On Mar 15, 3:11*am, 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? *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

Well, I 'm not an expert, but it seems that with a transmitting
antenna, the idea is to transfer as much power as possible to increase
efficiency, and so the antenna needs to be closely matched to the
output of the transmitter for best results. But the receiving antenna
is a different problem, since no power from the antenna is needed to
drive the receiver, and so who cares about the match? The idea with
the receiving antenna is to get the most voltage and highest S/N ratio
with no load. The input to the receiver should be buffered with a high
impedance FET amplifier, or some such, so the receiver draws almost no
power from the antenna. This leaves you free to design the antenna and
input tuning circuit for the highest Q and lowest noise figure without
worrying about impedance match.

Just my opinion.

-Bill