The 10pf would probably work for the cans where the notch is on the low side of the pass ie hi-pass BpBr. The capacity required for the notch on the high side is 20-30 pf. I have paralleled 3 of the 10pf caps (which can be had surplus for $1 or so each) to replace the 30pf caps. It does help when modifying it in this manner to have the proper tap for the threads (15/64x64tpi). It is not exactly easy or cheap to find such a tap.
To explain the action of the capacitors, look at the wide response (+/- 15MHz) of the can. You will see that there are two notches, one on each side of the pass peak. The notches are at least 10 MHz apart. The two notches move in tandom with changing capacity. The more capacity the lower the notches are. With low capacity the lower notch will cosy up to the peak. In order to lower the upper notch down close to the peak requires a lot more capacity. Good luck with your tuning. Burt VE2BMQ>> tomnevue wrote: >snipped > Now for general information, the nut is just a locking device and the > capacitor is screwed into the threaded mounting plate. Also it was > pretty easy to find Johanson hi-Q piston caps at 0-10pf which are about > 1/4 inch shorter. I was curious to see if this was enough capacitance > to resonate the loop at the right frequency, so I temporairly installed > one of these. I added a strip of copper to take up the added length. > With the 10pf capacitor it resonated well into the Ham band, but I'm > sure it wouldn't make it down to the cavity lower spec of 137MHz. So in > a pinch, I think the shorter 10pf version would probably useable for 2 > meters. > > Thanks again. Tom
