Topband: Shunt feed short tower

[Larry Rockfield]

> Summary:  42' tower w 4'mast, XM240, (12) 64' radials on ground, 2 shunts 
> of 1/2" copper tubing spaced 1 foot from the tower face> Kelly - N0VD

Kelly, a rough model shows feedpoint Z=2+j330 for the lower end of 160M. 
Expect ground losses to be high, and matching network losses alone to run 
16% or greater, most likely greater.  One of the L-network schemes show 
components of 0.89uH shunt and 256pF series. The shunt component is 
connected across the coax termination at the tower.

With only 100W, the peak voltage across the capacitor will exceed 3kV, and 
the RMS current in the incuctor will be in excess of 6A.  With 1500W, look 
for a capacitor voltage of 12kV with inductor current approaching 25A.

My estimated feedpoint Z could easily be wrong, but at least a starting 
point.

Larry W6UB