On Sat, 15 Mar 2008 04:11:21 -0700, 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

In areas where it matters receive antennas are matched, and are something
other than random wire. This is why you can go to Radio Shack and buy TV
antennas* -- they're designed** to be both directional and a good match
over the broad frequency ranges of TV signals.

Get into amateur radio or military communications and you'll find many
different permutations of directional, matched antennas on receive.

Note: For many LNA designs, the best signal/noise ratio occurs at an
impedance that is close to, but not really, a perfect conjugate match.
The signal is coupled to the amplifier best at the conjugate match
impedance, but sometimes the noise is enhanced even more.

* or could -- does Rat Shack still carry antennas?
** kinda -- they're really designed to _look_ like they'd make a good
antenna, but they're better than rabbit ears.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html