Gary,
Without getting too deep into cavity theory, the rf is generated between
the plate and grid in that tube. The rf flows on the cavity inner surfaces
only. The rf will not pass through any of the metal parts.
Following this reasoning, any rf present at the plate flows from the tube
contact ring into the plate bypass fingerstock and out and on to the inner
surface of the bypass plate. At this point, the rf sees a very low impedance
and exits the bypass plate and on to the inner surface of the cavity upper
wall. The rf continues on to the outer wall of the cavity and sees a short
circuit there. The short circuit causes a phase shift of 180 degrees and
sends the rf back toward the tube in reverse order from above.
When every thing is in resonance, the returning energy is in phase at the
tube plate connection.
The same action occurs on the lower side of the cavity from the grid to the
cavity wall except there is no bypass capacitor plate in the rf flow path.
There is heavy coupling between the upper and lower cavity walls as they
are very close in terms of wavelengths.
The concept of ground essentially disappears inside the cavity. There is rf
everywhere inside and ground is meaningless. Also, the action performs a
balun function perfectly so that the output coax, which does have a ground
reference, can couple to the rf inside the cavity.
There is a standing wave inside the cavity due to the short at the outer
wall this is nearly infinite. The loss in the cavity is the limiting factor
in this vswr. The inherent high Q of cavities is related to this high vswr.
Pretty cool huh?
73,
Gerald K5GW
In a message dated 11/20/2009 9:48:31 A.M. Central Standard Time,
gpatterson53@hotmail.com writes:
I have a n6ca 2c39 1296 cavity amp and have trouble understanding how it
works. The 2c39 plate has a large teflon sheet capacitor to ground. I dont
understand how any rf gets into the cavity when the plate is dead shorted
with this capacitor. any help?
gary
w4af
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