In this particular example,
The cavity outside is square. The outside forms a square box.
The cavity inside is round.
On some other cavities the outside is round.
All cavities have the power-generator in the center of the cavity, and it is usually cylindrical.
The power generator is a tube. You can see the underside of the cooling fins in the area of the red
capon. The cooling fins are also the plate, and are at many thousands of volts positive. The plate of the tube
is insulated from the rest of the tube by ceramic.
New Tube sitting in a cavity.
The cavity top is formed by the top of the tube: the cooling fins.
The top of the cavity is connected to the walls by way of a four sided capacitor. The capacitor is made of several
layers of the red milar capon which is an insulator.
Although the milar material is rated at 20kv, a small hole can be "blown-through" by excessive
voltage, if the cavity is mistuned, by having loading too light, or adjusting tuning controls while under full power.
The walls extend down untill the walls intersect the "honeycomb" floor. The floor touches the walls with
"finger-stock", (spring loaded flexible fingers). The finger-stock makes contact with
the walls, and also the inside cylinder.
A large cylinder is in the center of the cavity that the tube rests in. The cavity is really a big donut: the center
section reserved for the power generation.
Below the honeycomb floor, the inside tube cylinder continues down; containing the rest of the tube and tube
connections and electronics. Connections for tube filaments, grid, and screen, come up into this center assembly.
Which are not visible in this cavity.
Plate tuning adjustments are made by a motor turning the threaded plastic screws. The honeycomb floor moves up
and down, and causes dimensions to change in the cavity. The dimensions of the top and floor do not change.
But the distance between the top and floor - do change - to properly tune the cavity. Electric fields radiate radially
outward at the top and bottom, but 180 out of phase. Magnetic fields rotate clockwise and anticlockwise simultaneously:
one field near the top, and one near the bottom.
Lower portion of the inside cylinder.
The floor of the cavity is constructed as a honeycomb to allow forced air inside the cavity, and
in particular, for cooling of the tube fins. Air is also forced inside the center cylinder (housing the tube)
for cooling of tube connections.
The contact finger-stock is made of silver, as well as the inside of the cavity walls. You should never
move the tuning unnessesarily. Keep adjustments deliberate, and to a minimum. Keep all - but finial adjustments -
under low power. If, while adjusting, a finger looses some contact due to dust or heat, the finger can instantly burn
away. The resulting finger loss will result in increased current demands on remaining finger stock. The effect will
cascade; and you may suffer intire sections of finger-stock destroyed. Instant loss; highlighted by smoke and popping breakers.
Also, tuning parts must be kept tight. If any thing vibrates, unwanted FM noise in the signal may result. If any thing moves;
parts of the cavity may overheat, SWR sensors may shut down the transmitter, or output power may simply go away.
Seen in the picture is an "s" shaped Coax cable. The coax cable brings in over a thousand watts of input-drive.
The drive is applied to the Input Tuning control, Input Coupling, and Input Loading Controls. The Input Tuning and
the Input Loading are the white ceramic variable caps. There is a metal paddle between the capacitors forming the
The rod inside the center assembly is one of the filament connections. It has 180 amps of current.