"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.)