Topband: Shunt-feeding

[Tom Rauch]

Two points to remember:

1.) 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.

With modelling so easy, I'd take the time to check out the voltages 
before burning up insulators! 

2.) The broadest bandwidth (and almost always) lowest loss 
configuration is a single reactance in series with the gamma (or 
shunt) feed, especially if everything is done to minimize the 
required reactance of the single component. 

Loss may not be significant, but it is a good idea (if possible and if 
you have the time and energy) to look for that 50 ohm point if you 
want maximum bandwidth.

The differences in voltages across capacitors, or between elements 
and a boom, can be amazing in different shunt-fed tower systems.
73, Tom W8JI
W8JI@contesting.com