Date: Fri, 25 Sep 2015 18:16:17 -0700
From: "Doug Ronald" <doug@dougronald.com>
To: <TowerTalk@contesting.com>
Subject: [TowerTalk] Expected VSWR of antenna near the ground
I have assembled my 19 element LPDA with the front 12 elements about 3 feet
off the ground and parallel to the ground. The back elements are laying on
the ground, but electrically connected in the usual LPDA way. Today, as a
check before I erect it, I swept with a VNA, the antenna from 1 MHz through
32 MHz. The VSWR is horrible! It has peaks and deep dips at the low end, and
then at 14 MHz or so, rises to over 5:1 and stays there. Now I didn't
expect even 2:1 over the range, but I expected the VSWR on the high end to
be at least reasonable, because the front shorter elements are off the
ground.
Anyone care to weigh in with an opinion? I hoping an expert says this is
perfectly normal for an antenna just a few feet off the ground. If not, I'm
kind 'a stuck as I have checked all the connections with an ohm meter, and
they are all okay.
Thanks,
W6DSR
### What is the make and model of the LPDA ? Or is it HB ?
What freqs does the LPDA cover ?? Is it 14-30 mhz.... or 10-30 mhz,
or 7-30 mhz ?? How LONG is the boom ?
## I assume you mean low freq end of boom is on the ground, while the other
end of
boom is aprx 3 ft above the lawn ?? That proves nothing. If far end of boom
is 3 ft above
ground, all the inboard eles will be LESS than 3 ft above ground. IE: 0-3 ft.
Sure, You could point
it straight up to the sky, with low freq end of boom aprx 1-3 ft above the
lawn..and also
temp guying the boom with non conductive guy rope..pita.
## with my 15m yagi sitting 3 ft above ground on wooden sawhorses, it
resonates way too
low. Typ 20/17/15/12/10m yagi will shift a huge amount higher in freq when
raised up from
3’...to 20’.
## If it’s a commercial built LPDA, just install it to its final height on
tower. Or at least temp raise
it to 10-15 ft, with boom parallel to ground. If its still screwed up, you
may well have a problem
with the assy / construction process.
Jim VE7RF
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