Dear Bill (no call sign):
It depends. The goal of a receiver is to have a satisfactory
signal-to-noise & distortion ratio. Many factors enter into that ratio.

Let us consider just the antenna: Below something like 30 MHz, it is
easy to have receivers that are quiet relative to noise picked up by most
antennas. In other words, below something like 30 MHz, the SNR is
determined outside of the receiver and tweaking the tuning between the
antenna and receiver just increases or decreases the volume. At VHF and
higher frequencies, the noisiness of the receiver tends to be a significant
player. It has long been known (see MIT Radiation Lab volume 18) that the
maximum SNR occurs close to a conjugate match between the antenna and
receiver's first stage.

However, many consumer VHF-UHF receivers (FM broadcast, TV, scanners)
are presented with such large signals that there is little point in tweaking
the interface between antenna and receiver for an even larger SNR.

In the early days of radio astronomy, when we were still using vacuum
tubes, a lot of attention was paid to having the right amount of mis-match
so as to maximize SNR.

So: HF and below: be concerned about the directivity of the antenna
(is it reasonably sensitive in the direction of interest).
VHF and above: have the antenna's impedance, transmission line's
impedance, and receiver's input impedance be within a factor of 2 of each
other AND point the antenna in a desired direction.

Regards, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"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