In a message dated 6/11/01 8:25:29 PM Pacific Daylight Time, k7awb@qwest.net
writes:
<<
I recently bought an M2 eight element fm 2 meter beam to install at the top
of my mast on my tower. Where do I purchase an insulated mast section for
the 2 meter beam that I can connect to my 2 inch steel mast at the top of
the tower? I don't remember reading in the m2 literature that I needed an
insulated mast though. But, I don't want to affect the pattern.
Steve K7AWB
>>
You didn't say if it was to be vertically polarized which I assume it is or
you wouldn't be requesting the insulated mast. Try the quad people for their
fiberglass poles.
However there is another problem. The coax has to be run in the same area as
the insulated pole so there will indeed still be almost the same interference
to the beams pattern if a metal mast is used connected to the boom center.
There are two ways to solve this. Support the 2M beam from the rear and run
the coax to the back along the boom and run it down a supporting metal mast
at least a 1/4 wave from the reflector. The boom may have to be beefed up
for an end support. Or run a fiberglass support pole at 45 degrees from the
mast. The beam F/B will give a fair isolation from the mast in the rear.
Add another band beam on the other side pointing the other way to balance it
up.
At a Ham Convention in Sea Side, Or last week a vendor had 2 neat LP's for
144 and 450 MHz only of about 6 elements each. It was fed in the front. He
had a 19" (1/4 wave) piece of PVC pipe holding the coax out away horizontally
to keep it from the vertical elements and then it dropped down and back to
the mast. He said "it maintained the 1:1 SWR doing this" which I didn't
doubt. I've seen this before, used it in a hurry 30 years ago with a single
vertically polarized beam but haven't checked what it does in Eznec for sure.
(Since then I've always used 2 vertical beams on a horizontal boom since
with both spaced away from the tower evenly and at a spacing that gives the
cleanest pattern). It does give a fairly good pattern but I'll run an actual
pattern and also in Eznec on a 3 element quad and a yagi to see exactly what
it looks like with the loop around coax.
It's the only way one can run a single vertical yagi or quad center mounted
on an insulated mast and get a decent pattern without major interference from
the coax. This coax support has to be fairly rigid to take wind and ice/wind
loading.
3 element vertically polarized yagis have been supported on the DE with a
J-Pole support and feed.
The logs were mounted on a neat Y PVC pipe support on the boom and he was
able to tip it either vertical or horizontal or 45 degrees. k7gco
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