Believe what you will, the law of reciprocity will ignore your beliefs and
continue to function.

"Frank" wrote in message
news:01c3b0a2$989de120$0125250a@cqvdqntcxxawvjpo.. .
Wes Stewart ...

^ Antennas are reciprocal, if they wouldn't work well for
^ transmitting, they will work equally poorly for receiving.

I don't believe that. It's been my experience that an antenna used for
receiving will function satisfactorily over a much broader range of
conditions (environment, antenna length, etc.) than it will if used for
transmitting under those same conditions.

Frank

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".

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.

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.


"Frank" wrote in message
news:01c3b0a2$989de120$0125250a@cqvdqntcxxawvjpo.. .
Wes Stewart ...

^ Antennas are reciprocal, if they wouldn't work well for
^ transmitting, they will work equally poorly for receiving.

I don't believe that. It's been my experience that an antenna used for
receiving will function satisfactorily over a much broader range of
conditions (environment, antenna length, etc.) than it will if used for
transmitting under those same conditions.

Frank



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

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.

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."
| Home:
| Work:

On Sat, 22 Nov 2003 10:23:52 +0500, "John Doty" wrote:

|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.

I never made that statement.

[snip]
|
|Directivity matters equally for receiving and transmitting. Was your wet
|string as directive as your other antenna?

I don't know. Remember I live in the desert; I couldn't keep the
string wet long enough to find out.

|2 meters is also quiet enough
|that there's not much room for inefficiency: in some directions the sky
|temperature is 200K.

Yep. Love that quiet sky.
|
|
| 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.

Tall tree? What's a tall tree? The best I have is some 35 foot tall
Saguaro cactii. They're a bitch to climb, although when the coyotes
went after the cat, she managed. Let's see, I could tie a string to
the cat's tail and find a coyote.....

[snip]

| 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*.

See, one of the groups this got cross-posted to is an *Amateur Radio
Antenna* group. I'm reading and writing it from this group and
commenting from that perspective. I normally don't cross post but did
the first one by accident and since I've developed such a loyal
following I didn't want to lose anybody G.

Regards,

Wes Stewart, N7WS

Wes Stewart ...

^ Tall tree? What's a tall tree? The best I have is some
^ 35 foot tall Saguaro cactii.

That might serve as a 40m vertical if the roots are dry and shallow.

Frank

In article , "Wes Stewart"
wrote:

|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.

I never made that statement.

Oops, you're right.. It was "w4jle". But you took his side :-)

See, one of the groups this got cross-posted to is an *Amateur Radio
Antenna* group. I'm reading and writing it from this group and
commenting from that perspective. I normally don't cross post but did
the first one by accident and since I've developed such a loyal
following I didn't want to lose anybody G.

But we were discussing Frank's observation:

It's been my experience that an antenna used for receiving will function
satisfactorily over a much broader range of conditions (environment,
antenna length, etc.) than it will if used for transmitting under those
same conditions.

Certainly below 30 MHz this is represents a correct observation,
verifiable both by calculation and experiment. Why should this reality
change with the newsgroup?

Tall tree? What's a tall tree? The best I have is some 35 foot tall
Saguaro cactii. They're a bitch to climb, although when the coyotes
went after the cat, she managed. Let's see, I could tie a string to the
cat's tail and find a coyote.....

Sounds like you're in "Beverage on the ground" territory. That works too,
I'm told (I live next to a swamp, so the tall maples are my friends :-).
I've never tried a Beverage on the ground, although I have used a long
skinny island as a slot antenna (worked very well from longwave through
tropical bands, useless above 10 MHz).

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