Jim,
I would be surprised if your 10 - 100nF 400VDC capacitors are capable of
taking 6 amps at 1.8 Mhz, without excessive heating and drifting.
Therefore your only option is to parallel the 100 to 500pF capacitors. If,
for some reason the voltage rating is insufficient on some of these
capacitors, you may have to consider a number of series combinations as
well to make up the voltage rating.
If you are using ceramic doorknob capacitors, make sure they are of the NPO
type.
Have you considered using a 70 - 100 foot coil of RG213 or RG17 as the
capacitance? Both are around 30pF per foot and have voltage ratings of 5000
and 11000 RMS respectively. Sounds like overkill, but it becomes dead easy
to permanently adjust the capacitor value with a pair of sidecutters!
I often use this trick with smaller pieces of coax as part of the parallel
combination, but you must make sure that the coil of coax is kept well
insulated from any metal parts, or there can be breakthrough the outer
sheath.
Cheers
Peter VK3QI
----- Original Message -----
From: "Jim Brown K9YC" <k9yc@audiosystemsgroup.com>
To: "TopBand List" <topband@contesting.com>; "Tower Talk List"
<towertalk@contesting.com>; "NCCC" <nccc@contesting.com>
Sent: Wednesday, November 28, 2007 6:41 AM
Subject: [TowerTalk] Capacitors To Tune 160 Vertical
> Crew,
>
> I'm thinking of following N6LF's advice on a rebuild of my 160
> vertical by tuning it low and adding series capacitance to tune out
> the resistance to match it to 50 ohms. Legal power will put about
> 220VAC across 2.2 nF, at a little less than 6A. I think my greatest
> concern is dissipation from the ESR of the cap. Once I get this
> working, I'll probably do something similar for 80M.
>
> My question is about what capacitors will work. I'm looking for
> about 2.2 nF (2,200 pF), and I'll run legal power with the fairly
> high duty cycle of a contest. I've got some 3kV and 6kV caps in the
> 100-500 pF range, and some 400VDC caps in the 10-100 nF range. I
> could series the LV caps or parallel the HV caps. Does anyone have
> experience or an educated guess?
>
> 73,
>
> Jim K9YC
>
>
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