[AMPS] RF Choke Forms

[measures]

>
>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?
>
As I understand it, Larry, some varieties of PVC are lossier than others. 
 The best test I  found was to hang a sample of the plastic  about 6 
inches from the end of my 40m dipole.  Sending ''dits'' with the 
''Plywood Box'' amplifier, common pvc would ignite in seconds.  .  Oddly 
enough, Less RF potential existed at the end of the antenna wire. 

cheers

-  Rich..., 805.386.3734, www.vcnet.com/measures.  


--
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