Date: Fri, 28 Oct 2016 15:37:58 -0400
From: N1BUG <firstname.lastname@example.org>
To: 'Amps' <Amps@contesting.com>
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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