In article , "Wes Stewart"
wrote:

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?

You didn't ionize the air in the range :-)

Seriously, for your purposes you did nothing wrong. Just don't call
reciprocity a "law", OK? It's a useful idea of wide applicability, but
physics does not require it in general. Calling it a law confuses people.

Many years ago in grad school my advisor would vex visitors to his office
with a little disk that looked like a piece of tinted glass. Put a quarter
on the table, put the disk on top of it, the quarter looks black. Flip the
disk over, put it back on the quarter, the quarter looks shiny. What was
the construction of this thing? Some world class physicists couldn't
figure it out.

|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

Directivity matters equally for receiving and transmitting. Was your wet
string as directive as your other antenna? 2 meters is also quiet enough
that there's not much room for inefficiency: in some directions the sky
temperature is 200K.


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

Throw a thin wire with dark brown insulation over a tall tree, up over one
side, partway down the other (shaped like a "?"). Tie it in place with
nylon fishing line. It will be invisible unless you're very close. Couple
to coax with a grounded 9:1 broadband matching transformer. Bury the coax
run to the house.

Not only will this be much less conspicuous than a Yagi, but it will
outperform your Yagi as a receiving antenna for nearly every signal over
the range 100 kHz - 30 MHz. A Yagi is just too specialized an antenna for
a listener.

A trailing wire under sea water receives just fine at ELF and doesn't
work worth a damn for transmitting. But those are special cases that
can always be manufactured. Beverage antennas are also not something to
be used for transmitting but you won't be disguising one as a chimney
cap either

For the listener from ELF to HF these are not manufactured special cases,
they are the general case. MW and tropical band listeners often target
their regions of interest with Beverages, either temporary or permanent. A
Beverage is another antenna that can be very inconspicuous: if your soil's
dry you can even bury a Beverage! For listening, a simple broadband
antenna like a Beverage is much more practical than a complicated
narrowband antenna like a Yagi.


In the general sense of h-f to microwave, I stand by my claim.

For the special case of confinement to a small number of narrow bands (as
in ham radio), you are reasonably correct above 10 MHz. To me as a
hobbyist listing to LW/MW/SW, that isn't the general case. Of course the
game changes when I'm operating a satellite, but that isn't my *hobby*.

--
| John Doty "You can't confuse me, that's my job."
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