I posted a note on this a while back, but will recap here. I mounted four
dipoles from a DB-224 on one leg of two sections of Rhon 25 and mounted the
tower on an antenna range turntable. The pattern was a perfect circle, using
DB Power readings around the 360 degrees. I thought I remembered the offset
being 3 dB, but I must have been mistaken since it was pointed out that could
not be the gain off the back of the antenna mounted this way. All four dipoles
were stacked vertically on one leg of the tower and oriented directly away from
the tower. The gain was measured as 9 dBd in the favored direction, coming
down to 6 dBd at 90/270 degrees, and I thought I remembered the gain as 3 dBd
off the back side of the tower, but it must have been a lower number. In any
case, looking at the plot it appeared to be a perfect circle with an offset
center. The center offset must not have been 3 dB. I was surprised that the
tower did not cause a null off
the backside, just caused a reduction in gain in that direction.
This plot was submitted to the FCC to get repeater license WR5ADU and WR5ADV
back in the '70s when antenna patterns for a repeater had to be submitted.
73 - Jim W5ZIT
--- On Sun, 8/9/09, MCH <m...@nb.net> wrote:
From: MCH <m...@nb.net>
Subject: Re: [Repeater-Builder] 2M Vertical Dipoles
To: Repeater-Builder@yahoogroups.com
Date: Sunday, August 9, 2009, 6:33 PM
Lots of comments on the unidirectional pattern which I
suggested might
not work well.
Any comments on having the elements on one side of the tower right on
the leg?
Joe M.
n...@no6b.com wrote:
> At 8/9/2009 05:47, you wrote:
>
>
>> 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.
>
> I once modeled this arrangement in NEC-Win: the resulting pattern looked
> like a warped pancake. On-horizon gain was all over the place.
>
> Bob NO6B
>
>
>
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