Topband: Shunt fed tower loading riddle

[James C. Hall, MD]

Thanks to all for the great replies. I particularly like the link on the
doorknob capacitors. Interestingly, I had a carbon track on the lid of the
Hoffman box. The box is grounded to the tower and the doorknob capacitors
were attached to the back of the box. Interesting – could be the box itself
was getting in the act !

 

Anyway, thanks for all the info – very useful.

 

73, Jamie

WB4YDL

 

From: Brian Machesney [mailto:nekvtster at gmail.com] 
Sent: Sunday, November 14, 2010 05:38 AM
To: James C. Hall, MD
Subject: Re: Topband: Shunt fed tower loading riddle

 

Jamie,

I've had sad experience with so-called doorknob "transmitting" caps from a
reputable brand name. They don't seem to want to handle any useful
quantities of RF current. Not much use for transmitting!

See http://www.qsl.net/i0jx/capacit.html for a useful discussion.

73,

Brian K1LI

On Sat, Nov 13, 2010 at 5:15 PM, James C. Hall, MD <heartdoc at nwtcc.com>
wrote:

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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