The boom is actually a structural truss, not involved with feeding the dipole
elements. All the dipole elements are fed from a linearly tapered coaxial
transmission line transformer, the details of which, I don't fully understand.
There is a BALUN built into the mix too, all done with 1 5/8 " EIA rigid coax
which runs up and down the length of the boom. The center conductor of one
length of the EIA has a tapered center conductor. Calculating the Z0 from one
end to the other based on dimensions is 50 Ohms to 73 Ohms.
I have read the reference you provided when I was trying to figure out how mine
works. This antenna, having been designed for the military, is rated for 25 kW,
so they do things differently than W8JI discusses.
-W6DSR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of K0DAN
Sent: Monday, September 28, 2015 2:09 PM
To: Doug Ronald <doug@dougronald.com>; towertalk@contesting.com
Subject: Re: [TowerTalk] Expected VSWR of antenna near the ground
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|