I built the "Field Day Special" (by W7EL) 20m antenna which uses 330pf
500vdc silver mica capacitors in each lead at the feedpoint to match
50ohm coax. It seems to work fine, so now I'd like to upgrade the power
handling to full limit. How do I size silver micas for the voltage,
current and power dissipation for full power? I've got the 500vdc rated
CDE CM/CD series in most values at 500v rating (new 1kv ones are
pricey) so some sort of series/parallel arrangement is what I expect is
needed.
20m and 40m versions are planned.
From various CDE pdf's it seems the ESR for <1000pf is around 0.005
ohms at 14MHz, and DF of 0.009 at 1MHz. Another pub has the max current
at 5MHz 85 deg C for 330pf as 1.5a and for 680pf as 2.3a which by my
understanding (not sure about it) is much less than pure ESR losses i.e.
(5a^2 * 0.005ohms = .125 watts). I would guess that a silver mica could
dissipate 1/8 watt without distress, but don't understand what might
limit current capacity per the CDE data. I probably need to series caps
for 1000vdc to provide some SWR margin at full power.
Insight and advice appreciated.
Grant KZ1W
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|