billcalley wrote:
Hi All,

I always hear that antennas have to be matched to their radio, but
in receivers (such as FM and shortwave radios) I see mostly long
random length antennas used, and these antennas -- be they a
telescoping whip or a long wire out a window -- are used over some
really wide bandwidths. How is this possible if an impedance match
must always be maintained for radios

Andy writes:

As a practical matter, if the background noise heard in the receiver
increases when the antenna is attached, the antenna is good enough.

This means that the atmospheric noise, in the frequency range that
the receiver is tuned to, is greater than the internal receiver
noise....

It also means that any signal that is to be received that exceeds
the
atmospheric noise, will be heard......

Unless you are using some signal processing that can detect signals
below the atmospheric noise level, this is a very good rule of
thumb....

Consider an airborne LORAN antenna, used on aircraft, to receive
100 khz signals. It works out that around 22 inches is the length
where
the atmospheric noise exceeds the general receiver noise for most
receivers. Making the antenna longer will pick up more desired
signal,
but also more atmospheric noise, in the same ratio.....so the SNR
doesn't
get much better. Note that a matched antenna for 100khz will be
many many hundreds of feet long.... but is never used either in boats
or airplanes, since a "matched" antenna serves no advantage to
sensitivity ( SNR)....

These are rules of thumb, and useful approximations, but, in
effect,
you don't need a great antenna unless you are trying to receive a weak
signal....or have a method to increase the SNR by signal processing.

Andy in Eureka, Texas W4OAH