On Fri, 21 Nov 2003 23:17:33 +0500, "John Doty" wrote:

|In article , "w4jle" W4JLE(remove this
|to wrote:
|
| Believe what you will, the law of reciprocity will ignore your beliefs
| and continue to function.
|
|1. It's only a "law" for scalar radiation (like sound), not vector
|radiation (radio). To be sure, it's a fine approximation for most HF
|antenna systems, but watch out a microwave frequencies, especially if your
|antenna system contains a "circulator".

Heh heh. When the guys from MIT come out to argue with you, you know
you're in trouble. But fools rush in...

I have made thousands of measurements in anechoic antenna ranges and I
have never seen a difference between measuring s21 and s12. (Without
the circulators, and accounting for mismatch effects of course)

Where did I go wrong?


|2. In the cases where reciprocity applies you would be correct to say that
|it requires that the antenna directivity and efficiency are the same for
|transmitting and receiving. It does not follow, however, that a poor
|transmitting antenna is necessarily a poor receiving antenna. Efficiency
|matters much more when transmitting than it does when receiving.

It also does not follow that a lousy receiving antenna is good enough.

For example, I *always* got better moon echos on 2-meter EME using the
same antenna for transmit and receive. When I tried a wet string on
receive I didn't hear nuthin' g

I have observed the same on 20 meters. My Yagi at a modest height of
50 feet is *always* better than an indoor wire.

|
|Franks's observations are correct, and can be verified if you do a
|detailed signal to noise calculation. You could also try the experiment
|yourself.

Two experiments cited above.