VHF Multipath: More Than One Signal Path

Multipath distortion is a common phenomenon that I've become accustomed to living in a mountainous region. It can mess up your signal and create some interesting and confusing distortion. Multipath can also occur in other areas when signals bounce off objects such as tall buildings. The figure below shows several different ways that multiple signals can propagate from the transmitter (a mobile station) to a receiver (a handheld radio).

Multipath distortion occurs when the signal from the transmitting station takes multiple paths and recombines at the receiving station. Signals may travel directly by "line of sight," scatter from irregular surfaces, diffract over or around sharp edges, and reflect from some surfaces and materials. Each signal path takes slightly different time, potentially scrambling the signal phase relationships of the various signal paths at the receiving station.

Scattering occurs when radio waves encounter objects or irregularities—such as rough surfaces, small obstacles (e.g., leaves, raindrops, or rocky edges), or atmospheric variations—that are comparable to or smaller than the wavelength. The waves are redirected in multiple, often random directions, spreading the signal’s energy.

Diffraction is the bending of radio waves around the edges of an obstacle or through an opening, allowing signals to reach areas that would otherwise be shadowed. It happens when a wave encounters a barrier (like a hill, building, or knife-edge structure) and curves into the region behind it.

Reflection occurs when radio waves strike a large, relatively smooth surface—like a mountain face or tall building—and bounce off at an angle equal to the angle of incidence (think of a mirror). Reflection is wavelength-dependent but typically involves surfaces much larger than the wavelength.

Constructive or Destructive?

These signals take different paths, some longer than others, arriving at slightly different times at the receiver. These multiple signals combine at the receiver and may cause multipath distortion because of constructive and destructive interference. The figure below shows how two sinusoids can combine. In the upper diagram, the two waves are in phase and add together to create an even stronger waveform. In the middle diagram, the worst case is shown with the waveforms 180 degrees out of phase, causing them to cancel out when combined. Any phase difference other than 180 degrees causes partial effects, including partial signal cancellation as shown in the lower diagram. In the general case, we may have multiple signals of various amplitudes arriving at different times (phases), causing complex interactions. The net result is multipath distortion of the recovered signal, usually manifesting as garbled or distorted audio to variable degrees of severity.

When two signals combine, waveform amplitudes sum together. There can be constructive interference, destructive interference, or partial effects including cancelling interference. The upper diagram shows perfectly constructive interference, boosting the amplitude of the combined signal. The center diagram depicts perfectly cancelling interference, 180-degree phase difference between the two waves. The lower diagram depicts a partially cancelling interference, diminishing the amplitude of the resultant combined signal.

Time and Space

These multiple paths can vary with time, due to changes in atmospheric conditions. You can be sitting at home listening on your 2-meter FM transceiver, and another station's signal varies in strength and distortion. An even more pronounced variation happens when you move your receive antenna a short distance (some fraction of a wavelength). Doing so may move you from an area of destructive interference to one of constructive interference. This is easier to do with a portable station such as a handheld transceiver or mobile unit in a vehicle, and not so easy with a fixed base-station antenna.

Experienced VHF operators change their position slightly, searching for a "hot spot" to improve their reception. These interference patterns tend to set up based on the wavelength of the signal, with maximum constructive interference (a hot spot) about 180 degrees away from maximum deconstructive interference (a bad spot). For the 2-meter band, the wavelength of a radio signal is, well, about 2 meters long, so moving 180 degrees corresponds to 1 meter (or about three feet). Sometimes, when signals are weak on 2 meters, you'll hear the suggestion of "move a few feet and try again." Note that it only takes about 3 nsec for a radio signal to propagate 1 meter, so the time shift between the two signals can be very small.

Confusion

Multipath propagation can be quite confusing. Here's a real-life story of three radio amateurs communicating using a 2-meter repeater located within a mountainous area. (We'll call these hams Paul, Sally, and Leroy.) The terrain in the area caused significant multipath distortion between the hams and the repeater and between each other. This resulted in Paul's signal sounding distorted going into the repeater, which was heard as a distorted signal coming from the repeater. At first, Sally and Leroy thought Paul's signal had a problem and perhaps his radio was malfunctioning. While diagnosing the problem, Sally listened to Paul's signal directly on the repeater input (simplex) and found that his signal sounded just fine. This made them wonder if the repeater was having a problem. But no, Sally and Leroy's signals were coming through the repeater without a problem. Leroy also tuned to the repeater input frequency to hear Paul direct but he found that Paul's signal still sounded distorted. However, the distortion was not quite the same...the amount of distortion was less and the tone of it just sounded different.

What was going on? The path from Paul's radio to the repeater was distorted due to multipath distortion. The repeater just repeats this distorted signal, so Sally and Leroy heard a distorted signal from Paul. When Sally listened to Paul's signal directly, there was no multipath and the signal sounded fine. But when Leroy listened to Paul directly, that path also had multipath distortion with somewhat different characteristics, which really confused things.

Dealing With Multipath

So what can we do about this problem? How do we deal with multipath distortion? Here are a few tricks and tips to consider.

1. First off, be aware that multipath effects exist and can cause on-the-air distortion in radio signals. So don't assume the other station has a transmitter problem.

2. As mentioned earlier, moving a few feet (depending on wavelength) can cause a large change in signal amplitude and quality. For example, the wavelength for 2 meters is about 6 feet, so moving about one-quarter to one-half wavelength (1.5 to 3 feet) can make a big difference. Or not.

3. Using a directional antenna can really help as it tends to focus on one signal path and attenuate others. Point the antenna for best reception, and it may not be obvious which direction will be best.

4. Along those same lines, be aware that the best signal path may not be in the apparent direction as signals bounce off mountains or structures.

A key takeaway is to move around a bit to try to optimize your signal strength. It is not totally predictable, so try something and see what happens. The same thing applies to pointing your directional antenna: the "obvious" heading may not be the best heading. Try something and see what happens.

Wrap It Up

Multipath distortion might sound like a tech nightmare, but once you get the hang of it, it’s just another quirk of the radio landscape. Whether you’re tweaking your ham radio setup, boosting your Wi-Fi, or trying to catch a distant TV station, a little know-how and some trial-and-error can go a long way. So, next time your signal drops, don’t despair—grab your antenna, take a few steps, and outsmart those bouncing waves.