[TowerTalk] High VSWR

[Paul Christensen]

As bad as the SWR plots look after the repairs, it's not that far out of spec.  The USAP LP-1005 SWR spec is less than 3:1 between 3-4 MHz, and less than 2:1 between 4-30 Mhz.  On 75m-80m, it really needs an ATU no matter what based on their 3:1 SWR spec.

http://www.usantennaproducts.com/antennas/models-lp-1005-lp-1001-lp-1002/

I would also have to call into question USAP's gain spec.  Providing a gain spec based on ground reflection is misleading.  Just give customers the gain in either dBd or dBi in *free space.*  That's it.  Why the need to back into a meaningful free space number by subtracting out the reflection gain?  Let the customer determine reflection gain based on terrain using refection prediction software (e.g., HFTA).   

Based on the LP-1005 boom length, frequency coverage, and reasonable sigma/tau parameters, I would estimate that gain is no more than about 5.5 dBd across the spectrum.  I base that on Roger Cox's LPCAD35 log periodic design software.  

When running a LPDA model in NEC software, one sees that only 2-3 elements carry the majority of RF on any given frequency.  In essence, the LPDA is an electrical equivalent of a short-spaced 2-3 element Yagi on a short boom -- even when the boom is 70 ft long.  A big price in performance is paid to have continuous frequency coverage.  

Just my opinion, but this type of massive government/military system should only by turnkeyed with a performance guarantee.  By doing all, or some of the work yourself, you end up owning each and every problem that develops no matter how large or small.  With a well-written turnkey contract, the risk is significantly reduced.  

Paul, W9AC

-----Original Message-----
From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of Jim Brown
Sent: Wednesday, August 28, 2019 11:27 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] High VSWR

Thanks for the link, Wes. Yes, the process was interesting to follow. 
Don't feel like the Lone Ranger though, Doug -- over not very many years, K5RC has had a lot of trouble keeping his 3-el 80M yagi at 175 ft operational at W7RN.

And here's another thing on which Wes and I agree. I know a lot of hams who have had a lot of fun with far less mechanically heroic antenna farms.

73, Jim K9YC

On 8/28/2019 7:14 PM, Wes wrote:
> For those that may have missed the link to Doug's build photos here it is:
> 
> http://w6dsr.com/LP1005AA/index.html
> 
> Doug, I'm sorry about your loss.  But if the mast isn't bent, which 
> the one post-failure photo I've seen (Dave I'd like to see the others) 
> seems to indicate, then you still have tower, rotor, and mast.  I'd 
> put up a less heroic "ham-style" stack and get on the air.

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