"axolotl" wrote in message
...
Ed Huntress wrote:


For one thing, it's a matter of coherent radiation.

Say what?

That refers to the fact that you don't want to transmit harmonics if you can
avoid it. You want to transmit a single frequency. In receiving, it doesn't
usually matter. In fact, antennas made strictly for receiving aren't usually
tuned at all.

That's why a car antenna can recieve a bandwith of 3f (500 kHz - 1600 kHz)
without tuning. If you tried that with a transmitting antenna, your
impedance at the feedline end would be all over the map, and the reflected
power would, at some point, probably blow up your output stage.



Ed, antennas are reciprocal devices.

A transmitting antenna will be capable of transmitting and receiving. A
receiving antenna usually is not capable of it, unless it's also designed
for transmitting. Car antennas are not.

One of the more convienient ways to
measure the characteristics of an antenna is to build two identical
antennas, point them at each other, using one as receive and one as
transmit, and divide the gain. An impedance mismatch on the receive side
means some of the energy you are trying to gather will be reflected back
out the antenna. The primary difference between transmit and receive is
that the transmit structure has to handle more power- bigger conductors
with more or better dielectrics to take the voltage. The Ls and Cs dont
change.

Again, receiving antennas for the AM band usually work better the longer
they are. Most receiving antennas are low-Q designs so impedance matching is
less of an issue. Still, they can have a gain advantage due to length
(longwire effect), and the gain may outweigh the loss due to mismatch.

In any case, the point is that, in practice, you can get good performance in
receiving with very simple antennas that have unknown impedances, which
wouldn't work in transmitting without a separate matching circuit.
Relatively few receiving antennas are matched, unless they're also used for
transmitting.

--
Ed Huntress