Hi Jim,
Whilst polypropylene dielectric capacitors can be put to good use in RF
bypassing circuits, the one point you overlooked is their capacitance shift
with temperature.
Whilst I have not seen the CD-942P data, most polypropylene capacitors have
around 0.5% variation at ambient temperature with an increasing variation as
they begin to heat up.
This means that in a critical mode as Jim was attempting, where he was
wanting to establish a resonance, he would be constantly retuning. NPO
Ceramic capacitors are they way to go in his application, unless he wants to
use air or vacuum variables. In the days when large block mica capacitors
were readily available (as in WWII tuners and transmitters) that would have
been the best alternative.
That's why every ham swap meet I go to, I always scan the tables for
transmitting type ceramic caps and block micas and bargain!
Cheers
Peter VK3QI
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "David Gilbert" <xdavid@cis-broadband.com>
Cc: <towertalk@contesting.com>
Sent: Friday, November 30, 2007 3:09 PM
Subject: Re: [TowerTalk] Capacitors To Tune 160 Vertical
>
>
> -----Original Message-----
>>From: David Gilbert <xdavid@cis-broadband.com>
>>Sent: Nov 29, 2007 11:15 AM
>>To: Bill Turner <dezrat@copper.net>
>>Cc: towertalk@contesting.com
>>Subject: Re: [TowerTalk] Capacitors To Tune 160 Vertical
>>
>>
>>I don't understand that comment. Murphy's Law has nothing to do with it
>>... Ohms Law does. As N6RK said, where high voltage DC is present,
>>capacitors in series are typically bridged by high value parallel
>>resistors to make the voltage divider dependent upon the resistors
>>instead of the much higher (and indeterminate) DC resistance of the
>>dielectric. For RF, the impedance of the capacitors themselves provides
>>that same shunting action as far as the bulk resistance of the
>>dielectric is concerned.
> <snip>
>>
>>All that being said, not many inexpensive RF capacitors handle several
>>amps very well, so for high power applications we probably end up with
>>some configuration that includes both series and parallel devices ...
>>and I'd probably shoot for more margin on the parallel portion than on
>>the series portion.
>>
> Back when I was building high power tesla coils and Marx banks, I
> discovered that capacitor prices (and sizes) tend to be proportional to
> energy storage. Say you needed 1 uF at 10kV.. You could parallel 10 0.1
> uF caps rated at 10kV or series 10 10 uF/1kV caps to get there, and the
> physical size and cost would be about the same (within limits, and
> assuming the same basic capacitor technology).
>
> In fact, high power pulse capacitors from,e.g. GA/Maxwell, are made of
> subunits hooked in series parallel to get the required ratings. The
> voltage rating tends to be determined by the dielectric/electrode system
> and there's a certain sweet spot for thickness vs capacitance vs breakdown
> (thinner dielectrics have higher breakdown fields in a Volts/mil sense).
>
> These days, the best all around performance for largish values (tenths of
> uF) at medium voltages (1-2kV) is with extended foil polypropylene
> dielectric (e.g. CD 942P series for example).. they're very low ESR
> intended for things like high frequency switching supplies. Probably
> pretty good up to MHz-ish (I haven't measured one at HF... I'll have to
> get around to it), but their outstanding characteristic is the ability to
> handle high rms AC currents (amps) without melting or blowing sections of
> the foil off.
>
> There's a similar PP dielectric system where the electrodes are metallized
> onto the film as a series of plates. These have a little "fuse" (actually,
> a very thin trace) between the plate and the bus along the edge, so if a
> fault develops, the fuse blows, and the capacitor keeps on working, with a
> slight reduction in C.
>
>
> In more traditional RF caps, you're looking at things like doorknobs which
> are barium or strontium titanate ceramic from companies like TDK or
> Vishay/Ceramite .These too have DC and RF versions that look a lot alike,
> but are VERY different internally. The RF ones are a lot less dense (pF
> per volume, pF per dollar, etc.). The internal differences (aside from
> the intrinsic loss differences in the dielectric) are much like those for
> film capacitors with the DC ones not being able to tolerate as much RF
> current and having higher losses.
>
> However, even better these days are the stacked caps from companies like
> ATC. Surface mount style with tabs, not wires, very low loss, etc.
> Pricey, but work really well, and if you only need a few (as for a DC
> block in a kilowatt amp) they might be worth it. As with others, these
> seem to be sized and priced by volume, which in turn corresponds to stored
> energy (double the voltage rating is 4 times the size, etc.)
>
> Jim, W6RMK
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