Bishoop wrote:
snip

10 or 12 db of forward gain means your recieved signal is about 10 to 16
times as strong as a dipole hanging in the air. If you interpret the
negative number as the amount the signal is down from the forward gain,
the numbers given (9 to 17db) would indicate reception off the back side
would be somewhere near a dipole in open space (1 db net) to -5db (about
1/3 of the signal of a dipole) pickup from the back. I have used aluminum
sheet (tested prior with aluminum foil) tied to the mesh to completely
block reception from the back (a near infinite front to back ratio) in an
extreme case where I had significant multipath reflections coming in from
the back. It really cleaned things up. In your case you may not need to
go to this rather severe step.


snip

Good luck!
--Rick AH7H


10 or 12db of forward gain goes NOT equal 10 "times" the received signal
strength.


OK, so it's semantics.
a 3 3db positive change is a doubling of "power", which I relate to
signal strength on reception, hence my use of the term above.
3db ix 2x power
6db is 4x power
9db is 8x power
10db is essentially 10x power.... (and a generally accepted
approximation).

Given the nebulous measurement methods used, stating that 10-12db of an
antenna gain is nearly the same isn't that far off, assuming the stated
10-12db is even remotely accurate to begin with....

--Rick AH7H

In article t,
"Ralph Mowery" wrote:

"Bishoop" wrote in message
(snip)
10 or 12db of forward gain goes NOT equal 10 "times" the received signal
strength.

Then what do you think it is equal to ?


Well, 10dB of gain is a 10x increase in absolute signal, but 12dB of
gain would be about 16x the signal.

dB = 10 * log(S1/S2)
where S1 and S2 are the absolute strengths of two signals you're
finding the dB of difference between.

The regular absolute-value scale and logarithmic scale of dB "cross" at
the factor of 10; so 10dB = 10x, but that's the only place that's true
(at least, I think it is, off the top of my head).

-Kadin.

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

On Jul 6, 9:43 am, 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
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

Your antenna has around 8 dB more gain than mine and you're closer to
some of the stations. How long is your cable? If you have Line Of
Sight to the towers you shouldn't need much gain at all, basically to
make up for cable losses like 5-10 dB. If you're behind obstructions,
that game changes a lot though.

Instead of the 8275 I would use the AP-8700. There is less gain but
more headroom and I suspect that will be more of an issue.

http://www.winegard.com/offair/preamps.htm

GG

I haven't followed this thread very closely, but the statement that a
preamp is being used when stations are 15 miles away got my attention.

I'm roughly the same distance from TV broadcast towers here, and I have
to use an attenuator to prevent overloading my TV when using a small
antenna in the attic. Without the attenuator, the TV goes blank on a
couple of channels. I'd be very surprised if your preamp and/or TV
aren't being overloaded, and that might be the cause of problems you're
having.

Roy Lewallen, W7EL

On Jul 6, 3:14 pm, G-squared wrote:
On Jul 6, 9:43 am, 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
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 gainUHF, 2.9 dB
noise VHF, 2.8 dB noiseUHF). 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

Your antenna has around 8 dB more gain than mine and you're closer to
some of the stations. How long is your cable? If you have Line Of
Sight to the towers you shouldn't need much gain at all, basically to
make up for cable losses like 5-10 dB. If you're behind obstructions,
that game changes a lot though.

Instead of the 8275 I would use the AP-8700. There is less gain but
more headroom and I suspect that will be more of an issue.

http://www.winegard.com/offair/preamps.htm

GG

Hello and thanks for the reply!

We are mainly concerned with stations that are all between 35 & 45
miles away (don't care about the one that's 15 miles away); and some
seem to be weak as current signal strength is about half, and they
drop from time to time. Given this, would the AP-8275 work well, or
would you still recommend the AP-8700? I have an attenuator just in
case.

I really appreciate your feedback on this..... Thank you!!!

--
Chris

On Jul 6, 5:11 pm, szilagyic wrote:
On Jul 6, 3:14 pm, G-squared wrote:

Your antenna has around 8 dB more gain than mine and you're
closer to
some of the stations. How long is your cable? If you have Line Of
Sight to the towers you shouldn't need much gain at all,
basically to
make up for cable losses like 5-10 dB. If you're behind
obstructions,
that game changes a lot though.

Instead of the 8275 I would use the AP-8700. There is less gain
but
more headroom and I suspect that will be more of an issue.

http://www.winegard.com/offair/preamps.htm

GG

Hello and thanks for the reply!

We are mainly concerned with stations that are all between 35 & 45
miles away (don't care about the one that's 15 miles away); and
some
seem to be weak as current signal strength is about half, and they
drop from time to time. Given this, would the AP-8275 work well,
or
would you still recommend the AP-8700? I have an attenuator just
in
case.

I really appreciate your feedback on this..... Thank you!!!

--
Chris

Again, I'm at 35 miles with line of sight to the transmitters and a
low gain antenna. The reason I suggested the 8700 is the reduced gain
compared to the 8275. Keep in mind that amplifiers are not a panacea
and are wideband devices. Just because the local (15 mile ) station is
on whatever channel doesn't mean it can't cause overload issues _many_
channel numbers away. At 35 miles with good coax 100-200 feet, I
wouldn't expect to need any amplifiers at all, much less 29 dB worth
but if you believe you need it, the 8700 should be more tolerant of
overload issues. You might even need to put the attenuator _ahead_ of
the amplifier to keep _it_ from overloading.

GG

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!!

In alt.tv.tech.hdtv "Roger (K8RI)" wrote:

| 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.

If the desired signal is a single channel, two antennas connected 180
degrees out of phase (or flip one upside down), where they are spaced
an odd multiple wavelength from the desired source, and equadistant to
the multi-frequency side source (if there is a specific noisy source),
might do the trick.

You might look into these antennas:

http://simplicitytool.com/mu_series_uhf_quad_array.htm
http://simplicitytool.com/log%20periodic%20arrays.htm

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / |
|------------------------------------/-------------------------------------|

Hi Chris

I think others have pretty well answered your questions.

My gut feel is that given the manufacturer even mentions NF is a good
thing. ie those that don't may be of poor performance in this area. As
others have seen be careful of overload due to closer stations. I once
lived in a VHF TV area where signals on 165/180MHz were strong but most
people also wanted one on about 49MHz. Many a preamp had problems with
the strong stations. I ended up setting up some 1/4 stubs on the preamp
input that notched the strong signals enough to stop the bleed through.

Cheers Bob

szilagyic wrote:

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.
----