It sounds like a connection that is touching but not making solid contact
due to a fracture or corrosion. Assuming that this is your only 15m antenna
I would try banging on the elements while watching the receiver next.
John KK9A
Drew Vonada-Smith wrote:
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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