Topband: shunt feed help needed

[Herbert Schoenbohm]

Pete,


The ratio of the #14 wire to the size of the tower, as well as the lower C3E
(even though the elements are insulated may be adding to your problem.  My
suggestion is to use a larger diameter shunt such as discarded or surplus CATV
.750 hard-line or even build a cage of at least four wires as your shunt feed.
This will trick the RF into the belief that the tower is very fat and broaden
the bandwidth a bit while increasing the apparent feed impedance.

Based on the free standing complex top loading of your particular tower  your
shunt feed may need to be a bit higher on the tower.  Some theorist claim that
the cage need not be higher than the shunt but in most AM BC station
applications the cage runs the entire length of the tower.  The tap is then
selected to get as close to 50 ohms as possible and a series cap is utilized to
remove the shunt feed or cages inductive reactance.  In some cases where 50
ohms is not attainable a bridge T matching circuit is needed.  (This consist
generally of a fixed cap to the coax and a variable cap to the shunt wire or
cage with a tapped coil to ground at the center junction of the two caps.  This
will allow
a wider ranger of impedances and still deal with the inductive reactance of the
shunt wire or cage.

If the lower yagis are creating a problem, and the C3E at 69 feet really is
only the boom since the elements are insulated, you may have to try and run
some tests by insulating the boom to mast clamp with several layers of tape and
by temporarily disconnecting the coax feed line.  It would be interesting to
determine if the lower yagi is decoupling the higher ones at 1.8 Mhz.  I doubt
it but you can measure the change to be sure.

Shunt feeding towers with stacked arrays and side mounted antennas can produce
some exotic results as to the shunt feed impedances by making the tower
actually appear at RF to be shorter than it really is and driving the impedance
down.

Please let me know the results of your tests if you decide to try any of my
suggestions.


73


Herb Schoenbohm, ex-KV4FZ