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Re: [Amps] RF voltage on VHF strip line

Subject: Re: [Amps] RF voltage on VHF strip line
From: Steve <>
Date: Tue, 1 Nov 2016 11:06:26 +0000
List-post: <">>
What effects/damage are you seeing? Does it always happen in the same place?

Can you run the amp with reduced plate voltage? That might show up whether it's a marginal effect where rf on top of dc is just enough to cause breakdown or if it's something purely related to the rf.

A couple of thoughts to add to John's comments:

With a few kV of dc+rf, you need air leakage paths >0.2"/5mm to avoid breakdown. If there's anywhere in the assembly where the teflon doesn't overlap the brass far enough it might be a weak point.

Teflon is very soft and tiny bits of swarf can become embedded in it compromising the insulation.

Air gaps can be a weak point too. For example, consider if the teflon is .01"/.25mm thick and the brass is slightly out of flat so there's a .002"/.05mm air gap somewhere when it's clamped together. In that area the air and teflon are like two capacitors in series and the voltage divides between them inversely with capacitance. In this example, roughly 30% of the (rf + dc) voltage appears across the air part which is likely to cause breakdown and lead to damage to the teflon.


Suppose you have a 4CX1500B sitting in the middle of a half wave
plate strip line for 144 MHz. Some may prefer to think of it has two
quarter wave lines with the tube between them. This is a "teflon
sandwich" line where the upper brass plate is connected to the tube
anode, the lower is not. The lower brass plates are grounded at the
far end from the tube. Between the upper and lower plates we have a
thin sheet of teflon that forms the dielectric for the DC blocking

Can someone help me understand whether (and if so why) there would be
significant RF voltage across the blocking cap? Obviously RF voltage
is going to vary along the *length* of the line, being high at the
tube and near zero as we get toward the grounded end. But what about
voltage across the teflon sheet between the upper and lower plate *at
any given point along the line*?

I am trying to understand WHY this thing would want to arc or break
down only when RF is present. My mind wants to believe that the RF
voltage across the capacitor at the point where the teflon shoulder
washers insulate the screws holding it together would not be
significant, and that the voltage would be just B+ regardless of
whether RF is present or not. However I see evidence this is not the
case and want to know what it is that I don't understand.

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