Topband: Shunt fed tower loading riddle

[James C. Hall, MD]

Hello All:

 

I’m continuing my attempts to load my shunt fed tower which is electrically
longer than ¼ wavelength. At the tower, I managed to get the SWR down with a
500pf 10kV vacuum variable as the series capacitor and a couple of 50pf 5kV
doorknob capacitors as bypass (omega) caps. However, despite showing that it
should load fine in the shack, when applying any sort of power over about 50
watts, the SWR would suddenly spike.

 

This morning, I decided to eliminate the doorknob capacitors and dip the SWR
with just the series capacitor. This produced a dip at 3:1 and a resistance
showing 10 ohms. I decided to see if I could tune it out back in the shack
with my TenTec tuner. This turned out to be fairly straight forward and,
when tuned, the resistance showed 35 ohms. I’m now able to crank up the
power without difficulty.

 

Now to add a bunch of radials. I have another vacuum variable I could add
back at the tower as an omega bypass capacitor if that would add anything.

 

73, Jamie

WB4YDL





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