TopBand: Shunt-fed tower - HELP !!!

[George Guerin]

--IMA.Boundary.705984588
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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
--IMA.Boundary.705984588--

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