I sent this message last night, but haven't seen it posted yet.
Here it is again. Forgive me if it turns out to be a dupe.
Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com
On Mon, 5 Apr 1999 21:09:26 -0400 thompson@mindspring.com writes:
>
>Well I have gotten to the end of the KT34XA saga. All re-capped and
>new
>balun in place. All measurements set to the 6/19/85 dimensions (Bruce
>said
>these where the latest. 10 and 20 are below 1.4 over the band and
>match the
>factory curve. The problem is 15 meters where the SWR is 2.4 to 3 to
>1
>across the band. It is almost resistive at the top and bottom of the
>band
>but 2.4 to 1 not 50 ohms. The only thing I did not have before I got
>the
>new revisions from Bruce is the B dimension on page 9 which sets 15
>meters.
>I will measure the length to the strap on the dual driven elements and
>the
>reflector first as I can reach those elements from the roof with the
>tower
>cranked down. The capacitors all check less than 1/2 ohm and the 20
>meter
>freq is not affected by the other side of the B strap. KLM says 20
>meters
>is based upon length A (to the tip) and only the length changes 20
>freq. I
>lengthened the rear reflector as they stated and got 20 meter to a low
>SWR.
>
>Anyone have any other ideas?
Regardless of what the KLM techs or "From the Horse's
Mouth" say, I wouldn't screw around with reflector or
director measurements to reduce swr. I would put those
measurements back on nominal.
It's pretty obvious from the design of the antenna that
the rear driven element controls swr at the low end of
the bands, and the front driven element controls swr
at the high end of the bands. You can vary the B dimensions
on those two elements to see if you can improve the 15 meter
swr on the low and/or high end. I would try to use the procedures
of "From the Horse's Mouth," except where they address the low
end of the band, make the adjustments on the rear driven element,
and for the high end of the band, adjust the forward driven
element. Don't mess with the parasitic elements.
Some other things to check:
Measure dc resistance across the
two halves of the rear driven element. You should see a low
resistance through the phasing straps, through the match
section and through the balun.
If you're using one of the new antenna analyzers to measure swr,
try using a conventional swr bridge.
See if the swr varies with beam heading to determine if the antenna
is getting detuned by something nearby.
I've never had to vary from the factory-recommended settings
to get a low swr on all bands on the XA, but a lot of other people
have reported problems just like the ones you are having. Take
good notes and report your results. You may save a lot of other
people a bunch of work. Good luck.
Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com
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