On Fri, 06 Jul 2007 09:43:24 -0700, szilagyic
wrote:

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

Another way to think of it is if you put the amplifier at the TV set
and feed it a weak, noisy signal you will get a strong noisy signal
out of the amp. At the antenna you have a stronger signal which has a
better signal to noise ratio (s/n) which means you don't need as much
gain, and can actually get by with a less expensive preamp. With
less gain the preamp is not as prone to signal overload from strong
stations either.

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

Again, if you have a good signal with good s/n ratio you *may* be able
to get by without having to resort to a preamp with the lowest noise.

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

So, the antenna mounted *may* let you use less gain and not resort to
the lowest noise amplifier which both give better performance when in
the presence of strong signals.

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

With many remote part 15 devices operating in the UHF band it is
becoming more common. There are several Amateur Radio bands in the UHF
range but those signals although much more powerful than part 15
devices are usually cleaner and are restricted to specific bands
unlike part 15 devices.

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

And stacking antennas makes the patterns even worse. I ran a quadature
array (4 antennas mounted 2 high and 2 wide) of UHF antennas which had
tremendous gain and a very good front to back, but not just the
secondary lobes, but several to either side of those were nearly as
strong as the main lobe
..
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!

The ones on my quadature array were a lot stronger than that. I
finally gave up as it's too difficult to get the proper spacing from
side to side across the entire UHF band. Besides at 90 feet I point
them (I have one to the NW and one to the S) to the weak UHF stations
and they do very well on the much stronger VHF.

http://www.rogerhalstead.com/ham_files/tower21.htm They look close
together, but those are big antennas separated by about 12 to 15 feet.
they probably would have worked better with about 15 feet of vertical
spacing and 20 feet horizontal.

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

I use the RS amps on mine, but with the gain turned down. I end up
replacing them every couple of years any way due to lightning. The
tower has been taking an average of 3 *verified* hits per summer and I
am surprised they last that long. That and the current installation is
only about 35 feet below a pair of 144 MHz antennas being driven with
a kilowatt and about 20 feet above a vertical being driven with 50
watts on 144 and 35 watts on 440 MHz.

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'd think 15 miles to a VHF station would provide a pretty strong
signal.


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

Myself I'd prefer the Winegard preamps over the RS ones and there is
very little difference in price as I recall. I have the RS preamps
only because they were handy to try.


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