Topband: Mica capacitors for antenna matching.

[K9AY]

I'll add to Tom's recommendations. I have used multiple ceramic discs in 
parallel for many years. Amplifier makers have been doing this for quite a 
while, as well (although the factory originals failed in in my Alpha 99...).

I also use 6 kV discs, but have one caveat -- use the physically largest 
devices you can find. Whenever I travel to Silicon Valley, I make a trip to 
Halted Specialties (HSC) and stock up on the ones they carry, which are tan 
colored and 0.6 inch diameter (1.5 cm). Pricing is in range of 2- or 
3-for-a-dollar. I see the same type at some hamfests. Many distributors 
carry HV discs that are significantly smaller in size and, presumably, in 
current handling.

My present vertical uses approx. 1200 pF, made up with (6) 180 pF discs and 
(2) 56 pF. Past Inv-Ls were matched with approx. 2200 pF shunt caps , using 
(8) to (10) 240 pF or 270 pF discs. I found that fewer devices of larger 
value would overheat.

After observing the need for many in parallel, I avoid values higher than 
250 pF, and have plenty of small values for fine tuning. If you already have 
larger values, use them in series/parallel to increase current handling.

73, Gary
K9AY



> There's been a thread on here about using silvered mica capacitors for 
> antenna matching networks. I have had no success at all using these as 
> they don't appear to handle any significant amount of current and usually 
> disappear in a puff of smoke. For several years I have used 'L' matching 
> networks for my 160M inverted 'L' which use high voltage disc ceramics 
> sold by Farnell (Newark in US). These are rated at 6.3KV and have survived 
> decent amounts of RF power without getting hot. Farnell/Newark part 
> numbers are 952-7311 (220pF), 952-7303 (100pF), 952-7290 (47pF) and 
> 952-7281 (22pF). Most capacitance values required can be made up by 
> series/parallel arrangements of these.
>
> Another issue regarding use of 'L' matching networks - it's better to use 
> a low-pass arrangement (parallel capacitor to ground) from a harmonic 
> rejection viewpoint rather than the high-pass configuration (inductor to 
> ground), BUT the high pass offers better lightning protection.
>
> 73
> Tom G3OLB