Colin -- The RF current carrying ability of mica dielectric caps
typically decreases as freq increases. Ceramic-dielectric caps are
typically able to carry more current as freq increases.
On Dec 4, 2004, at 5:44 AM, k7fm wrote:
Wishing to complicate the discussions of the current capacity of
transmitting capacitors vs. frequency, I am looking at the Cornell
Dubilier
advertisement for Type 86 Transmitting Capacitors. This ad appears in
the
1938 Radio Handbook. Most of you will know what the type 86 is, as it
is
the large WW II mica cap with screw terminals on the top.
What complicates things is that they have a chart showing the
frequency vs.
max. current in amps. Each value has a separate chart, indicating
life is
not so simple. For example, Cat No. 31A-86 has a capacity of .0001 at
12.5
kv. At 30 MHz, it is rated at 5 A, while at 1.8 MHz it is rated at 2
A.
Contrast that with Cat No. 21A-86 which has a capacity of .001 at 12.5
kv.
At 30 MHz, it is rated at 5 amps, while at 1.8 MHz it is rated at 12
amps.
Even more peculiar is that the 35A-86 .0005 cap is rated as follows:
30 MHz 5 amps
15 MHz 8 amps
7,5 MHz 9 amps
4 MHz 8 amps
1.8 MHz 7 amps
I suspect the unique problem is that each value is constucted quite
differently and the internal lead length causes the variation.
Nevertheless, it blows the simple formulas away. And, some of us
still like
to use the WW II micas since they like as old as our 304-TL finals.
K7FM, Colin
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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