Multipath: the same signal, twice
A VHF signal reaching a receiver has usually taken more than one path — direct, plus reflections from buildings, terrain, vehicles, and the ground. Those copies arrive at slightly different times, and where they meet they add or cancel.
Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?
Multipath propagation cancels or reinforces signals.
A few feet is the tell. At 146 MHz a wavelength is about two metres, so moving a metre can swing you from constructive to destructive interference. This is why walking three steps with a handheld sometimes fixes a marginal contact, and why it is worth trying before assuming you are out of range.
The mobile version has its own name:
What is the meaning of the term “picket fencing”?
Rapid flutter on mobile signals due to multipath propagation.
A moving vehicle sweeps through the interference pattern continuously, so the signal rises and falls several times a second — a rasping flutter that sounds like driving past a picket fence.
Multipath also hurts data. Delayed copies smear symbol boundaries, so error rates are likely to increase. It is the reason digital modes designed for mobile use build in heavy error correction.
Polarisation, again — and it costs you
What is the effect of antenna cross-polarization over a line-of-sight VHF or UHF path?
Received signal strength is reduced.
Substantially reduced — typically 20 dB or more. Hence the two conventions:
- FM repeater work: vertical, because handhelds and mobile whips are.
- Long-distance CW and SSB on VHF: horizontal.
What antenna polarization is normally used for long-distance CW and SSB contacts on the VHF and UHF bands?
Horizontal.
Ionospheric propagation is different. Signals that have bounced off the ionosphere have had their polarisation scrambled along the way, so either vertically or horizontally polarised antennas may be used for HF work. The polarisation discipline is a VHF and UHF line-of-sight concern.
What the atmosphere does at each band
The pattern to hold: higher frequency, more affected by physical obstacles and weather.
| Effect | 10 m and 6 m | UHF and microwave |
|---|---|---|
| Fog or rain | little effect | precipitation decreases range at microwave |
| Vegetation | little effect | absorbs signals, hurting weak reception |
How does vegetation affect UHF and microwave signals?
Absorbs signals, leading to poor reception of weak signals.
At microwave the wavelength approaches the size of leaves and raindrops, so they scatter and absorb. At 6 metres the wavelength is metres long and a tree is barely there. This is the single most useful reason to prefer VHF over UHF in wooded terrain — and why MURS at 151 MHz beats GMRS at 462 MHz in the woods.
Getting around obstructions
When using a directional antenna, how might your station be able to communicate with a distant repeater if buildings or obstructions are blocking the direct line of sight path?
Try to find a path that reflects signals to the repeater.
Point the beam at a building, a hillside, or a water tower and bounce off it. This works far better than intuition suggests, and it is the standard trick for working a repeater from behind a ridge.
The related mechanism:
Which of the following effects may allow radio signals to travel beyond obstructions between the transmitting and receiving stations?
Knife-edge diffraction.
A sharp obstruction — a ridge line, a building edge — bends the wave around it, so signal appears in the geometric shadow. A sharp edge diffracts better than a rounded hill, which is why a knife-edge ridge is sometimes an asset rather than an obstacle.
Ionospheric fading
What is a likely cause of irregular fading of signals propagated by the ionosphere?
Random combining of signals arriving via different paths.
The same multipath mechanism as VHF, on a continental scale: signals taking one hop and two hops arrive with different delays and combine randomly as the ionosphere shifts. The ionosphere is the region of the atmosphere that can reflect HF radio waves — and the next lesson is about what it does and does not do for a Technician.
Check yourself
- Your handheld barely reaches a repeater. What costs nothing to try first?
- You are choosing between 2 metres and 70 centimetres for a wooded park. Which, and why?
- A friend with a horizontal Yagi cannot hear your vertical whip on 2 metres, line of sight. Explain.
Answers
- Move a metre or two. Multipath means signal strength can vary greatly over a fraction of a wavelength.
- 2 metres. Vegetation absorbs UHF and microwave far more; at VHF the wavelength is long enough that foliage matters much less.
- Cross-polarisation. Vertical to horizontal over a line-of-sight VHF path costs 20 dB or more of received signal strength.