Topband: feeding 1/2 wave vertical

[Terry Conboy]

At 02:54 AM 2007-11-10, John K1GUN wrote:
>gee i'm sorry folks in my haste to ask the question how to feed my 
>new tower I didn't mention that it's grounded,but the the guys are 
>broken up with insulators i don't think its possible to shunt feed a 
>1/2 wave vertical,1/4 is easy but the voltages and currents are 
>different in 1/2 verticals sorry for the confusion.John K1GUN....

 From a quick model in EZNEC, it looks like it shouldn't be too hard 
to shunt feed a grounded half-wave vertical.  The model had a 5 inch 
diameter tower and the same diameter for the shunt feed wire, spaced 
out 3 feet from the tower.  The 256 foot tower was tapped at 78 feet 
(~30%) to get a 51 ohm real part (with 485 ohms of inductance).  A 
180 pF series cap tunes out the reactance.  Bandwidth is about 60 kHz.

The large diameter feed wire could be approximated with a skeleton of 
2 or 3 wires spaced 6 inches or more.  A larger effective diameter 
shunt wire gives more bandwidth.  The diameter also changes the 
resistance and reactance at the bottom of the shunt feed wire, so 
some experimentation will be needed.

Going further up the tower raises the resistive component, so it 
shouldn't be too hard to move a temporary tap up or down to get 50 
ohms.  A 365 pF variable could be used to resonate for 
testing.  However, with 1500 watts, a 180 pF cap has 2650 volts (RMS) 
across it, so for operating, chose a cap with a suitable voltage rating.

73, Terry N6RY