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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