[BC] Eimac Cavities

[RichardBJohnson at comcast.net]

Sight picture: Move a current-probe along the length of a 1/4 wave radio tower. The current is highest at its bottom where its voltage is the lowest. It is the lowest at its top, where the voltage is highest. The reason is that a resonant structure is not a single component where the input current equals the output. Instead, it is a complex R/L/C network with different currents and voltages at different points along its length.

In the case of the plate-blocker, at the tube end of the resonant line, the current is close to the displacement current within the tube from the plate to its screen, which is the RF common "ground." If the plate-blocker is at the cavity box end, the Q of the resonant structure increases it substantially and its reactance would need to be near zero to have no RF voltage across it. With the box we modified, the current at the "shorted end" of that resonant line was estimated to be about 200 amperes when running 70 kW. There was nothing that would take that current except metal so we shorted that end and put the blocker where it should have been in the first place.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "John Lyles" <jtml at losalamos.com>

Question: 
How can a plate blocker dielectric be failing from RF current. Dielectric heating would occur in an intense E field but not a magnetic field. <= =>If the capacitor were not made with enough C, so that it had appreciable reactance at the fundamental frequency, 
then I could imagine a voltage appearing across it. Is this what you mean? This would imply that it was underdesigned however. 

John Lyles