Fellow Hams,
I am hoping someone might have an idea on this. It's a stumper. With All Asia
SSB approaching, I am getting concerned. The antenna in question is a JK
Mid-Tri, a multiband interlaced Yagi using a pair of "transmission line" bars
to feed the 10/15/20 driven elements.
Symptom - Both SO2R radios show intermittent received signal strength on 15M
only. The change seems to be about 2 or 3 S units of popping in and out. SWR
is changed little, if at all, during these events. It sometimes settles down,
but turning the antenna usually causes more mayhem.
Evidence:
*
Both SO2R radios act similarly
*
In the unlikely event that BOTH my low power band filters are bad, there was no
change when bypassing them.
*
In the event that the remote 2 x 8 switch is a problem, changing ports for the
15M selection made no difference.
*
Bypassing my triplexer and routing directly to the antenna made no difference.
*
The 15M specific cable from triplexer to the switch (and all others outside)
has been replaced.
These facts along with rotation stimulating the problem would seem to indicate
the antenna itself. I was finally able to climb yesterday and check the 15M DE
mounting to the transmission bus bars. All seemed corrosion free and tight.
Nevertheless, I applied DeOxit and banged and manipulated the connection to
break up any hidden oxide. No effect. This is not too surprising as I had
never seen a SWR change, but I thought it perhaps possible that the effect was
RX only due to a corrosion film layer, as sometimes happens in relays.
The only remaining issue I can think of is an intermittent in parasitic
elements. The Yagi is 4 el on 15M, so it is hard for me to imagine that a
director going non-resonant due to a bad joint could kill 3 S-units. It just
becomes a non-optimized 3-element Yagi, maybe worth 2-3 dB? Even if the
reflector is somehow intermittent to become shorter and act as a director,
which I'd think is the worst case scenario, 3 S-units seems a stretch. In
addition, there are no short tip lengths in the design - all tips are about 2
ft long and use good machine screw fasteners w/nylock, not hose clamps. But I
am left without other theories.
Most of the antenna is unreachable so I can't just inspect every joint.
Nothing amiss is visible using good binocs at 24 ft Yagi height (crank-up tower
retracted). I am trying to think about a way I can get audio in the field and
perhaps smack every element with a long pole. I have a 30 ft fiberglass
telescoping mast I could use for that, perhaps with my FT-817.
If any of you can think of anything else to try, I'd sure love to hear it.
This one is a STUMPER!
73, Drew K3PA
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