Barry,
If you want to avoid cutting your feedline just to test the 1/8
wavelength shortening hypothesis, you can add the equivalent of 3/8
wavelength of feedline (as measured at the center frequency of the 10
meter phone band). In terms of how the antenna tunes on 10 meter SSB,
that should be electrically equivalent (to a first order) to subtracting
1/8 wavelength of feedline. As far as how the antenna tunes up on the
other bands, the equivalence between subtracting an 1/8 wavelength for
10M SSB and adding 3/8 wavelength for 10M SSB will no longer hold, but
at least you'll know whether or not the 1/8 wavelength shortening scheme
will solve your 10 meter matching problem.
As concerns your larger question about measuring and analyzing all this
stuff, I suggest you take your question to the Towertalk reflector
(towertalk@contesting.com). Just this morning there was a whole crew of
highly knowledgeable RF guys responding to a question about antenna
impedance mismatch and certain aspects of impedance analyzer measurements.
73, Mike W4EF............
On 2/5/2023 9:57 PM, Barry Jacobson wrote:
Hi, I need advice on getting one of the antenna analyzers on the market. I
have an 80 meter dipole fed with ladder line from an MFJ 989D tuner. I
seldom use 10 meters, but tried to go on the 10-10 phone contest today.
Would not tune in that segment of band. Tuner instructions say most
commonly need to shorten feedline by 1/8 wavelength at band that won't
tune. Before cutting line and possibly messing up other bands that work
fine, I would like to know exactly where I stand on each band. I could use
a device that will tell me exact length of line currently, and what effect
shortening will have on all other bands, and whether the new impedances
will be within range of capability of tuner. I don't mind learning complex
impedances and Smith charts if necessary, but want to do it systematically.
Don't know if devices out there do this for you. Some possibilities I have
heard of are NanoVNA, Rigexpert, MFJ antenna analyzer. I don't know if I
should go on EZNEC. The last 15 feet of each end of dipole droop
vertically, but don't think that's the main problem.
There is a whole page of devices. Don't know where to begin.
https://www.eham.net/reviews/view-category?id=31
Any advice from an expert greatly appreciated.
73,
Barry
--
Barry Jacobson
WA2VIU
bdj@alum.mit.edu
@bdj_phd
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