[TowerTalk] 6m halo capacitor

[Mike Fahmie]

How much effect that the paper has on the capacitance would depend on how much of the space between the electrodes was air and how much was paper.  Paper has a dielectric constant of about 2 but if impregnated with oil it could be more than twice that amount.  Assuming no air space, then the capacitance would be directly proportional to the dielectric constant of the paper but if the air space is significant then the effective constant would be much lower and less dependent on the paper.
Knowing the dimensions of the cup structure, you can calculate the capacitance range with all air, then comparing that to the desired 70pf result you can decide on what dielectric can be used.
If you can build it with air dielectric, then the maximum power is dictated by the flashover voltage of the capacitor (which will vary with weather).  Adding a dielectric may not improve power handling since more stress will be applied across any airgap.
-Mike-WA6ZTY

      From: Jim Hoge <knowkode at verizon.net>
 To: Towertalk Reflector <towertalk at contesting.com> 
 Sent: Friday, December 4, 2015 9:22 AM
 Subject: [TowerTalk] 6m halo capacitor
   
I have an old 6m halo of unknown manufacture. (Think Rohn 40 vintage.) It is about 37" in diameter and made from 3/4" aluminum tubing. The mounting bracket is cast aluminum in a tee shape. The gamma rod is fixed at 11". The capacitor, mounted on the halo and connecting to the gamma rod, is made like a small aluminum cup with what appears to have been a paper or cardboard like material as a dielectric between the cup and the inner, movable electrode (adjusted via a leadscrew.) With an MFJ-269 and leads less than 2", the cap reads 70pF at 50.120mHz. 

Given the condition of the capacitor with the dielectric hanging out about half an inch, dried out, curling, and brittle, I am reluctant to use the antenna with any more than 10 watts at which it has not presented any problems. I would like to rebuild or replace the cap. If rebuilding the cap, what effect will a different dielectric material have on its value? I am guessing it would not be severe enough that the cap could not still be tuned back to a value of 70pF. Also, any ideas on the power handling capability of this unknown antenna?
Tnx es 73,Jim, W5QM

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