On Sat, 15 Mar 2008 09:35:22 -0500, Tim Wescott
wrote:

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.

I once cranked out an NEC2 model of a Radio Shock TV antenna to see
what it really did over the 54-890MHz range. It was fairly horrible.
There were actually a few frequencies where the impedance was close to
300 ohms. There were also a few frequencies where it actually had
some gain. At some frequenies, it had more gain in the reverse
direction than forward. As an example of a "directional and a good
match" antenna, that typical Radio Shock TV antenna doth truly suck.

I'll see if I can find the model. Unfortunately, it may have been on
a hard disk that crashed a few years ago.

Not having a good match between the antenna and LNA has several
effects. The mismatch will affect the system noise figure thus
reducing sensitivity. Some LNA's are not unconditionally stable and
will oscillate when presented with a weird source impedance. At HF
frequencies, the atmospheric noise level is above the receiver input
noise level, so considerable mismatching can be tolerated. About
about 20MHz, this is no longer the case, and a match is required.

Antennas are also affected by their load impedance. A highly
directional yagi antenna pattern can easily be ruined by mismatched
coax or LNA input impedance. For the antenna to work as advertised,
it has to see the rated load.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558