[AMPS] RF Choke Forms

[Larry Molitor]

At 11:13 PM 1/14/00 -0700, Arne Gjerning wrote: 
>
> I have been advised NOT to use Teflon, Delryn, and a few other insulating
> materials for an RF choke. Instead only use ceramic. It was then stated the
> "D" factor was the reason. 
>
> OK, but why as I could not find info on this subject in the ARRL Handbook.
> Any help to point me in the direction of knowledge on this subject would be
> appreciated.  May as well learn something while building my amp. And I want
> to do it RIGHT. 
>
> 73 de Arne N7KA       


Arne,

I have successfully used Teflon and G10 epoxy forms for Plate chokes on HF at
1500 Watt power levels. Delrin and nylon are no-no's. There are other plastics
that are suitable, but probably not easily obtainable. For less critical
applications, don't discount common PVC. I have used successfully schedule 20,
40 and 80 PVC pipe, CPVC pipe, and Schedule 20 and 40 PVC electrical conduit
(sunlight resistant). These materials are not suitable for HV chokes in the
anode compartment because they would probably melt (if not actually explode
into flames due to the HV RF). On the other hand, they make dandy forms for
mobile antenna loading coils. I have even used them for 160 meter top loaded
verticals running 1200-1300 watts. 

By the way, I have never been able to get common PVC to warm up or get soft in
the microwave oven. This tracks with my successful use as a loading coil form.
The voltage gradient on a mobile antenna coil has got to be even worse that
that found in an amp anode circuit as evidenced by the amount of corona on the
top of the antenna. So, my question is: If PVC doesn't heat up in the
microwave, and doesn't burst into flame when used as a loading coil, why is it
so problematic when used as an anode choke?  Rich, Ian, anybody?

73,

Larry - W7IUV



--
FAQ on WWW:               http://www.contesting.com/ampsfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm