I want to make a couple of L networks to match a 20m delta loop to 50
ohm coax on 20m and 17m; 100W max. I know I can use a transformer on
20m, but I figure I'll save my ferrites if I'm building L networks
anyway, and to be economical of junkbox parts, I'd also like to save my
air variables.

Would capacitors made of PC board be decent transmitting caps at these
frequencies, provided they didn't break down? The board is hamfest
surplus special... so I don't know what it is, but it may be G10...
it's a dark green fiberglass material, about 2mm thick dielectric.
Comparing the capacitance of a 2 inch square of the material (88pF) to
a parallel plate cap with air dielectric of the same physical
dimensions (12pF) gives a dielectric constant of about 7.3.

I'm going to try it anyway, but I'd appreciate any input anyone has
about the suitability of unknown PC board as HF transmitting capacitor
material, primarily with respect to dielectric losses.

If it matters, the 20m network is a 1.14uH series inductor with an 88pF
capacitor shunting the antenna side. The 17m network is a 3.08uH
series inductor with a 29pF shunt. The 20m antenna impedance is about
250+j0 and the 17m impedance is about 950+j1200. I figure there might
be some voltage breakdown issues, but worst case, I smooth the edges of
2 or 3 2x27pF or 3x27pF caps and put them in series.

Thanks and 73,
Dan