I am using vacuum capacitor, you can find them on e-bay around $200. But I
have used small NPO capacitor in parallel for several years. With the right
material that can handle current on 1.8 MHz. Most ceramic capacitors, like
doorknobs can handle 3 RF Amperes on 3.5 MHz but only 1A on 1.8MHz.
The capacitor I used for several years with no failure is this
http://www.electronicsurplus.com/cera-mite-564cc0gaa302el620j-capacitor-cera
mic-62pf-3-000v
62pf NPO 3KV, for 1200 PF you need 19 units in parallel, very important the
capacitor cannot touch each other, keep them at least 2 mm apart, or 80 mils
separation between them. You can find them on DigiKey as well or other
vendors, at Eletronicsurplus they cost U$ 1/each.
NPO is a must otherwise they will change capacitance with the temperature,
some 500pF capacitor can really change the tuning a lot. Three 170pf NPO in
parallel does a much better job them one 500ps that is not NPO.
My two cents
N4IS
JC
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