Nov. 16th 2004 Initial Visit to the Stone Mountain Site
WGTV channel 8 Transmitter Tour
(click on thumbnails for bigger images)
Robin Cutshaw,AA4RC, Mark Fehlig,WA6NGC, and Jack Watts(Chief Engineer soon to be ham??)
Channel 8 control room. 30KW solid state Harris amplifier on the left wall tucked away in a bunch of drawers.
Jack informing Moe,AE4JY that he can't use the channel 8 transmitter for an ATV repeater.
No its not the sewage treatment plant, but the power combiner where the aural and video signals are combined before going out the 8" hardline to the antenna.
A hot water heater of sorts. Its a dummy load and power "meter" that measures power by temperature changes in water heated by the RF.
Atlanta Weather Service 162.55 MHz 100W transmitter sharing space in the back room.
The "Ice Bridge" from which hangs all the feedlines coming from the transmitter building out to the base of the antenna tower.
The base of the tower and the grounding system.
Channel 8 antenna on top. The unused FM antenna is below it with all the mesh reflector assemblies.
Unused 90.1MHz FM antenna just waiting for something to drive it.
Intermod city. This is the maze of public service antennas and who knows what else stuck up under the roof.
Now on to the FM transmitter building
Entrance to abandoned WABE FM transmitter room. The blue cabinets just inside the door is the old Harris 20KW transmitter.
The first task was to turn on the lights and measure power coming back down the feedline to get an idea of what we were dealing with. We decided that the nice flat surface on top of the main 20KW power supply would serve as a workbench. The furnace looking thing to the right of the ham dummy is the old FM transmitter dummy load
Behind the FM transmitter cabinets is one of the antenna switches where we were allowed to tap into the 3" hardline going to the FM antenna. This used to switch between the dummy load and the antenna. Jack saved the day and supplied the nice N adapter to hook into the feedline.
Next a scope was connected and the voltages were measured coming out of the feedline terminated in a 50 Ohm load.
This shot is from the scope in FFT mode showing the relative strengths of all the signals coming back down the line. Amazingly, only about 7dBm (5mW) of RF was measured even though the antenna is sitting right below a 30KW transmitter antenna. Ironically the same amount of power we were planning on transmitting on 2m.
The SWR was measured by keying an HT on 2meters and 440MHz with an SWR bridge in line. Unfortunately, the RF coming back down the line made the MFJ analyzer useless in sweeping the antenna. The SWR was around 2.5:1 on 2 meters and about the same on 440.
The 5mW beacon was hooked up and a DCI 2meter bandpass filter inserted in line with the antenna feed to keep as much RF from getting back into the beacon and possibly mixing to create intermod products.
Technical details of the original beacon can be found here. http://www.qsl.net/ae4jy/versabeacon.htm
Turn on the beacon and make our way off the mountain.