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Re: [Amps] load capacitor ratings?

To: "R. Measures" <r@somis.org>,"amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] load capacitor ratings?
From: Will Matney <craxd1@ezwv.com>
Date: Tue, 31 Aug 2004 01:52:19 -0400
List-post: <mailto:amps@contesting.com>
Rich,
This insulation material that they said was nylon, looked as if they dipped the stator assembly in it as a liquid and let it dry. It still gave enough clearance for the rotor plates to move freely. They guaranteed it to be rated for X2 the rated air voltage. I never tried one so can't say how they worked. I couldn't see paying that kind of price for a small unti when going up one or two sizes in an air variable was all that was needed and was almost the same price as the smaller one.


I wasn't aware of the problems they had with the roller inductors as I never did try one of theirs to use it. I knew they made them but had never heard about any problems. Of course this is something a manufacturer is going to announce either.

Will Matney

R. Measures wrote:


On Aug 30, 2004, at 5:37 PM, Will Matney wrote:


Rich,
Exactly. I talked to the owner of Oren Elliot (Orens son Stephen) several years ago about this and what he thought the rating should be. He had several thoughts on this but told me that most were using a X2 rating the same as the coupling capacitor for tune C.


- A x2 Vdc Tune-C (6kV) is what the TL-922 uses and the result is frequent loss of the bandswitch - which has a 5kV withstanding ability. It is better to have a Tune-C flashover than a bandswitch flashover.

He recommended using a load C with a voltage rating some higher than the peak voltage rating across the load, generally having a 50 ohm load as standard. I think he figured by a sometimes varying load, it could take more voltage to create the same power across a mis-matched load. Of course this could sure be the case and is something I would not have thought of at the time. Not all antennas are a perfect 50 ohms or close to it. His capacitors are tested in open air with a hipot tester. They had a jig made up with a nut driver handle made onto a coupling to turn the rotor with. They applied the hipot and turned the rotor through its rotation to catch any arcing. My concern was the physical size of air variable capacitors and what was the least size I could safely get by with. He did mention that one was available where the plates had a "nylon dielectric" to insulate the stator plates raising a .016" air gap to 1100 V (X2).


Will --
- Nylon is a good DC insulator, but it has the worst D (dissipation)-factor I know of. Another lossy RF dielectric is Delrin -- which is what Elliot used in their roller-coils a few years back. MFJ used one in their "3kW" tuner. When this tuner was tested in the ARRL Lab, it caught fire with 900w of AØ/NØN. An easy fix is to make the roller-coil form out of G-10, G-11, or ABS.


cheers
...


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