Topband: Shunt-feeding

[k6se@juno.com]

Tom, W8JI wrote:

"Insulated elements can have an arcing problem when used on a shunt-fed
tower. This is especially true when moisture is present, and the
insulation poor (like fiberglass or PVC).  Voltage between the insulated
element and the boom can be very high, and is as much a function of the
height and other antennas on the tower as 
the power involved..so without modeling there is no accurate way to
predict voltage."
==========
What Tom says is very true.  PA3DZN had arcing from his insulated
Force-12 CX-34 tribander 20-meter reflector to the boom when he
transmitted on 160m to his shunt-fed tower.  I suggested that he ground
the centers of all of the parasitic elements of the tribander to the
boom, which he did and it solved his arcing problem (as well as gave him
more top loading on 160m).  He didn't notice any difference in the
performance of the tribander on 20, 15, and 10m.

The highest RF voltage always occurs at the farthest end of an antenna
from the feedpoint.  In the case of a shunt-fed tower with an HF Yagi
with insulated elements used for top loading, this highest RF voltage
point would be at the ends of the Yagi's boom (where the CX-34's 20-meter
reflector is).  If the center of the elements are grounded, beam
performance should not be much different and the highest RF voltage on
160m would now be at the ends of the Yagi's elements where no arcing will
occur.

73, de Earl, K6SE