...commercial television translator modifications for 1296 MHz EME

In order to obtain high power at 1296 MHz I chose to modify the final amplifier stage of a surplus UHF commercial television translator that was originally designed to operate at 1 KW continuous power output in the 470-860 MHz broadcast band (Television Technology Corporation, Model XL-1000U). The final amplifier stage in the translator uses a Thomson TH-347 tetrode tube in a grounded-cathode coaxial cavity.

Photo above: Commercial television translator before modifications.

Photo above: Unmodified cavity amplifier components and TH-347 tetrode tube.

In order to modify the cavity amplifier to operate at 1296 MHz I decided to lengthen the output cavity to enable it to resonate in a 3/4 wavelength mode. As it happens, the original long-tube input cavity section is adjustable over a very large frequency range and can be adjusted to resonate at 1296 MHz without modification, so I elected not to make any modifications to the input section at all. I elected to remove completely the secondary shaping cavity of the amplifier and discard it. New output coupling and tuning probes had to be constructed as the original output was through the discarded shaping cavity. I elected to use two capacitive tuning probes for greater tuning versatility and range. The main coaxial cavity resonator that contains the TH-347 tetrode tube needed to be extended a distance of approximately 2-5/8" in order to resonate at 1296 MHz in a 3/4 wavelength mode. The precise extension distance is not especially critical as the original cavity shorting-slider method of adjusting the effective cavity length over a limited range is retained.

To extend the length of the cavity I fabricated a new outer wall for the cavity from a 4-1/2" diameter x 6-1/2" long x 1/8" wall seamless brass tube and fabricated an extension tube for the inner cylinder from a 2-3/4" diameter x 2-1/2" long x 1/8" wall seamless brass tube. On a metal lathe I turned down the OD of the 2-3/4" OD inner tube approximately 0.060" to approximately match the OD of the existing inner tube that was being extended. The inner extension tube is soldered onto ends of the original inner tube of the cavity. The original outer wall assembly was cut off from its mounting ring and the new longer outer wall tube was soldered onto the original mounting ring. As the ID of the new outer wall tube is slightly larger than the original cavity ID it was necessary to fabricate from a 1/2" thick brass plate a new, larger-OD, slider disk with finger stock grooves in order to achieve a proper finger stock fit to the outer wall. A silver chloride paste (Cool-Amp Conducto-Lube Co.) was rubbed onto all interior surfaces of the cavity, all surfaces of the new slider disk, and surfaces of the output coupling probe and capacitive tuning probes to form a thin silver coating to enhance surface conductivity.

Photo above: Modified and newly fabricated translator cavity components are shown, including the extended inner tube assembly with its new slider ring installed, new finger stock for the outer groove of the slider ring, new single piece outer wall for the cavity with probe ports installed, two capacitive tuning probes, output coupling probe with its 7/8" EIA assembly, and the cavity top plate that contains the plate ring with its finger stock and the teflon (PTFE) plate bypass dielectric ring. Interior surfaces of the cavity components and the probes were coated with a thin layer (approximately 0.0001" thick layer) of silver to ehance surface conductivity. The diameters of the capacitive disks for the output probe and the primary tuning probe are 1". The auxiliary (smaller) second tuning probe diameter is 1/2"; a larger end knob was used for easier manual adjustment.

Photo above: Capacitive output coupling and tuning probes are shown installed. Note that the output probe and the primary tuning probe must be in place before installing the inner tube assembly since the 1" diameter capacitive disks are larger than are the pass-through holes in the cavity walls and therefore cannot be installed after the inner tube assembly has been installed.

Photo above: Modified cavity and the output coupling and tuning probes are shown assembled. The coaxial output probe assembly and its transition to the 7/8" EIA flange maintains appropriate inner conductor O.D. and outer conductor I.D. dimensions along the entire assembly to maintain a 50-ohm characteristic impedance.

Photo above: Assembled modified cavity showing the TH-347 tube socket, fingerstock connections, and the HV plate bypass disk assembly.

Photo above: Modified/extended cavity amplifier installed in the translator rack.

The modified television translator amplifier currently runs at 1500 watts output with 300 watts rf input drive. Modifications to the original power supplies included making the HV supply variable by installing a Variac control on the input windings of the HV transformer and modifying the screen and grid bias supplies to achieve higher voltages than originally designed. TH-347 tube operating parameters (presently) are: V(plate) = 3KV idle, 2600V key down; I(plate) = 0.25 A idle, 1.8 A key down; V(grid) = -90 VDC regulated, I(grid) = 50 mA key down; V(screen) = +600VDC regulated, I(screen) = 15 mA key down. Under these operating conditions the cavity does not get warm even during extended operation, suggesting that the resistive losses in the cavity are not excessive. (entered November 2, 2007)