Mark Robinson wrote:
> ----- Original Message -----
> From: "Rick Karlquist" <richard@karlquist.com>
> To: "Mark Robinson" <markrob@mindspring.com>
> Cc: <richard@karlquist.com>; "Jim Brown" <jim@audiosystemsgroup.com>; "Tower
> Talk List" <towertalk@contesting.com>
> Sent: Tuesday, 17 February, 2009 7:33 PM
> Subject: Re: [TowerTalk] 43' vertical mod for true resonance on 80M
>
>
>> Mark Robinson wrote:
>>> I am planning something like this. What sort of capacitor do you need to
>>> use
>>> as the series feeding element.......a vacuum capacitor?
>>>
>>>
>>> Mark N1UK
>> I use WWII surplus "transmitting mica" caps. If you want to make
>> the antenna tunable, you can switch in various fixed caps. A vacuum
>> variable typically doesn't have enough capacitance, and in any
>> event, you don't need the high voltage rating.
>>
>> Rick
>
> So maybe a polyester film capacitor? I imagine that the current is major
> issue in the selection of the capacitor here
>
In particular, you probably want a polypropylene dielectric (lower loss
than polyester) with extended foil like the tesla coilers and high power
PWM switcher designers use. Low ESR, Low ESL, good current handling
capability (amps), good voltage (kilovolts). However, finding one small
enough in capacitance might be challenging.
Cornell Dubilier 942 series is one to look at. Run about $2-3 each for
.15 uF @ 2kV.
Jim, w6rmk
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