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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