I would stay away from those old WW2 caps. I have almost the
same kind of antenna. 95 ft vertical, 4 top loading wires,
resonant at approx 1725 kHz. Series cap is a 500 pF fixed
vac cap paralleled with a very large "bread slicer" cap being
approx 300 pF. I am using approx 250 radials. With this
SWR is approx 1.5-1, I would need some additional L, however
I feed antenna with 7/8 inch hardline so I don´t worry too
much about it. My only problem is I´m not active enough.
This 95ft vertical used to be base loaded with a tuning
box. I have not made any measurements but my gut feeling
is that this top loaded version is much better, i e I do
feel "louder".
73 Jim SM2EKM
-------------------
Jim Brown wrote:
> On Tue, 17 Feb 2009 17:33:41 -0800 (PST), Rick Karlquist wrote:
>
>> 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.
>
> Right. The most important specification is the equivalent series
> resistance -- this is a fairly high current point, so the capacitor
> will be carrying a lot of RF current. A lossy capacitor will let the
> smoke out very quickly when you put some power into it. :)
>
> I don't have any of those old WWII caps, but there's an electronics
> surplus house in my area that stocks a bunch of good RF capacitors,
> and sells them cheap, so I'm using a series/parallel combination of
> those capacitors. Most of the time I'm running 1.2kW on 160, and
> during a contest I'm transmitting a lot. :) Once I found low loss
> caps, they run cool. It's pretty easy to do that -- you put the cap(s)
> in series, transmit for a while, and then feel them to see of they're
> hot (or if they're still there). :)
>
> Rick and I are nearly neighbors and compete with each other in
> contests. I can vouch for his antennas -- he often does nearly as well
> with 100 watts as I do with a kW!
>
> 73,
>
> Jim Brown K9YC
>
>
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