Make sure the booms are electrically separate. Each 1/2 element should be
staggered onto the other boom from the previous one. Some LPDA's have issues
with the feedpoint choke, and how the coax is routed. You might want to look
for those articles (such as this:
http://www.w8ji.com/baluns_on_log_perodic_antennas.htm).
Wow that is one big antenna!
Good luck!
73
dan
k0dan
-----Original Message-----
From: Doug Ronald
Sent: September 28, 2015 14:42
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Expected VSWR of antenna near the ground
Well, okay, I'm convinced, thanks to everyone's advice, that I cannot expect
a VSWR sweep to yield anything meaningful with the antenna on the ground. So
I've done everything I can on the ground to ensure the antenna has been
constructed correctly. I have verified that there is continuity on every
dipole, with the correct phasing to the transmission line. I have verified
that the coaxial transmission line isn't shorted, and has continuity
throughout. Since the antenna is a commercial product, shipped completely
disassembled, I have to assume it will perform when at its design height of
100 feet.
Unfortunately, I'll only get one chance to get it right, since I have to
schedule a crane and helpers, around the constant wind which blows up here
almost incessantly. Judging by the wind forecast, this week is pretty much
out. The antenna is pretty big; 72 foot boom, 105 foot longest rear element,
about 2800 pounds weight, so once I manage to get it up there, its staying
up there. It covers 3 - 30 MHz, although below 4 MHz, the VSWR can rise to
over 2:1. Over the rest of the range the VSWR is < 2:1.
Thanks for all the answers to my question,
-W6DSR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Thomson
Sent: Monday, September 28, 2015 9:27 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Expected VSWR of antenna near the ground
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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