[CQ-Contest] Antenna Analyzer Expert

[Barry Jacobson]

Thanks very much for the help, guys. I am going to try to join Tower Talk
which I never heard of before. Yes, the manual mentions either adding an
1/8 or subtracting an 1/8. Adding 3/8 would be similar to subtracting an
1/8. I do have some slack and even before this based on the rule of not
making an 80 meter dipole an odd 1/4 wave I had been telling myself to cut
more. I bought a 125 foot roll of ladder line. Cut down middle 67 feet to
make a T. (Trusted DX electronics that the roll was exactly 125 feet
because didn't have good tape measure at the time, so used midpoint as 62.5
feet and cut 4.5 feet more.) That leaves 58 feet for feedline which is
close to the 66 feet no-no. So I cut an additional 10 feet off which is 48
feet. Manual says 34-40 feet us ideal, but may need to be adjusted to
accommodate higher bands, which is my current situation. So I am deciding
whether to just go for it and cut 4 feet plus few inches to make 1/8 wave
based on 1/2 wave is 468/f MHz, or to order an analyzer. I will also need a
balun as my MFJ 989D tuner doesn't have a bypass setting for balanced line,
a big deficiency in my opinion since their cheaper ones do. Will get
expensive. Plan to try to download Rigexpert manual and possibly NanoVNA to
see what it does. Else may just snip 4 feet without analyzing anything and
try it out.

Appreciate your time.

Barry WA2VIU

--
Barry Jacobson
WA2VIU
bdj at alum.mit.edu
@bdj_phd

On Mon, Feb 6, 2023, 10:18 AM Michael Tope <W4EF at dellroy.com> wrote:

> 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 at 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 at alum.mit.edu
> > @bdj_phd
> > _______________________________________________
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