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Ron Stone wrote:
"Greetings chaps.
I got some excellent reports yesterday from the East coast on my
repaired shunt fed tower. But every now and then the amp would trip out.
Never done that on the dipole at 120ft (which is 2 s-units down on the
vert).
Also something very odd was happening. If I sent a string of dots, the
reflected power would creep up (and sometimes the "resonant frequency"
would creep up too) to the point where it would go up to amp cut-off
point in about ten minutes.
...snip all the changes and checks made so far ...
I have just tomorrow to fix it. After all day today and half of
yesterday I'm clean out of ideas. Anyone got any suggestions on a fix
please.
It's driving me round the bend !!!
73 and good luck in the contest. You may or may not hear me on !
Ron Stone
Vine Antenna Products - for World-Class RF Communications
Telephone +44(0)1691 831111
Mobile 0585 785659
Facsimile +44(0)1691 831386
Mail - The Vine
Llandrinio
Powys SY22 6SH
Wales, UK
Hi Ron:
If Yuri's suggestion doesn't work out, I wonder about two
further things:
Do you have a good metal connection from the top of the
tower, presumably 120 feet, to the rotary mast coming out
the top? Sometimes RF through a thrust bearing or rotator
can heat up a dirty connection and change the resistance in
the path. Also, are the connections from the mast to boom
and from boom to elements on the yagi on top of the tower
are OK? I use a piece of flexible flat braid from a bolt on
the top of the tower near the thrust bearing to a U bolt on
the boom to mast connection plate for my yagi beam to keep
RF out of the rotator and thrust bearing.
Is the dipole on a separate support? If so, does taking it
down make the problem go away? I would then look at the
connections and Balun or center insulator and end insulators
of the dipole to see if it is affecting the very near field.
If the dipole is supported off the shunt fed tower, I would
guess the RF choke or balun at the end of the coax feed line
between the tower and dipole is taking off and heating or
changing value to not isolate the dipole. Again, take it
down or reisolate it. One of the Chicago area hams had to
make a common mode choke to isolate a dipole supported and
fed off a shunt fed tower. He built a coil of coax on a
plastic pipe form maybe 6 to 8 inches diameter and 18 inches
long and put enough capacity across the shield from one end
of the coil to the other end to resonate it as a trap on 160
to isolate the feed line for an 80 meter dipole supported
from his shunt fed tower to keep it from taking 160 meter
energy.
I guess I would check the dipole first and the mast
connections second.
Good luck and I hope you sleep OK tonight Ron,
CU in the contest from W0CD,
George, K8GG
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