"billcalley" wrote in message
...
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? And since there cannot be a
good match over such wide bandwidths with any (typical) wire antenna,
what is the downside to using these completely unmatched long antennas
for receivers? (Poor gain patterns with lots of nulls? Lower
sensitivity due to bad noise figure or gain match for any LNA or
frontend amp? Degraded overall antenna gain)?

Thanks; I'm very confused on this subject!

-Bill

The best transfer of energy is achieved when the antenna impedance matches
the input circuit impedance of the receiver.
This should be the case for the entire band of the desired received signal.
It is not always feasible to do this and it is often not necessary. Today's
receivers have very high gain and excellent selectivity.
They need very small signal strengths to operate and excess signal is
attenuated.
So a piece of wire used as an antenna in an area where the signal strength
is large will not do any worse than a perfectly matched antenna.
In an area where the signal strength is weak (like Mars) matching of the
antenna to the front end of the receiver is desired.
There are other considerations for transmitters.

Tom