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[TowerTalk] Trimming 75 ohm hardline

To: <towertalk@contesting.com>
Subject: [TowerTalk] Trimming 75 ohm hardline
From: k6ll@juno.com (k6ll@juno.com)
Date: Wed, 13 Jan 1999 20:13:38 -0700


On Wed, 13 Jan 1999 19:17:45 -0700 Steve Zettel <zettel@homer.libby.org>
writes:
>
>A question for the combined wisdom of the reflector:
>
>I was trimming some 75 ohm hardline, 1/2" Commscope variety, to get a 
>half wave at 7.0 mHz so I could feed my stacked F12 C-3E tribanders at 
>something resembling 50 ohms on the even harmonics. I am terminating 
>one end with a 50 ohm non-inductive resistance, the other end I've 
>terminated in a plumber's special to PL-259 fitting. I am measuring 
>SWR with an Autek RF-1 Analyst; I am not measuring in any strong BCB 
>or other RF fields that I am aware of.
>
>I started off quite a bit longer than the length predicted by the 
>formula:
>
>        1/2 X WL X VF
> 
>I've got a 1:1 SWR right now falling between 7.00 and 7.3 mHz, 
>although centered somewhat higher than I'd like around 7.175 mHz; and 
>1:1 points falling in the 20, 15, and 10M bands, too. This is at 232 
>feet of hardline. Interestingly, there is a 1:1 SWR point below 7.175 
>mHz at around 6.1 mHz. Does this indicate that I should continue to 
>trim until I've "raised" this lowest 1:1 SWR point to 7.025 mHz, or 
>leave well enough alone? At 232' I'll have some excess to coil up and 
>I can live with that; what I don't want to do is cut the hardline too 
>short and have to splice some back in.

Rather than using swr, I think it would be more accurate to use
the impedance method described in the RF-1 manual. Put a temporary
short on the far end, and adjust the frequency of the RF-1 until
you see minimum Z.

Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com

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