As frequency decreases, so does the importance of keeping the dipoles
exactly above one another. This is why you can get away with mounting
the bays of a LB array around a smaller tower (like Rohn 25) and still
have very good omni-directional performance. Positioning the bays
around a central supporting mast of a UHF array creates considerable
pattern distortion and gain is lost. DB Products and others used a
method of exciting two dipoles at the same elevation - the double
dipole. This makes the radiation center of the array the mast pipe, and
at higher frequencies there is a big advantage to this.
Frank Rossi, N3FLR and I did testing at my Seven Springs site in the
early 90's with a Cushcraft UHF exposed dipole array. (Yes the ones
with the RG-58 harness, poor connectors, and all that) I had a UHF
repeater hooked to it and it was side mounted on the tower with
stand-off brackets to minimize the blocking effects of the tower. I
started out with the dipoles arranged one directly above another. In
Greensburg and Pittsburgh (about 30 and 45 miles from the site) the
repeater was usually full scale on Frank's S meter and he was received
very well even on 5 watts. My house in Friedens (about 20 miles away)
was in the null and I received the repeater on average about S-7.
Testing was done for several days to get a benchmark.
I then tried the antenna in omni, positioning the bays around the mast
as Cushcraft suggested. The result was an increase toward Friedens of
two S units (from S-7 to S-9) and a drop towards Frank from usually full
scale to S-5 to S-9 - a considerable loss. It was soon learned that on
UHF with single dipole arrays that you need to keep them one above
another unless you can stand the considerable reduction in gain when
going omni. The antenna worked so much better in every other direction
than the exact null, that I went back to the "directional" pattern and
dealt with it.
The Cushcraft antenna was relegated to remote base use, and still works
some 15 years later. It was sealed as well as could be done. The
repeater was connected to a top mounted Stationmaster look-alike home
brew 23 half-waves in phase with 3 degrees of downtilt. This antenna
buries the S-Meter in all directions.
<http://www.repeater-builder.com/antenna/wa6svt.html>
I know of one high profile VHF repeater in Pittsburgh that uses a dipole
array with the dipoles faced into the tower. It has amazing
omni-directional range.
Kevin Custer
MCH wrote:
I'm sure someone on *this* list will be able to correct me if I'm wrong,
but I think you can mount virtually any of the dipole type antennas on
the tower leg as long as you're not looking for a 360 pattern. An offset
or figure-8 pattern, though, no problem. In fact, I've done this for
commercial users, and in some cases such as Low Band dipoles, that's the
only way to can mount them.
You might even be able to get away with it mounting one on each leg for
a 360 pattern (with a little more gain in one direction for a 4-bay
antenna), but your gain would be messed up since the signal would then
be out of phase for each element in any given direction.
Joe M.
Doug Rehman wrote:
I remember seeing a vertical dipole from one on the amateur antenna
manufacturers that was designed to bolt onto your mast or tower leg. I think
it might have come in a kit of 2 or 4.
I thought it was Cushcraft, but I couldn't find it. Does anyone have any
idea what antenna I thought I saw?
Thanks,
Doug
K4AC
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