[600MRG] Vacuum variable capacitor, low-pass/band-pass filters

[patrick hamel]

I used oil-filled breadslicer capacitors in my experimental service (part -5) 600 meter amplifier. 
I made the enclosures. The writeup is in QST, I don't remember the issue.
They worked until the ham band was approved and I moved on to something else.
The amp is available if anyone would be interested. See QST mar 2008. It is four 811s in class B push-pull. It provides a clean output signal. 
It runs a KW DC input for 300 watts RF. It ran for years 8 to 10 hours steady beaconing and CW QSOs a night.
The key to the operation of the oil-filled variable is to get a seal for the shaft and provide a pressure-equalizer that won't let in moisture. 
The finger from a glove worked fine for me.
There is a world of good information from real experimenters at www.500kc.com especially the technical downloads. (see also the burning PVC insulators on a link).
73
Pat W5THT & ex WD2XSH/6

----- Original Message -----
From: lstoskopf at cox.net
To: "Joe" <nss at mwt.net>, "Frank O'Donnell" <600 at inkbox.net>, "600MRG" <600mrg at mailman.qth.net>
Sent: Friday, June 21, 2019 3:46:48 PM
Subject: Re: [600MRG] Vacuum variable capacitor, low-pass/band-pass filters

A totally uninformed answer...   I have several small, cheap caps that have a floppy dielectric film floating between the plates.  Not exactly precision caps, but small.  I wonder if the effect of air-plastic-air is similar to a small cap-large cap-small cap in series unit so not worth the effort or stability.  If it works you would think that some of the guys selling large loop antennas would add some plastic to their heavy variable caps.  Looking for the guys that know!
N0UU 
> But why can you not take a regular large bread slicer variable, and fill 
> in the air gaps with a dialectic material. Gets two advantages
> 
> the arc over voltage point can be raised considerably!
> 
> and the amount of capacitance can be raised many many fold depending on 
> the dialectic. some insulating plastics can be like 15 times!
> 
> Why is this not done ever that I have seen?
> 
> Joe WB9SBD
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