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[Amps] RF voltage on VHF strip line

Subject: [Amps] RF voltage on VHF strip line
From: John Lyles <>
Date: Fri, 28 Oct 2016 18:43:53 -0600
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>Message: 5
Date: Fri, 28 Oct 2016 15:37:58 -0400
From: N1BUG <>
To: 'Amps' <>
Subject: [Amps] RF voltage on VHF strip line
>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 capacitor.

>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.

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. 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.

A good design might have a few hundred volts of peak RF voltage superimposed across the blocker with a significant DC voltage present. If the blocker has some sharp edges then the RF voltage can easily initiate partial breakdown, i.e., corona effects around the edges. The most intense electric fields are near the point where the dielectric and the electrode (metal plate) are exposed with air at the edges, the so-called triple point. This is where capacitors will usually fail with RF voltage first. And the shoulder washers of a Teflon blocker are places for E field enhancement due to geometry.

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.

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