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Signals on each band travel between stations in somewhat different ways, depending on various factors, discussed here.
VHF and UHF signals travel primarily in what is termed "line of sight", although this does extend somewhat beyond a true optical path. However, unless you are operating between mountains tops, range is typically limited to tens of kilometres, maybe a hundred or so to a well located repeater. Using SSB or CW, and high gain directional antennas, range can extend beyond this.
More correctly, VHF signals leave your antenna, and continue into space, literally "going off on a tangent". The trick is to get some of that signal to return to earth.
On 6 metres, at times signals can refract from the ionosphere, and this can also occur on 2 metres and 1.25 metres irregularly, to the extent one such mechanism is called Sporadic E. 10 metres, and even 12 metre ionosphere propagation is sporadic. The F2 layer also provides VHF DX at times.
The trails of ionised material left by meteors can reflect signals, allowing for short bursts of extended radio propagation. While this occurs on various bands, the examiner believes that 6 metres is the best.
Tropospheric Scatter was a commercial system where a high power UHF or microwave signal is fed into a large dish, directed towards the receiving station. A small portion of the signal refracts due to moisture and dust in the atmosphere, reaching the receiving station which consists of several large dishes, spaced some metres apart, for diversity. With a range of over 1000 km, this system has however fallen out of favour, in favour of satellite systems. In Amateur use this signal scattering is also called "tropo-scatter", or just "tropo".
Tropospheric ducts occur when a temperature inversions cause signals to be trapped between the earth and the inversion, meaning that VHF and UHF signal can travel a significant distance. Sometimes ducts can be formed between layers above the ground. Another effect of an inversion can be that a repeater on a mountain can become inaccessible to those on the plain below.
If one has had the privilege of seeing an aurora, you will notice that it shimmers and flutters. This rapid changing thus exhibits itself if VHF radio signals bound off it, with rapid signal level changes, and distortion. The term "backscatter" applies to this propagation.
How about we put sheets of metal about 10km above the ground, and bounce signals off them? We do! Signals can reflect from either the bodies of large aircraft, or their condensation trails (that's contrails, not chemtrails, the latter being the result of the combustion of Tetrahydrocannabinol*). VHF signals can pass well long paths, such as Sydney to Melbourne, when aircraft are around half-way along the route.
*THC - probably a little unkind to stoners, but you would have to be off your scone to believe 40,000 pilots and ground-crew would poison their own families. And HAARP is nothing more than a system to experiment with the ionosphere, not a weather or mind control device.
Many of these phenomena provide both DX for broadcast FM and TV, and can lead to interference between stations. Some of these can also cause interference between VHF marine bases.
Long distance propagation works best with narrower signals, such as SSB, single-sideband, and CW / Morse. There are also "weak signal" digital modes for VHF and UHF DX. All these modes are more often used with horizontal antennas, such as Yagis, cubic-quads, and omni-directional "halo" designs. However, don't let this put you off trying SSB, if you only have a vertical.
On HF bands, there is a generalisation that, the higher the sun, the higher the frequency which should be used. This also varies with time of year, and the 11 year sunspot cycle. For 10 metres to have regular ionospheric support, there typically must be both a period of high sunspot activity, and daylight. At other times 10 metres has similar properties as 27 MHz CB, with only local coverage.
"Gray-line" relates to MF and low HF propagation. It allows the reception of distant MF stations in the early evening and around dawn. An example is interstate high power ABC stations in Australia, and 2ZB on 1035 kHz from NZ to Australia. Norwegians can hear BBC Scotland on 810 kHz. This is something you can try in most areas, just by tuning an AM radio around the band in the evening. The AM radio on an older car, especially one with an antenna a metre long, works well, with the ignition off. That said, in the US, there may be a lot of clutter with local stations, a requirement often exists to reduce power at night. Perhaps Spanish from Cuba will be audible on the lower part of the band.
While some countries (including Australia under the current right-wing regime) have killed of shortwave broadcasting, this also uses the ionosphere to send signals across the globe. Shortwave was also used in Australia to provide news and weather, plus general programming across the Northern Territory, but I suppose residents of remote communities don't matter to this government.
Rain and fog have little impact on HF and VHF signals, but rain and snow has significant impact on the microwave bands. Given satellite TV distribution, including to many terrestrial transmission sites, is also microwave spectrum, this can be affected by rain. To counter this, national broadcasting organisations use large (6 metre) dishes, and in Darwin they have two sites, for "spatial diversity".
Especially at 10 GHz (3 cm), signals can be bounced off storms to make longer distance contacts. Meanwhile on 6 metres, South African Amateurs have found that two stations at a safe distance from a lightning storm can reflect signals off it, perhaps from the ionised columns of air following lightning strikes.
Things like metal fences and buildings can reflect signals and the direct signal and the reflections can interact, with cancellations causing weak signals in some locations. Moving your radio, or its antenna, a little can help overcome these "multipath" related cancellations.
On VHF and UHF, the motion of a vehicle can result in you passing through peaks and troughs in signal level several or a few dozen times a second, causing fluttering of the received signal.
On HF, at times signals may reach your station via groundwave and skywave, in other words, via direct VHF-like propagation, and off the ionosphere. Or a signal can travel via the short and long path at the same time, say Britain to Australia over Asia, and over Antarctica and the Atlantic. Two things can happen, being that the signal can be cancelled, or that the time of arrival is different. In the latter case, two elements of a data signal can arrive at the same time, confusing the data demodulator, resulting in errors.
Actual exam questions, from the published NCVEC Technician pool.
T3A01
Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?
A. The signal path encounters different concentrations of water vapor
B. VHF ionospheric propagation is very sensitive to path length
C. Multipath propagation cancels or reinforces signals
D. All these choices are correct
Multipath propagation can cause "nulls", where the signal is weak, answer C.
T3A02
What is the effect of vegetation on UHF and microwave signals?
A. Knife-edge diffraction
B. Absorption
C. Amplification
D. Polarization rotation
Vegetations, especially if it is wet, can absorb radio sugnals, answer B.
This is noticed in TD-LTE and similar wireless internet access systems, where a path through vegetation is lost when it rains.
T3A03
What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands?
A. Right-hand circular
B. Left-hand circular
C. Horizontal
D. Vertical
This operation usually uses horizontal antennas, answer C.
This is said to work better, but the other reason is convention. Using the same polarisation as other stations generally means signals are stronger.
T3A04
What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?
A. The modulation sidebands might become inverted
B. Signals could be significantly weaker
C. Signals have an echo effect
D. Nothing significant will happen
In this case signals are can be significantly weaker, answer B.
The loss figure is said to be an extra 20 dB.
T3A05
When using a directional antenna, how might your station be able to access a distant repeater if buildings or obstructions are blocking the direct line of sight path?
A. Change from vertical to horizontal polarization
B. Try to find a path that reflects signals to the repeater
C. Try the long path
D. Increase the antenna SWR
VHF and UHF signals can be reflected off terrain or buildings, especially if they are metallic, answer B.
While the Snowy Mountains hydro-electric scheme was being built, they set up plain metal billboard-like structures, which were used to bounce microwave signals off.
T3A06
What is the meaning of the term "picket fencing"?
A. Alternating transmissions during a net operation
B. Rapid flutter on mobile signals due to multipath propagation
C. A type of ground system used with vertical antennas
D. Local vs long-distance communications
While driving, at times you may pass through an area where there are peaks and nulls in the signal level, due to multi-path propagation. Especially on the 2 metre band, you may pass through 10 of these per second, resulting in rapid fluttering on the signal, sounding like varying reflections from a picket fence, answer B.
A similar audio effect occurs applying the Size 12 while climbing through Galston Gorge, with exhaust noise echoing between the rock wall and the heavy square timber posts supporting the mesh on the cliff edge, resulting in pulsing of the sound.
T3A07
What weather condition might decrease range at microwave frequencies?
A. High winds
B. Low barometric pressure
C. Precipitation
D. Colder temperatures
Rain snow, and hail, etc, can attenuate microwave radio signals, answer C.
T3A08
Which of the following is a likely cause of irregular fading of signals received by ionospheric?
A. Frequency shift due to Faraday rotation
B. Interference from thunderstorms
C. Intermodulation distortion
D. Random combining of signals arriving via different paths
If signals arrive via two or more paths, at some times the signal will add, to give a strong signal, and at other times they will cancel each other out, resulting in a near zero signal, answer D.
T3A09
Which of the following results from the fact that skip signals refracted from the ionosphere are elliptically polarized?
A. Digital modes are unusable
B. Either vertically or horizontally polarized antennas may be used for transmission or reception
C. FM voice is unusable
D. Both the transmitting and receiving antennas must be of the same polarization
Once the signal has refracted from the ionosphere it is possible to receive it using an antenna of either polarisation, and likewise, both vertical and horizontal antennas can be used to transmit with, answer B.
Selection of vertical or horizontal antennas depends, for instance, whether you are hoping to work long distances, or take part in a net covering a few hundred kilometres. Available space is also a consideration.
T3A10
What effect does multi-path propagation have on data transmissions?
A. Transmission rates must be increased by a factor equal to the number of separate paths observed
B. Transmission rates must be decreased by a factor equal to the number of separate paths observed
C. No significant changes will occur if the signals are transmitting using FM
D. Error rates are likely to increase
Signals arriving at different times may cause errors, answer D.
T3A11
Which region of the atmosphere can refract or bend HF and VHF radio waves?
A. The stratosphere
B. The troposphere
C. The ionosphere
D. The mesosphere
This is the ionosphere, answer C.
T3A12
What is the effect of fog and rain on signals in the 10 meter and 6 meter bands?
A. Absorption
B. There is little effect
C. Deflection
D. Range increase
This weather has little effect at these frequencies, so B.
T3C01
Why are simplex UHF signals rarely heard beyond their radio horizon?
A. They are too weak to go very far
B. FCC regulations prohibit them from going more than 50 miles
C. UHF signals are usually not propagated by the ionosphere
D. UHF signals are absorbed by the ionospheric D layer
UHF signals are rarely reflected (or refracted) by the ionosphere, if ever, answer C.
The FCC can't prohibit a signal going anywhere, but they used to prohibit deliberate CB communications beyond 250 km (155.3 miles). At least they are not like the failed merchant banker*, Turnbull who claimed that "the laws of Australia override the laws of mathematics", in reference to encryption.
* Interesting how reality matches rhyming slang for politicians.
T3C02
What is a characteristic of HF communication compared with communications on VHF and higher frequencies?
A. HF antennas are generally smaller
B. HF accommodates wider bandwidth signals
C. Long distance ionospheric propagation is far more common on HF
D. There is less atmospheric interference (static) on HF
Long distance ionospheric propagation is common on most HF bands, while "Sporadic E" providing VHF-DX is rare, answer C.
Access to several HF bands, especially if this includes 60 metres, can provide quite reliable communications over many paths.
T3C03
What is a characteristic of VHF signals received via aurorall backscatter?
A. They are often received from 10,000 miles or more
B. They are distorted and signal strength varies considerably
C. They occur only during winter nighttime hours
D. They are generally strongest when your antenna is aimed west
Signals fluctuate rapidly, and may be distorted, answer B.
Aurora occurs at both poles, but are only visually observable during darkness, and darkness at night only occurs in winter at the poles, as there is "midnight sun" in summer.
T3C04
Which of the following types of propagation is most commonly associated with occasional strong signals on the 10, 6, and 2 meter bands from beyond the radio horizon?
A. Backscatter
B. Sporadic E
C. D layer absorption
D. Gray-line propagation
Sporadic E is the only propagation type offering good signals on these bands, answer B.
Backscatter signals are weak, and often distorted; and grey-line is for MF and low HF bands.
T3C05
Which of the following effects may allow radio signals to travel beyond obstructions between the transmitting and receiving stations?
A. Knife-edge diffraction
B. Faraday rotation
C. Quantum tunneling
D. Doppler shift
VHF and UHF signals can "trip" over cliff edges and sharp ridges, in a process called "knife-edge diffraction", answer A.
T3C06
What type of propagation is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?
A. Tropospheric scatter
B. D layer refraction
C. F2 layer refraction
D. Faraday rotation
Tropospheric scatter can provide communication over around 500 km on a regular basis, answer A.
This occurs on things like Sydney to Melbourne weekend morning skeds. F2 is much more occasional.
T3C07
What band is best suited for communicating via meteor scatter?
A. 33 centimeters
B. 6 meters
C. 2 meters
D. 70 centimeters
6 metres is best for meteor scatter, reflecting signals off the ionised trail, answer B.
This does not rule out other VHF and UHF bands for such operation.
T3C08
What causes tropospheric ducting?
A. Discharges of lightning during electrical storms
B. Sunspots and solar flares
C. Updrafts from hurricanes and tornadoes
D. Temperature inversions in the atmosphere
The troposphere is the layer at which weather occurs, including temperature inversions. These provide long range propagation of VHF, UHF, and SHF signals, answer D.
These are related to a stable atmosphere, which occurs in eastern Australia when there is a high pressure cell off the coast.
T3C09
What is generally the best time for long-distance 10 meter band propagation via the F region?
A. From dawn to shortly after sunset during periods of high sunspot activity
B. From shortly after sunset to dawn during periods of high sunspot activity
C. From dawn to shortly after sunset during periods of low sunspot activity
D. From shortly after sunset to dawn during periods of low sunspot activity
High sunspot activity means F layer supports the propagation of 10 metres during the day, answer A.
T3C10
Which of the following bands may provide long-distance communications via the ionosphere’s F region during the peak of the sunspot cycle?
A. 6 or 10 meter bands
B. 23 centimeter band
C. 70 centimeter or 1.25 meter bands
D. All of these choices are correct
Sunspots alter the ionosphere, and thus it is the two bands which are in the HF to VHF transition, 10m and 6m, answer A.
T3C11
Why is the radio horizon for VHF and UHF signals more distant than the visual horizon?
A. Radio signals move somewhat faster than the speed of light
B. Radio waves are not blocked by dust particles
C. The atmosphere refracts radio waves slightly
D. Radio waves are blocked by dust particles
The atmosphere refracts radio waves slightly, answer C.
On to: Transceivers
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Written by Julian Sortland, VK2YJS & AG6LE, February 2018.
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