Polarization for long distance?

[DTKG]

Hi everybody, I have a little question...Which polarization it is better to use for long distance(3km-more)? Suppose that we are free to use any ( not interferences) . I want to know this because someone told me to use Horizontal Polarization but I read an article that said that is better to use vertical polarization. I hope someone can help and maybe give good reasons.


Thanks,
DTKG

[M/Q]

Good question and there are proponents firmly placed in each camp. In the microwave frequency range there is actually a lessor degree of importance placed on polarization than at lower frequencies. The one very important concept is to make darn sure both ends of the link are exactly the same. Even 10deg variations seem to have an affect.

Also one has to be careful about antennas that they use in either polarization. For example a vertical dipole can be used horizontally but the radiation pattern is no where near the same. Check out the dipole graphs in the link I have provided below. Most other antennas have a symmetrical radiation pattern when switching polarizations. But to be sure, check out the radiation patterns supplied by the antenna's maker. If there are none, do not purchase that device.

There is one other polarization that comes into play at least in most cases when you are dealing with satellite comms. I am an amateur radio op and I have a few circular polarized antennas especially for that kind of work. The reason is that there is some rather bizarre things happening to RF in the ionosphere and in most cases you can not determine the antenna polarization of a satellite as it is a very movable target. There are some proponents that state this polarization works well on land based systems as well. There is little scientific proof to support that claim. The only logical or intuitive assumption one can make is that circular polarization may work better if there are multi-path signals involved. I have tested much of this and am a sceptic as of yet.

When setting up your own exclusive communications link, it may be wise to first test the link with vertical and then horizontal polarization to see which yields the best performance (if any). If there are any reflections in the area, especially from structures or towers, one polarization may outperform the other. Furthermore, if there are other RF signals in an area, using a polarization opposite the predominant high level signals will give some isolation.

On another note, when radio waves strike a smooth reflective surface, they may incur a 180 degree phase shift, a phenomenon known as specular or mirror image reflection. The reflected signal may then destructively or constructively affect the direct LOS signal.

Here is a link to one of my mentors. He is what I consider a very good expert on antenna and RF theory.

http://www.trevormarshall.com/byte_articles/byte1.htm

[spiderbite]

Thank you for the enclosed article. I especially enjoyed the disclaimer on the main site....

[M/Q]

I remember the first time I read that. I then proceeded to read it about 10 more times before I got a good grasp of it. Kind of covers all the bases. :-)