On Jul 3, 5:27 pm, Bob Bob wrote:
Hi Chris

Much has already been said about your problem. I just have a few more
points to add..

- One of the initial "reasons" for masthead preamps was to reduce/remove
the loss of the coax run. Quoting the preamp gain in this case is useful
but in reality it is much more important to have a preamp device that is
low noise or will give you a better signal to noise ratio at low signal
levels. There is actually a hard limit based on Boltzmanns constant and
the system bandwidth that is the thermal noise on Earth. (ie you can use
it mathematically to check your system) If it is spec'd look for the
lowest preamp noise figure (NF).

Unfortunately low NF tends to go hand in hand with not so good large
(undesired) signal performance. If however you don't have other strong
signals around you it works well. Some bad preamp designs even "take
off" producing their own interference and contributing towards the problem.

- One of the killers for low level signal reception is that the local
noise near your antenna varies above the "constant" mentioned above.
Although rare onUHF, electrical interfering sources can "raise the
noise floor" such that the s/n of the wanted signal gets smaller. Ways
around this include a good antenna install with very good directivity to
the signal source. In some cases you can use the antenna nulls and
polarization to reduce interfering signal and of course finding and
fixing the interfering source.

Your "couple of channels" dropping out could be a local noise problem or
equally a propagation/bending/reflection issue. (Reflection problems
tend to be short lived though) It may be worth logging the failures to
see if they fit a pattern. What frequency the channels are on can also
help in the hunt. (eg someone using a 900MHz cordless phone while your
weak signal is at the top of theUHFband...)

It might be worthwhile finding out what channels the distant stations
are on, then purchasing an antenna more centered on that frequency. As a
general rule high gain is inverse to bandwidth so for the same amount of
metal/size an antenna made to cover (say) 5 channels may have 5dB more
forward gain and better f/b performance than a wide band one. A narrower
band antenna may also have better undesirable lobes so a local noise
problem may be helped. I realize that you are more after a commercially
available system but your own design/build may even be an option.

In a perfect world an antenna would receive nothing from everywhere
except the desired direction! Unfortunately most radiation patterns look
like wildflowers on steroids! Wide band antennas are especially
horrible. The front to back ratio is not the only important figure. Some
antennas have lobes maybe 30 degrees off the back that are only a few dB
down from a dipole!

Hope this helps.

Cheers Bob W5/VK2YQA



szilagyic wrote:

Hello:

I have been trying to find the answer to my question on the front-to-
back ratio onUHFantennas, and so far have been unsuccessful.

..
I really appreciate the help and feedback on this!!

Thank you very much for the detailed information. All of these ideas
are VERY helpful, and I will be trying some of these very soon.

I have (for reasons that you mentioned) been thinking about replacing
the preamp (Radio Shack 30 dB). The manual for it doesn't give the
specs for noise, or anything useful for that matter. I have been
looking at a Winegard AP-8275 (29 dB gain VHF, 28 dB gain UHF, 2.9 dB
noise VHF, 2.8 dB noise UHF). Would this be a good alternative as far
as a low noise amp?? We do not live near any stations, I believe the
closest is 15 miles, so I am thinking a high gain preamp would be a
good fit.

I just wanted to get the feedback on this before I go out and buy
something.

Thanks again for all of the good info in this thread!!

--
Chris