[Tech-Assist] STL problem now critical and such...

[Burt I. Weiner]

>It doesn't matter from a path standpoint which end the larger dish is on.  The path loss will be the same.  However, if there is a receive interference issue, the larger dish may be needed on that end.  If your STL transmitter is interfering with some other system, the larger dish MIGHT be needed on that end.  On the other hand, the minor lobes will be at different angles, so sometimes a smaller dish will move the minor lobes to minimize interference.
>
>--chip

Well, that's not always true - Aside from possible off-axis interference issues to others, sometimes it does matter which end the larger dish is on.  De-coupling is a term used in microwave where the path is momentarily bent by thermals.  Imagine shining a flashlight on the wall and having the spot move away from where your pointing it!  Sorta like a streamer tied to the center of a fan.  It's what makes stars or far city lights appear to twinkle.  When you have a path with "de-coupling" issues, a large transmit dish can aggravate the problem because of a narrower beam.  My own experience has been at 950 MHz, 7 GHz and 5 GHz (in the Ham band) on paths 20-40 miles long.  Reversing the dishes, that is, putting a 4' dish at the transmit end and a 6' foot or larger dish at the receive end produces fewer and less deep fades due to de-coupling.  The bottom line in this situation is, if your going to have a "beam" that's flailing about, more better it should be a fat beam.

Having big dishes on both ends of a long path can have disadvantages as well as advantages.  A lot of it depends on the terrain the path is over.

In a couple of cases I've solved interference between co-channel STL systems by feathering dish azimuth slightly to put "the other station" in one of the nulls of a dish.  Sometimes it works, sometimes it doesn't.  It depends on the angles between stations and whether or not the change in azimuth is enough to cause any degradation problems for the desired signals.  Microwave is fun stuff!

Burt