[ARC5] Ant. Tuning Series Caps - Trials and Surprises

[David Stinson]

As most of you know, I put a fixed capacitor between the ANT post

and the center-conductor of the coax leading to a 50-Ohm load

when matching WWII Aircraft rigs to "ham" antennas.

I've recommended Hi-Voltage caps and, I confess, have done

so because it seemed intuitively "right" to do so.

I always meant to do some tests.  Finally did, and I'm 

very surprised at the results.

 

Tested at 3890 KC with a BC-696 transmitter on the 

dynamotor.  This set has soft finals, so puts out about

22 Watts when properly matched.  To do so, the series

cap value that works best is about 100-120 pFd.

 

https://goo.gl/photos/WSbxCuZwBKkPQsCm7

 

I gathered several caps:

 

100 pFd High Voltage, Russian, Orange tubular

100 pFd High Voltage,  Russian, Gray, Cylindrical

(2) 60 pFd Silver Mica in parallel, 500V

91 pFd Silver Mica, no voltage marking.

75 pFd, Silver Mica, 500V

100 pFd ceramic, small, no  other markings

120 pFd ceramic, small, no other markings

120 pFd Monolythic, TINY

 

Inserted each capacitor in turn and keyed-down 

for one minute.

I have not used a scope to get exact numbers,

but sorted the caps by those that worked well

with little heating and those that introduced 

excess loss and/or heated-up a lot.

 

The results were surprising.

"Good" caps on the left, "poor" on the right:



https://goo.gl/photos/tRDxqjMHUwQzaFVTA

 

The silver micas and the one Russian "Hi-V" cap

heated and made excess loss.  The 91 pFd was the 

worst and got very hot.  The two 60 pFd at 500 micas 

in parallel worked OK with the least heating of the bunch,

but the other caps were better.

 

Of the "acceptable" group on the left, there was

almost no difference between the big "Hi-V" cap

and the little ceramics.  The one that boggles 

my mind is the tiny 120pFd monolithic;

this thing is half the size of a grain of rice, folks.

I expected smoke.  After one minute key-down,

it had heated modestly with  less than a watt 

of reduction in power out.

I am at a loss to explain that one.

 

The take-away for me is:  Don't assume.

Try different caps.  One "120 pFd" isn't the 

same at RF Power as another "120 pFd.

 

73 DE Dave AB5S

 

 

 

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