Paul,
You should analyze the entire antenna system (tuner, feedline and antenna)
as a first order of business.
If you have an antenna analyzer, use it on the input of the tuner - tune for
a low SWR and record the settings.
If you don't have an antenna analyzer, borrow one and use it. Tune things
once and record the proper tuner settings for each band (and each antenna if
you have multiples) so you can easily return to those settings.
Such an inability to achieve a match with an antenna system could also be
because of some parasitic oscillation in the transmitter. If you cannot
obtain an antenna analyzer, try using a different transmitter temporarily to
check it out - if the tuner behaves the same for both transmitters, you can
rule out the parasitic possibility and work on the antenna system.
If you cannot achieve a match on any particular band (or bands), then yes,
try changing the length of the feedline (it will alter the tuner settings
for all bands) - this applies to any non-resonant antenna, not just a G5RV.
You can either lengthen or shorten the feedline to achieve a better match.
Change the length by about a quarter wavelength at the troublesome band as
an initial trial - that will change a high impedance feedpoint to a low
impedance.
You did not say how long each segment of the feedline is, but my
recommendation is to use 300 ohm (or 450 ohm or better yet open wire line)
for almost the entire run - right to the tuner if possible. If you must use
coax for part of the feedline, use a 1:1 current balun at the junction of
the coax and parallel transmission line.
73,
Don W3FPR
-Original Message-
I am wondering what the solution to my problem might be. I have a
MFJ 989 tuner and it works fine except on 17 meters. I have the
cross needles and they both shoot up to 12 0'clock and cannot get
a resonant point. Some have told me I should shorten the coax. I
am using a G5RV with 300 ohm twin lead and coax into the shack. I
am having suspicions that it is the tuner. Any suggestions...
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