Stacking the antennas vertically could work but will create some nulls /
signal attenuation /cancellation in the elevation plane (versus your current
problem in the azimuth plane). The elevation nulls can put some signals near
the horizon (where you want the antenna to provide its' gain) into a deep
trough. The effect would depend on frequency / channel, the physical spacing
of the two vertically stacked antennas, and the antenna designs themselves.
To get a rough idea about how to avoid these effects either in vertical or
horizontal separation of the two antennas, consider that 1 wavelength at the
end of the UHF band is very roughly 1 meter long and that it is roughly half
that at the high end of the UHF band. A minimum separation of quite a few
wavelengths is required before the so-called "near field" effects of the
antenna diminish, and I would guess that 10 wavelength = ten meters = about
35 feet would be very adequate for low band UHF and about 17 or 18 foot
separation if you were seeing cancellation / attenuation at the high end of
the UHF band only. I would NEVER suggest putting metal / insulating plates
or surfaces between the 2 antennas since this would act as yet another
element to cause attenuation / reflections / etc.

You are therefore left the options of spatially separating the antennas on a
single mast vertically 17-35 feet apart, or on 2 masts 17 to 35 feet apart.

My approach is very conservative and somewhat smaller spacings may work
adequately, but this could only be determined by experimentation. A lot has
to do with the specific antenna designs, the arrival angles of the desired
channels / signals, the coax line lengths and impedance matches, etc., etc.,
etc.


Smarty



"szilagyic" wrote in message
ps.com...
On Jul 23, 10:42 am, "Smarty" wrote:
Since both antennas cover UHF, point in opposite directions, and are
spaced
within a fraction of a wavelength at UHF frequencies and below, it is
extremely likely that the phase cancellations of the two antennas causes
the
drop in signal strength when mixed in a combiner. The solutions are far
as I
can tell would be to bring both feed lines (coax lines) down to the
receiver
and switch when you need one or the other, or to filter out UHF content
from
the log periodic with a step low pass filter, or separate the 2 antennas
more, since you appear to be creating a phased array even though you
never
intended to by the way you are configuring them. Phased arrays
deliberately
add and subtract RF energy to gain directivity at the expense of
attenuation
in other directions.

Smarty




Thanks for the quick reply. Right now the best alternative is the
spacing as I only have one downfeed that uses a preamp. What would be
the minimum distance between the two antennas that I should use for
UHF considering VHF doesn't appear to be a problem, or is
it better to stack these instead, and if so what would be the minimum
distance that should be used? Or, would a metal barrier between the
two fix this issue with spacing??

I really appreciate the help.
--
Chris