[TowerTalk] Shunt fed resonance changing.

[James Wolf]

Background:

For 160 meters, I shunt feed my tower.  The tower totals 120’ high with 20
ft. of Rohn 35 rotating above the 100 ft. level.   I have philistrand guy
lines.  All feedlines are grounded at the bottom of the tower and at the
antenna.

-          2 el. 40 Cushcraft 40-2CD @ 115’.  

-          TH6DXX @ 101 Ft.  

-          16 element 2 meter antenna at 107 ft.  

-          sticking out of the top of the tower is a Stainless Steel tube
bolted tight to the top with a 16 Ele 2 meter antenna at 122’ and a 432, 14
element at 128 ft. 

-          At ~75 ft. is a trapped WARC antenna and the elements are
isolated from the mast plate.  The coax goes to a switchbox at the ground
level.

 

My problem is that the resonance on 160 meters changes about 50 kHz.  This
usually seems to happen when using amplifier power.  There is nothing I can
do around the bottom of the tower, or the feed point to create the resonance
change.  However, I can at times move guy wires and see the resonance
change.

 

When I installed the tower 30 years ago, I painted the tower sections with
zinc tower paint, but did not paint the overlapping mating part of the
section where the bolts are.

I have bonded the mast pipe with a ¾” braided strap to the tower leg just
below the top tower bearing,  I have grounded the antenna mast plates to the
tower legs thinking that the paint might be insulating the mast plate from
the tower.

 

This is quite a problem as the Alpha 87a will kick off line in the middle of
a QSO if the resonance changes.

 

You can see a video of the tower here.

I have removed the three 220 MHz yagi’s that used to be used in the old days
for Packet Cluster back bone links.

https://www.qrz.com/ <https://www.qrz.com/lookup> 

 

Any suggestions as to what the problem is would be appreciated.  I am
assuming that it is a long shot that the tower sections would be RF
intermittent.

 

Jim,  KR9U