I would guess that antennas that don't have a dc short across the
feedpoint (such as a beta match) feed a lot of AM broadcast signal into
the SWR bridge. I had the same problem trying to tune a 10m Yagi here
in the midst of a lot of VHF/UHF energy from our commercial site. I
just couldn't get the SWR to read below 2:1 anywhere until I put a
standard TVI low pass filter on the MFJ, then everything worked great.
I noticed this effect also in tuning my 160 and 80 antennas that are on
a tower that has some paging transmitters. At first, I thought there
was a loose connection somewhere until I realized that it was the VHF
stuff keying up. Also saw the same problem every time I've tried to
tune an antenna at a multi-transmitter contest station, because somebody
is always eager to try out the rigs and antennas that are already
finished!
It would be a neat addition to SWR bridges to have a means of turning
off the oscillator so you could be sure that the meter dropped to zero.
This could even be automated by pulsing the oscillator at a fairly high
rate and looking for detector output when there shouldn't be any (MFJ,
are you reading the mail?).
This doesn't show up on the rig SWR measurement because the power level
is much higher than that of the SWR bridges. This problem shows up on
megabuck network analyzers, too, so don't feel that more money would
replace knowing what's going on.
As to shift of apparent resonance when you measure through a length of
transmission line, when the SWR isn't unity the impedance wanders all
over the place, and the frequency of "closest approach" to unity SWR can
be quite different depending on the length of the cable. There's only
one unique condition that is independent of line length, and that's a
load impedance equal to the transmission line characteristic impedance.
I'm intrigued by the Smith chart readout on the Kachina transceiver, and
by the network analyzer in the current QST. Swept frequency measurement
would be a big plus, but not at the slow rate of the AEA bridge.
Dave, W6NL
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