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Re: [Amps] RF voltage on VHF strip line

Subject: Re: [Amps] RF voltage on VHF strip line
From: N1BUG <>
Date: Tue, 1 Nov 2016 18:16:33 -0400
List-post: <">>
On 10/28/2016 08:43 PM, John Lyles wrote:
With respect to earth, the RF voltage swing is largest at the center of
the tube. But across the dielectric of the blocking cap, the RF voltage
should be low. The capacitive reactance is Xc=1/(6.28xFxC) and if the
blocker is built right, the C is large enough that Xc is very low, few
ohms or less of -j.

That's what I was thinking. In this case, Xc is about 5 ohms.

RF current through the capacitor should also be
reasonable there, being a current node (min) near the anode. Reactive
voltage across the cap would then be I x Xc.

This is where I get confused. I would easily understand this if the capacitor is of relatively small physical size and connected between the tube and the tank circuit. In this case the capacitor plates are physically large with one edge of each plate at the tube and the other edge almost at the cold end of the plate line. With respect to earth, RF voltage must be high at one edge of a capacitor plate while being relatively low at the opposite edge of the same plate... I think. Yet *between* the plates at any given point, I think there should be little RF voltage differential. Is this making any sense? I'm having trouble getting my mind around this.

A good design might have a few hundred volts of peak RF voltage
superimposed across the blocker with a significant DC voltage present.

The point of confusion above notwithstanding, that's what I thought. Maybe the extra few hundred volts is enough to initiate breakdown.

If the blocker has some sharp edges then the RF voltage can easily
initiate partial breakdown, i.e., corona effects around the edges.

Edges have been smoothed and slightly rounded (to the extent reasonably possible when working with .065 inch thick material).

And the shoulder washers of a Teflon blocker are
places for E field enhancement due to geometry.

That is interesting.

Another consideration is the harmonic voltages, if they are getting
enhanced by some 'feature' of the layout. There could be higher voltage
at one of the harmonics due to the standing wave structure across the
resonator. At higher frequencies, the blocker may not exhibit a purely
capacitive reactance, and could even have some parasitic resonance
associated. At this point, it could have an RF voltage superimposed on
the DC. I haven't measured such a capacitor myself with a network
analyzer or UHF Z meter so am just postulating.

Also interesting.

Thanks for the reply.

Paul N1BUG
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