Is Radio Wave Diffraction Causing Signal Fading While Driving in City Traffic?

[MikeGomez]

We’ve all experienced strong versus weak radio signals while driving a car and listening to the fm radio. The fading signal could be caused from traveling too far from the radio station transmitter, or from being blocked by hilly terrain or buildings.

Lately I have been noticing something interesting while driving in city traffic with tall buildings around. When I have to slow down and come to a stop due to a red traffic light or something, I notice that the radio signal fades in and out every few meters. If there is a song playing that I like I will either stop a little short of where I normally would, in order to keep the signal strong, or if it dies in the spot where I stop, then I will ease forward just a little so that the signal gets stronger.

When the light turns green and I start to move the signal comes in and out several times at regular intervals of a few meters. I know that this distance is in line with the wavelength of fm radio with frequencies from 88mhz to 108mhz, and I’m pretty sure that I am detecting radio wave diffraction patterns.

 

[UltrafastPED]

Usually it is radio wave reflection; sometimes it is refraction.

You can calculate the wavelength for your radio station; wavelength = speed of light/frequency; though the the "mega cycles" and "kilo cycles" for FM and AM have an extra factor of 2*pi.

If the wavelengths are of similar size to the spacing between buildings, then diffraction is taking place, which can result in phase differences ... and then you will have interference effects.

See http://en.wikipedia.org/wiki/Radio_frequency
108 MHz is about 3 meters, so a city canyon will have all of the possible effects.

 

[DrGreg]

Wikipedia has an article on this: Fading

 

[sophiecentaur]

Wave Interference and Diffraction are terms that are often used interchangeably, although I would say that Interference relates to situations where there are two or more identifiable sources and diffraction is the more general term for the more complicated effect of a single wave encountering a large distributed object or aperture. You can get what would be termed refraction when a radio wave 'spills' round a hill or obstruction, away from the line of sight direction.
The worst effect you hear when driving is usually called "multi path propagation" and it's a simple interference effect. It's the same thing, basically, as you get with the familiar two-slits interference. A direct signal and a reflected signal (or two reflected signals) off a hill or building, with roughly equal amplitudes, will add together to produce enhancement or near-cancellation, depending on the particular phase relation in the position you happen to be. If you move a few metres from one spot, you can move from where there is addition to where there is cancellation. Sod's Law works so that, when you are held up in your car, it's usually in a null position. TV (fixed) antennae are made with a directive pattern which increases signal level but also rejects many multi path signals from unwanted directions.

Multipath interference occurs mostly with VHF and UHF frequencies but you can also get Skywave Interference at MF and even LF frequencies when the Ground wave, that hugs the ground (hence the name) encounters the wave that bounces off the Ionosphere. This happens near the limit of reception where the ground wave is weak and about the same amplitude as the skywave (which will have traveled a long way to the Ionosphere and back). Signals come and go, as the Ionospheric layers float up and down, changing the phase of the received signal. This effect tends to be temporary because the skywave for MF and LF doesn't propagate during the day.

 

[MikeGomez]

Thanks all. There was actually a lot more going on than I thought.

 

[Mordred]

as antenna directivety is already mentioned, ground plane problems on an antenna mounting can also lead to poor directivety. Ideally the antenna should have a solid and central ground plane connection. However in vehicles that rarely happens, you might be able to improve your antenna from fading by cleaning the connections or relocation.

 

[sophiecentaur]

as antenna directivety is already mentioned, ground plane problems on an antenna mounting can also lead to poor directivety. Ideally the antenna should have a solid and central ground plane connection. However in vehicles that rarely happens, you might be able to improve your antenna from fading by cleaning the connections or relocation.

The sensitivity pattern of your antenna can make a difference to these effects (ideally, the pattern would be omnidirectional for a car but it will be typically like a fried egg). You have to take more or less what you are given these days with the antenna that the car manufacturer fitted (very fiddly to change an antenna - that damned head lining doesn't like to be disturbed. I remember reading an article in Practical Wireless (?) many years ago and it showed many car antennae have a very multi-lobed response. This means that the relative amplitudes of direct and reflected signals will be constantly changing - frequently giving you equal amplitudes, which can then cause deep multi path fading. You can often find that the reception is totally different when driving each way on the same road.

Apart from the fact that DAB has poor coverage on many UK roads, Digital Radio is much less susceptible to the effects of multi path. Life could be a lot better when they sort out the coverage. If you are in the UK, it could be worth while upgrading to DAB, if you have a lot of urban driving.