Almost 15 years ago I had an R7 mounted on top of a Universal 40' or 50'
Aluminum tower (sri, don't remember height at this point). By pure luck,
because of the length of the tower, the way it was grounded, length of coax,
and mast length (which was around 8'), the antenna system had a perfect SWR
null (1:1) right at around 3.800 MHz along with all the normal HF bands the
antenna was meant to cover. To my surprise I was able to work lots of DX
with that antenna and just 100 watts on 75m. It was wonderful as it allowed
me to operate that band on a tiny, tiny congested lot in northern Va. where
I had no room for any wire antennas.
Later on I moved and tried to replicate that setup but using a slightly
taller tower. The SWR characteristics changed completely and I had a very
high SWR on 75m. I never tried to use the antenna on 75m even though I
suppose I could have with a tuner in the shack. I guess my point is to let
you know that depending on the metal support beneath the antenna and it's
length, you might be able to operate an antenna like that down to 80m. The
two factors I do not know about was if operating that antenna on 75m was
possibly hurting the antenna matching network and what the actual efficiency
of that antenna was on that band. Great SWR does not always mean high
radiating efficiency.
73, Gedas, W8BYA
http://gedas.cc
http://www.w8bya.com
----- Original Message -----
From: "Bob Nielsen" <n7xy@clearwire.net>
To: "WA3GIN" <wa3gin@comcast.net>
Cc: <TOWERTALK@contesting.com>
Sent: Saturday, October 18, 2008 9:10 PM
Subject: Re: [TowerTalk] Vertical with no radial wires
> The Hy-Gain AV640, Cushcraft R8 (etc) and some of the Gap verticals
> have minimal counterpoises and if you elevate the base a bit above
> the roof, there shouldn't be any problems. I have used a AV640 for
> the past 7 years and am pleased with the results. It is mounted on a
> 15 foot length of pipe with the base about 2 feet above the edge of
> my roof. It's not rated for 80, but I have made contacts over 2000
> miles on both 80 and 160 running 100 watts under the right conditions.
>
> Bob, N7XY
>
> On Oct 18, 2008, at 5:39 PM, WA3GIN wrote:
>
>> Hi Folks,
>>
>> Looking for a commercial 80-10m for a building roof top install.
>> The building roof is rubber so we're using non-penetrating roof
>> mounts for the base of the vertical. The building owner prefers no
>> wires on the roof top.
>>
>> Vendor and Model recommendations are welcome. Please reply direct.
>>
>> 73,
>> dave
>> wa3gin/listinfo/towertalk
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