[TowerTalk] Expected VSWR of antenna near the ground

Mon Sep 28 17:33:48 EDT 2015

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 at contesting.com] On Behalf Of K0DAN
Sent: Monday, September 28, 2015 2:09 PM
To: Doug Ronald <doug at dougronald.com>; towertalk at 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 at 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 at contesting.com] On Behalf Of Jim Thomson
Sent: Monday, September 28, 2015 9:27 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Expected VSWR of antenna near the ground

Date: Fri, 25 Sep 2015 18:16:17 -0700
From: "Doug Ronald" <doug at dougronald.com>
To: <TowerTalk at 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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