Front-to-back ratio for UHF antenna
 

Hello:

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

I think I understand what the front-to-back ratio is, but the specs on
my Winegard PR-8800 (8 bay bowtie) show negative front-to-back ratios
(-9 dB @ ch 14; -17 dB @ ch 32; -11 dB @ ch 50; -9 dB @ ch 69). The
"dB over reference dipole" is: 10.7 dB @ ch 14; 12 dB @ ch 32; 11 dB @
ch 50; 12.5 dB @ ch 69. How can the front-to-back ratio be negative,
and is this good or bad? I am assuming this could be bad in my case
as I would like the signal at the back of the antenna to be minimal.

I am currently trying to improve our HDTV reception. I currently have
the PR-8800 antenna in our attic hooked to a Radio Shack 30 dB mast
amp. This antenna works very well overall. But from time to time, on
a couple channels the HD signal strength will drop all the way down,
enough to drop the signal or cause break-up. I've been considering a
Winegard HD9095P or PR9032 to use in conjunction with the PR-8800
since they have a higher gain than the PR-8800, then rotate the
PR-8800 the other way to receive some different stations.

I really appreciate the help and feedback on this!!

Thanks,
--
Chris

Front-to-back ratio for UHF antenna
 

Front-to-back ratio for UHF antenna
 

In alt.tv.tech.hdtv Bert Hyman wrote:
| (szilagyic) wrote in
| ups.com:
|
| I think I understand what the front-to-back ratio is, but the specs
| on my Winegard PR-8800 (8 bay bowtie) show negative front-to-back
| ratios (-9 dB @ ch 14; -17 dB @ ch 32; -11 dB @ ch 50; -9 dB @ ch
| 69). The "dB over reference dipole" is: 10.7 dB @ ch 14; 12 dB @
| ch 32; 11 dB @ ch 50; 12.5 dB @ ch 69. How can the front-to-back
| ratio be negative, and is this good or bad? I am assuming this
| could be bad in my case as I would like the signal at the back of
| the antenna to be minimal.
|
| Showing the F/B ratio as negative is non-standard, but I -guess
| they're just trying to emphasize that the signal received off the
| back of the antenna is less than that received from the direction
| it's pointing.

Looks like negative DB is used for any direction to indicate how much less
the signal will be relative to the main lobe. Whether side or back or any
other direction, the circles are labeled with negative DB, more negative
as you move to the inner circles.

See: http://www.hdtvprimer.com/ANTENNAS/cm4228.html

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

Front-to-back ratio for UHF antenna
 

On Jul 2, 10:49 am, szilagyic wrote:
Hello:

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

I think I understand what the front-to-back ratio is, but the specs
on
my Winegard PR-8800 (8 bay bowtie) show negative front-to-back
ratios
(-9 dB @ ch 14; -17 dB @ ch 32; -11 dB @ ch 50; -9 dB @ ch 69).
The
"dB over reference dipole" is: 10.7 dB @ ch 14; 12 dB @ ch 32; 11
dB @
ch 50; 12.5 dB @ ch 69. How can the front-to-back ratio be
negative,
and is this good or bad? I am assuming this could be bad in my
case
as I would like the signal at the back of the antenna to be
minimal.

I am currently trying to improve our HDTV reception. I currently
have
the PR-8800 antenna in our attic hooked to a Radio Shack 30 dB mast
amp. This antenna works very well overall. But from time to time,
on
a couple channels the HD signal strength will drop all the way
down,
enough to drop the signal or cause break-up. I've been considering
a
Winegard HD9095P or PR9032 to use in conjunction with the PR-8800
since they have a higher gain than the PR-8800, then rotate the
PR-8800 the other way to receive some different stations.

I really appreciate the help and feedback on this!!

Thanks,
--
Chris

30 dB is quite a bit of gain on top of a decent gain antenna. If
you're _far_ from the stations OR they're still running reduced power
it can be justified but as a point of reference, I use a Winegard
squareshooter (4-5 dB gain), no amplifier split 4 ways with 100' of
RG-6 35 miles from the LA transmitters. SIgnal strength reported as
80% on the HDTV Wonders. I wonder if perhaps you're almost overloading
either the preamp or the front-end of the TV.

As Bert mentioned, the negative front-to-back number just referencing
the front gain of the antenna.

Again, I wouldn't use so 'much' preamp unless I was 50+ miles from the
nearest station.

GG

Front-to-back ratio for UHF antenna
 

On Mon, 02 Jul 2007 10:49:39 -0700, szilagyic
wrote:

Hello:

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

I think I understand what the front-to-back ratio is, but the specs on
my Winegard PR-8800 (8 bay bowtie) show negative front-to-back ratios
(-9 dB @ ch 14; -17 dB @ ch 32; -11 dB @ ch 50; -9 dB @ ch 69). The
"dB over reference dipole" is: 10.7 dB @ ch 14; 12 dB @ ch 32; 11 dB @
ch 50; 12.5 dB @ ch 69. How can the front-to-back ratio be negative,
and is this good or bad? I am assuming this could be bad in my case
as I would like the signal at the back of the antenna to be minimal.

I am currently trying to improve our HDTV reception. I currently have
the PR-8800 antenna in our attic hooked to a Radio Shack 30 dB mast
amp. This antenna works very well overall. But from time to time, on
a couple channels the HD signal strength will drop all the way down,
enough to drop the signal or cause break-up. I've been considering a
Winegard HD9095P or PR9032 to use in conjunction with the PR-8800
since they have a higher gain than the PR-8800, then rotate the
PR-8800 the other way to receive some different stations.

I really appreciate the help and feedback on this!!

Thanks,

Get that antenna OUT of your attic. Put it outside on your roof, no
more than 12 feet mast. All your problems will go away.

Front-to-back ratio for UHF antenna
 

szilagyic wrote:
Hello:

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

I think I understand what the front-to-back ratio is, but the specs on
my Winegard PR-8800 (8 bay bowtie) show negative front-to-back ratios
(-9 dB @ ch 14; -17 dB @ ch 32; -11 dB @ ch 50; -9 dB @ ch 69). The
"dB over reference dipole" is: 10.7 dB @ ch 14; 12 dB @ ch 32; 11 dB @
ch 50; 12.5 dB @ ch 69. How can the front-to-back ratio be negative,
and is this good or bad? I am assuming this could be bad in my case
as I would like the signal at the back of the antenna to be minimal.

I am currently trying to improve our HDTV reception. I currently have
the PR-8800 antenna in our attic hooked to a Radio Shack 30 dB mast
amp. This antenna works very well overall. But from time to time, on
a couple channels the HD signal strength will drop all the way down,
enough to drop the signal or cause break-up. I've been considering a
Winegard HD9095P or PR9032 to use in conjunction with the PR-8800
since they have a higher gain than the PR-8800, then rotate the
PR-8800 the other way to receive some different stations.

I really appreciate the help and feedback on this!!

Thanks,
--
Chris

1) If you can, you'd be better off if you can get it out of the attic,
as UHF doesn't like roofing and other stuff between your antenna and the
signal source. (I think this has already been mentioned). Getting it
up in the air above the roof may help a bit, but it is always worth
trying various heights to find the "sweet spot". Antenna reception
isn't always quite like the theories would indicate...

2) UHF is also really susceptible to reflection. If you recieve the
direct signal and a reflected signal at the same time (the reflected
signal typically will be out of phase and tend to reduce the direct
signal, or if severe, look like ghosts on analog) you will have
difficulties during reception of the reflection. One place I lived had
a periodic loss of signal (didn't actually go away, just got
messy/snowy, it was in the analog days). Signal would get bad from a
half to a couple of seconds, and had an odd period. Certain times of
day it was every couple of minutes, other times it took hours to show
up. After beating around for a long time, discovered the problem seemed
to be related to reflection off of jets taking off from an airport
partway between us and the transmitter site... In messing about with
the antenna, found an "off axis" reflection off of something on a hill
offside to us (antenna was swung about 30 degrees to one side off of
direct line of sight) and, though the signal was a bit lower overall,
the periodic fading disappeared completely....

3) Radio Shack amplifiers tend to be way over-hyped. They rarely meet
the specs they claim, and tend to be somewhat unstable over time.
Better to get a commercial TV brand than Radio Shack, if you've a
chouce. If you're into home brew, you could use one of the
mini-circuits monolithic amplifiers and know you have decent gain.
They're pretty easy to work with, even at UHF frequencies, and stable to
boot. If you're really getting 30db from the Radio Shack amp, look at
the coax between your antenna and the receiver. Good quality coax can
help keep the signal you've taken so much care to obtain.

4) just for grins, try the exsisting system outside the attic without
the amplifier and see if you have a decent level of signal. You may be
surprised to see it isn't as bad as you think. As one person suggested,
you may actually have too much gain in the amplifier (though with RS,
you're actual gain may change with temperature and seemingly even the
phase of the moon...)

Not familiar with the model of antenna you have, though the "typical"
double bow tie (with 4 copies in your case)are mounted in front of a
mesh screen. In your case it looks like open elements as a reflector.
Sometimes you can get a better front to back ratio by adding a solid
reflector in place of the mesh/reflector rods. I would think in your
case it would help, and wouldn't take much trouble to try. Front to back
ratio is just that, the ratio of signal between what is received from
the front and the back of the antenna. A front to back ratio of 3db
means you have twice the amount recieved from the front than the back.
A high front to back means you will be essentially "blocking" potential
interference from the back side of the antenna by the ratio indicated.

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.

Looking at alternate antennas, the two you've indicated would likely
help, but you're only getting about 3db better signal (double the
power). They do have narrower beamwidth, so if your interference is
coming off axis, you may be able to use one of these to solve your
problem. Personally, I tend to like this type of antenna over the big
flat looking bowties. Especially out in the weather, there's only a
single connection to worry about, rather than the phasing array
connecting the 8 separate bow ties of the one you have now.

Another alternative to higher gain antennas would be if you have all of
your HDTV channels in the same relative band. Antennas like you've
looked at so far are compromises because they are set up for the entire
UHF channel set. If you have a relatively small set of close by
channels, you could get an antenna that is better matched to that set.
(I haven't looked in about 10 or 12 years, but you used to be able to
purchase antennas optimized for about a 10 channel spread, and they had
great gain and front to back specs.)

Hope you're not in an antenna restricted neighborhood, they can be the
pits to deal with. If you've currently got the bow tie in the attic due
to your neighborhood, look around and see if there's a place at one end
of the attic where you can pretty much shoot through a single wall.
Replacing the wallboard/t1-11/sheathing/siding with a plastic panel that
is about the size of the antenna reflector area could help increase the
signal strength without replacing the antenna. Of course, before
getting this drastic, I'd mount it temporarily outside of the attic at
that location first to see if it actually helps...


Good luck!
--Rick AH7H

Front-to-back ratio for UHF antenna
 

"Rick Frazier" wrote in message
...
szilagyic wrote:
Hello:

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

I think I understand what the front-to-back ratio is, but the specs on
my Winegard PR-8800 (8 bay bowtie) show negative front-to-back ratios
(-9 dB @ ch 14; -17 dB @ ch 32; -11 dB @ ch 50; -9 dB @ ch 69). The
"dB over reference dipole" is: 10.7 dB @ ch 14; 12 dB @ ch 32; 11 dB @
ch 50; 12.5 dB @ ch 69. How can the front-to-back ratio be negative,
and is this good or bad? I am assuming this could be bad in my case
as I would like the signal at the back of the antenna to be minimal.

I am currently trying to improve our HDTV reception. I currently have
the PR-8800 antenna in our attic hooked to a Radio Shack 30 dB mast
amp. This antenna works very well overall. But from time to time, on
a couple channels the HD signal strength will drop all the way down,
enough to drop the signal or cause break-up. I've been considering a
Winegard HD9095P or PR9032 to use in conjunction with the PR-8800
since they have a higher gain than the PR-8800, then rotate the
PR-8800 the other way to receive some different stations.

I really appreciate the help and feedback on this!!

Thanks,
--
Chris

1) If you can, you'd be better off if you can get it out of the attic, as
UHF doesn't like roofing and other stuff between your antenna and the
signal source. (I think this has already been mentioned). Getting it up
in the air above the roof may help a bit, but it is always worth trying
various heights to find the "sweet spot". Antenna reception isn't always
quite like the theories would indicate...

2) UHF is also really susceptible to reflection. If you recieve the
direct signal and a reflected signal at the same time (the reflected
signal typically will be out of phase and tend to reduce the direct
signal, or if severe, look like ghosts on analog) you will have
difficulties during reception of the reflection. One place I lived had a
periodic loss of signal (didn't actually go away, just got messy/snowy, it
was in the analog days). Signal would get bad from a half to a couple of
seconds, and had an odd period. Certain times of day it was every couple
of minutes, other times it took hours to show up. After beating around for
a long time, discovered the problem seemed to be related to reflection off
of jets taking off from an airport partway between us and the transmitter
site... In messing about with the antenna, found an "off axis"
reflection off of something on a hill offside to us (antenna was swung
about 30 degrees to one side off of direct line of sight) and, though the
signal was a bit lower overall, the periodic fading disappeared
completely....

3) Radio Shack amplifiers tend to be way over-hyped. They rarely meet the
specs they claim, and tend to be somewhat unstable over time. Better to
get a commercial TV brand than Radio Shack, if you've a chouce. If you're
into home brew, you could use one of the mini-circuits monolithic
amplifiers and know you have decent gain. They're pretty easy to work
with, even at UHF frequencies, and stable to boot. If you're really
getting 30db from the Radio Shack amp, look at the coax between your
antenna and the receiver. Good quality coax can help keep the signal
you've taken so much care to obtain.

4) just for grins, try the exsisting system outside the attic without the
amplifier and see if you have a decent level of signal. You may be
surprised to see it isn't as bad as you think. As one person suggested,
you may actually have too much gain in the amplifier (though with RS,
you're actual gain may change with temperature and seemingly even the
phase of the moon...)

Not familiar with the model of antenna you have, though the "typical"
double bow tie (with 4 copies in your case)are mounted in front of a mesh
screen. In your case it looks like open elements as a reflector.
Sometimes you can get a better front to back ratio by adding a solid
reflector in place of the mesh/reflector rods. I would think in your case
it would help, and wouldn't take much trouble to try. Front to back ratio
is just that, the ratio of signal between what is received from the front
and the back of the antenna. A front to back ratio of 3db means you have
twice the amount recieved from the front than the back. A high front to
back means you will be essentially "blocking" potential interference from
the back side of the antenna by the ratio indicated.

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.

Looking at alternate antennas, the two you've indicated would likely help,
but you're only getting about 3db better signal (double the power). They
do have narrower beamwidth, so if your interference is coming off axis,
you may be able to use one of these to solve your problem. Personally, I
tend to like this type of antenna over the big flat looking bowties.
Especially out in the weather, there's only a single connection to worry
about, rather than the phasing array connecting the 8 separate bow ties of
the one you have now.

Another alternative to higher gain antennas would be if you have all of
your HDTV channels in the same relative band. Antennas like you've looked
at so far are compromises because they are set up for the entire UHF
channel set. If you have a relatively small set of close by channels, you
could get an antenna that is better matched to that set. (I haven't looked
in about 10 or 12 years, but you used to be able to purchase antennas
optimized for about a 10 channel spread, and they had great gain and front
to back specs.)

Hope you're not in an antenna restricted neighborhood, they can be the
pits to deal with. If you've currently got the bow tie in the attic due
to your neighborhood, look around and see if there's a place at one end of
the attic where you can pretty much shoot through a single wall. Replacing
the wallboard/t1-11/sheathing/siding with a plastic panel that is about
the size of the antenna reflector area could help increase the signal
strength without replacing the antenna. Of course, before getting this
drastic, I'd mount it temporarily outside of the attic at that location
first to see if it actually helps...


Good luck!
--Rick AH7H

Gee, I want an isotropic antenna!
After all, it's the standard for comparison.
(If you don't recognize this as an attempt at humor, then God bless you.)

Front-to-back ratio for UHF antenna
 

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 on UHF, 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 the UHF band...)

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 on UHF antennas, and so far have been unsuccessful.

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

Front-to-back ratio for UHF antenna
 

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.

Front-to-back ratio for UHF antenna
 

"Bishoop" wrote in message
...
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.

Then what do you think it is equal to ?

Front-to-back ratio for UHF antenna
 

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

Front-to-back ratio for UHF antenna
 

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.

Front-to-back ratio for UHF antenna
 

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

Front-to-back ratio for UHF antenna
 

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

Front-to-back ratio for UHF antenna
 

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

Front-to-back ratio for UHF antenna
 

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

Front-to-back ratio for UHF antenna
 

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

Front-to-back ratio for UHF antenna
 

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!!

Front-to-back ratio for UHF antenna
 

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 / |
|------------------------------------/-------------------------------------|

Front-to-back ratio for UHF antenna
 

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.
----

Front-to-back ratio for UHF antenna
 

On 7 Jul 2007 11:38:12 GMT, wrote:

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.

The individual antennas I have listed above work quite well. Each has
its own coax and antenna mounted preamp. I just had problems with all
the lobes on the quadature array on some channels.

http://www.rogerhalstead.com/ham_files/Tower30.htm The UHF TV antenna
pointing to the NW is plainly visible. The one just below it pointing
straight south is just barely visible.

You might look into these antennas:

http://simplicitytool.com/mu_series_uhf_quad_array.htm

These antennas are wayyyy too close to really take advantage of using
the individual antennas as a quadature array. Regardless of their
claims for gain it can't be much more than one antenna alone. Front to
back *might* be legit and as they are so close as to perform like one
antenna they probably have only minor side lobes.

Unfortunately a company that puts big antennas that close together
makes me doubt their design criteria and performance figures.

Note how close their antennas are Vs boom length to the ones in
http://www.rogerhalstead.com/ham_files/tower21.htm and these are big
antennas.

One thing to remember when stacking antennas. A stacked pair will at
best be 3 db better than one. A quadature array (4 antennas) will at
best be 3 db better than the stacked pair or 6 db better than one.
Again that is _at_best_

Vertical stacking does not add side lobes over the single antenna
while horizontal stacking does. Horizontal stacking is also very
frequency sensitive so when covering the entire UHF band the
horizontally spaced antennas my exhibit quite different
characteristics on different channels. This is what makes building a
good quadature array that covers more than a few channels so
difficult.

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

Front-to-back ratio for UHF antenna
 

On Jul 2, 1:49 pm, 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 think I understand what the front-to-back ratio is, but the specs on
my Winegard PR-8800 (8 bay bowtie) show negative front-to-back ratios
(-9 dB @ ch 14; -17 dB @ ch 32; -11 dB @ ch 50; -9 dB @ ch 69). The
"dB over reference dipole" is: 10.7 dB @ ch 14; 12 dB @ ch 32; 11 dB @
ch 50; 12.5 dB @ ch 69. How can the front-to-back ratio be negative,
and is this good or bad? I am assuming this could be bad in my case
as I would like the signal at the back of the antenna to be minimal.

I am currently trying to improve our HDTV reception. I currently have
the PR-8800 antenna in our attic hooked to a Radio Shack 30 dB mast
amp. This antenna works very well overall. But from time to time, on
a couple channels the HD signal strength will drop all the way down,
enough to drop the signal or cause break-up. I've been considering a
Winegard HD9095P or PR9032 to use in conjunction with the PR-8800
since they have a higher gain than the PR-8800, then rotate the
PR-8800 the other way to receive some different stations.

I really appreciate the help and feedback on this!!

Thanks,
--
Chris

Thanks to all for the help. I have replaced my Radio Shack 15-1109
preamp with a Winegard AP-8275 preamp (I know the recommended model in
this thread was the AP-8700, but I want to put in a splitter and feed
about 5 separate devices at some point). After doing a direct swap, I
noticed a slight improvement in signal quality. We gained about 5-10%
signal strength for HDTV feeds, and about the same for our FM
reception.

Best,
--
Chris