Stone Mountain
Beacon Project
The Opportunity
The WABE-FM 90.1 transmitter had moved from the Stone Mountain facility and the antenna system was sitting idle on the 450 foot tower on top of Stone Mountain. Robin Cutshaw,AA4RC the president of the Atlanta Radio Club was approached with an interesting opportunity by the director of engineering of Georgia Public Broadcasting, Mark Fehlig and the Director of Engineering of WABE, John York ( both Hams). The idea was to allow the amateur radio community to have access to the unused commercial FM antenna and feed system sitting idle on the tower just below the Channel 8 TV transmitter antenna.
Ok, Now what to do with it?
Lets see, I'm a ham, I'm given the opportunity to attach a radio to an antenna about 1000' above the ground, who wouldnt jump at an opportunity like that?
The initial thinking was that the antenna being designed for 90.1MHz would not work at all or very poorly on the two nearest ham bands of 6 or 2 meters. Another major concern was how much RF energy from the 30KW WGTV channel 8 antenna located a few feet above the WABE FM antenna would be coming back down the feedline.
A November 16, 2004 trip was planned to go to the site and accomplish the following:
1. Measure the RF energy coming back down the feedline.
2. Measure the SWR on 2 meters.
3. Install a 2 meter 5mW beacon for some initial antenna performance tests.
The Initial Nov. 16, 2004 Trip
The beacon transmitted 5 minutes of regular CW on 145.600 MHz followed by 5 minutes of CW over FM, followed by an SSTV picture repeated 3 times. 5mW doesn't go far but it was heard out on Windy Hill and I75. Otherwise it was pretty much limited to reception on the NE side of Atlanta. The fact that it could be heard at all said that at least the antenna was radiating somewhat and so plans began for bumping the power to get a better feel for the antenna pattern and coverage.
The Slow Scan picture sent out by the 5mW beacon and a comparison of strength to the 146.76 repeater also located on top of Stone Mountain. Note the beacon is about 30dB(1000 times) weaker than the repeater implying that an increase in beacon power by 1000 to 5 Watts would make it nearly the same strength as the repeater. So we now had a goal for the next trip.
The next trip involved swapping out the 5mW beacon with a 5W version and making a few additional antenna measurements on 6meters, 220, and 440.
Initial results are promising as the beacon is now heard quite easily around Atlanta. The only negative so far is our temperature sensor that was hastily stuck through a hole in the side of the building was destroyed the very next day when a storm went through.
Here is a spectral view of the entire 2meter band that shows the peak signals in the area over about an hours time. As predicted, the beacon at 146.600 is now as strong as the Stone Mountain repeater, at least at this location in Duluth GA.
click on image to get full view