On Mon, 16 Feb 2009 00:32:22 -0800, "Sal M. Onella"
wrote:

Many good all-channel antennas exist but the absolute best-performing
antennas are limited to covering one band, sometimes just a portion of one
band. (This idea started a lot of do-it-yourself antenna projects.)

Sure. I can always trade bandwidth for gain. If you only watch one
DTV station, a single channel antenna is a good idea, especially if
it's distant and weak. If you don't like rotators, perhaps a
collection of single channel antennas pointed at each station might be
useful. I live in the mountains where reflections are a problem for
OTA reception. For a time, I was building and selling custom single
channel TV antennas, with built in preamps. They worked great, but
when Comcast started offering $13/month for local only TV, I gave up.

If you determine that you have some stations on VHF and some on UHF,
consider having more than one antenna. In a home where I lived in the
pre-cable days, I had four different antennas.

Same here. Back in the 1970's, I lived in Israel for a while. In the
cities, everyone lived in big apartment complexes. Nobody watched the
official Israeli stations. The good programming was on the various
Arab stations, which were diverse, distant, and politically incorrect.
There was no cable TV (because the government didn't want to make it
easy to watch the Arab stations) so every apartment had its own
collection of antennas. Typical was a 3 meter pole with at least 4
yagis. Multiply that by at least 4 apartments per building and the
roof tops looked like an aluminum forest. Somewhat later, I lived in
Smog Angeles where everything is on Mt Wilson. Only one antenna
needed.

Of note: The bigger the antenna, the more likely it will be highly
directional.

http://groups.google.com/group/rec.radio.amateur.antenna/msg/8535c986a167ac41
"small - efficient - broadband: pick any two."
Roy Lewallen, W7EL

I've dealt with some rather monstrous log periodic directional
antennas. They cover the entire HF spectrum, but only have about 4dBi
of gain at any frequency. Directionality is about what you would
expect for a 4dBi antenna (about 80 degrees -3dB beamwidth). Just
because it's big, doesn't mean it has gain or a narrow beamwidth.

The same thing applies to TV antennas. If you compromise on the
bandwidth by reducing the number of channels it covers, then you can
get some more gain and narrower beamwidth (for a given size). That
seems to be what's happening as the highest frequency goes from 900 to
700MHz. However, that's not going to make a huge difference is size
of gain. More useful is ignoring the VHF channels and making the
antenna UHF only. That really reduces the size of the antenna or
increases the gain (for a given size antenna).

If you expect to receive weak stations from more than one
direction, you'll probably need either a rotator or more than one antenna.

True. I once was involved in building direction finders. I got
greedy and decided to find a commercial application. What the world
needed was a direction finder tied to a rotator and yagi. The yagi
would automatically rotate to the direction of the strongest signal. I
first tried it with a TV antenna and it worked. Unfortunately, my
pre-microprocessor circuitry did not have any way to distinguish
between multiple peaks in the signal strength as well as antenna side
lobes. It would often pick the wrong lobe. Making a 360 degree scan
would solve this problem, but that increased the complexity beyond
acceptable limits.

So, I decided that perhaps it might be useful for commercial two-way
radio. I installed one on a mountain top site on a UHF commercial
frequency. Useful range was dramatically increased for the repeater.
However, during a typical conversation, the antenna would rotate back
and forth between each mobile and base. Commodity antenna rotators
are not designed for near 100% duty cycle. It was also a dice toss as
to whether a mobile could be heard, depending totally on the current
antenna orientation. I could have improved things with a better and
faster motor, but didn't. I know a bad idea when I design one.

I also built an omnidirectional TV antenna designed to be hung near
the top of the local 50 meter redwood and fir trees. There was enough
vertically polarized TV signal to make it usable. However, it also
picked up reflections from every possible direction, resulting in
multiple obnoxious ghosts. Bad idea, but fun to try.

It's probably better to connect multiple antennas through an A/B switch,
rather than combining them.

Yep. If the signals from multiple antenna could just as easily cancel
as they could add. However, if you installed a band pass filter
between the antenna and the combiner, the signals from one channel
will only appear on one port, thus eliminating any chance of
cancellation. I would filter and combine.



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