Topband: Shunt feed on a budget

[Roger Graves]

Re. alternatives to vacuum variables.
For my shunt feed system, I used an ordinary variable cap to tune 
the feed with qrp and swr meter, then measured the capacitance. I 
dug out the freshman college physics text and found the formula for 
capacitance of two parallel plates and designed a capacitor for that 
capacitance that is about 6 X 18 inches of alum sheet, spaced 
about 5/16 inch. It is supported on plexi strips with SS bolts that 
allow tuning by adjusting the spacing. It has been in place for years 
with no problems.
73,
Rod VE7VV