MIRCOWAVE MULTIPATH AND PHASE PROBLEMS



I thought it worth a mention on this web page the propagation problems that you might encounter while using 2.4GHz or 5.6GHz 802.11b networks. These are some of the examples you might come across.


Rain Fade

Rain fade occurs simply because there is more attenuation in the path due to water absorption. This also applies to a lesser extend the relative humidity in the air. On hot humid days or rainy days you may find that weaker paths fade or drop out all together.



Wind Fading

Wind can also cause fading. I do not understand fully the cause of it, but it is always noticable in strong winds over a distance. From my own observations it seems to be be related to wind gusts. The stronger the gust the bigger the fading, this appears to me to be air pressure refracting the waves.



Multipath Problems

If you live near a big hill or have lots of large objects around like buildings or structures like power lines, you might encounter higher than normal BER (Bit Error Rates) even though it is clear that you have a line of sight between antennas. This is multipath errors. It's caused by signals leaving your antenna in several directions and the direct path signal arriving as per normal, but has the rest of the bits in the data stream arrive, a second signal appears and starts to jam the first incoming bit stream. This second signal is actually the same signal from the same transmitter, but it has been delayed because it has taken a longer path usually bouncing of some object on the way. The less dirrectional your antenna is, the more likely you are to encounter multipath problems. Also, the longer your signal path is the more likely a multipath is.

Some examples of multipath are:
Ghosting on TV. The ghost is actually a signal that has bounced of something. The source of a ghost is very easy to trace. It is ((G-S)/W)*19.2=D where G-S is the distance between the ghost and the picture in centimetres, W is the width of your TV screen in centimetres, and D how far away the source is in Kilometres.
Aircraft flutter on TV is also multipath. You may notice (usually on Ch2) that the picture sometimes 'flutters' it fades in and out very quickly at first, and then it seems to slow down and almost stop, and then starts to speed up again. If you go outside and look up, you'll see the cause. Even small planes can cause it.
Car Radio Flutter. If you have a bad car radio antenna and you come to a stop at some traffic lights, the station your listening to fades out, yet if you let the car roll forward a metre, the signal comes back. This is because the signal takes multiple paths as it travels through the air and in some cases it causes interference wave fronts. This is the fading in and out you can hear every 1.5m. Which is exactly half a wavelength of the frequency you are listening to.



Polarity Shifts

There is another form of fading which causes major problems. Polarity Rotation or Polarity shifting happens mainly to radiowaves from 1GHz to 10GHz. I do not even understand the cause of the shifts, but I notice that they may last from a few seconds to several minutes. Things that usually effect polarity are electric or magnetic fields and dieletrics. It's my guess that one or more of these is causing it. Polarity rotation is usually only noticable over longer distances, I haven't noticed it on short paths.
There are two ways to overcome the problem. First is to use LHCP or RHCP (Left or Right hand circular polarisation). Two types of antenna will do this; Helical or Dielectric. The problem with that is that everybody must be using the same polaristation. Isolation between LHCP and RHCP is close to 1000 times. Also if other stations are using linear polarisations, they can use either horizontal or vertical against circular polarisations, but the loss will be 3db, this means half the signal strength.
The second option is if you have diversity antennas. Run one vertical and one horizontal, but I get the feeling that wireless lan cards use a thing called a hybrid and that you will still encounter a 3db loss. Such is the price for diversity.





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