Now you're Talking
John Callahan
A severe case of interference surfaced in Houston, Texas on RC channel 57. Several large, expensive models were lost at a number of fields.
Paul King obtained the RC Channel Analyzer from the District VIII frequency representative and started looking for the culprit without finding the RF interference. When two Hams, Bob McDonald and myself, heard of the problem and the availability of the AMA Channel Analyzer, we volunteered to assist.
The AMA Channel Analyzer (made by Aero Spectra) is a hand-held unit the size of a large calculator, run by a nine-volt battery, which makes it completely portable. It only covers one band (71.9–73.1 MHz). (Editor’s note: Newer models cover 50, 53, 72, and 75 MHz.)
Three displays are available:
- The first is alphanumeric and shows the frequency in MHz, the channel number, and signal strength in dB.
- The second shows the full spectrum from 72 to 73 MHz; each signal in that frequency range is a spike up from the baseband. The vertical axis on the display is calibrated in dB (0 dB at the bottom and 60 dB at the top of the screen).
- The third is called “zoom” and the selected channel is centered on the screen. You can see 30 kHz on each side of the selected frequency (channel). This is the primary display we used.
Analyzers are available from each District Frequency Coordinator for further information and availability.
The first coordinated attempt was to construct a two-element Yagi (beam) and use signal strength (alphanumeric display) to locate the interference. This resulted in a number of bearings that did not intersect and were unusable. It appears that reflections and the fact that the signal was shifting frequency (at a 120 Hz rate) prevented locating even the general area where the interference was located with the Yagi beam. Buildings and towers gave reflections that were stronger than the basic signal.
When observed on the zoom display, the interfering signal seemed to start around 72.94 MHz (a pager frequency) with a sweep down to 72.93 MHz (RC channel 57). The local office of the FCC was visited and a list of licensed users of 72.94 MHz was obtained. When these were checked in the telephone book, only one was a listed company. When this company was visited they advised they had given the frequency up last July and had only used it in California.
The FCC in Gettysburg, PA was then contacted, and the names of two current users of the frequency (72.94 MHz) in Houston were obtained, along with the latitude and longitude of their transmitters. When the analyzer was driven to these sites, neither was the culprit.
Bob McDonald then took the analyzer and found that the signal appeared to be strongest south of town. This corresponded to the area where the hardest-hit RC fields were located. I then used the analyzer and ham direction-finding techniques to find what appeared to be a “hot spot” near one of the Houston FM towers. I obtained the frequencies on that tower and an adjacent TV tower. These were input to an intermodulation computer program, with two “hits” identified. The frequencies involved in the hits on channel 57 were not of the type that would generate the interference that existed on channel 57.
An all-band scanner was also tuned to the interference and the output displayed on a scope. The audio signal sounded like raw AC. The scope showed a signal that appeared to be a repetitive, corrupted sawtooth pattern.
I took the data to another Ham, who is the chief engineer of a local FM station. After looking at and hearing the interfering signal, he said it was a bad power supply and part of the pager system (due to its location in the radio spectrum).
An outside-car Ham antenna was connected to the analyzer and I started driving toward a couple of 500-foot towers south of Houston that had pager systems installed on them.
As the first tower was approached, the signal strength (on the zoom display) continued to increase until it was 50 dB at the base of the tower. Bingo! It’s so easy when you finally find the source.
The engineer had access to the buildings at the base of the tower, and we found five large cabinets in one building. The analyzer did not show an increase in signal strength, however, when placed next to each cabinet.
The engineer then momentarily pulled the power cord from each cabinet. When the third cabinet’s cord was pulled, the interfering signal disappeared from the analyzer. When power was restored, the signal reappeared at 72.92 MHz and no longer impinged on channel 57.
The malfunctioning transmitter had an output of about 100 watts and the antenna was at 500 feet, so the interfering signal covered a wide area. The unit was used to phase transmitters in a large paging system; the company had switched to a satellite system, and this system was being kept as a backup. Since the satellite system had not failed, the malfunction of the backup was not detected until found by the team.
Suggestions for locating and correcting interference
- The analyzer loaned by AMA is very useful in observing the interfering signals.
- The best way to locate the interference seems to be by observing signal strength.
- Use intersecting roads where the interference can be seen on the analyzer:
- Determine the location along both roads where the signal disappears.
- Draw an ellipse through the four points on the map.
- Search near the center for towers and antennas; the interference might originate there.
- Use the analyzer to pinpoint the location from signal strength.
- Secure the help of professionals to identify commercial broadcast signals. Show them your data and the signal on the analyzer.
- To get the interference corrected, you will have to contact the tower owner to get the tenants of the tower, and contact them individually. We suggest you “soft-approach” these operators. “You can catch more flies with honey than with vinegar,” as my old Irish grandfather used to say.
John Callahan 430 E. Gaywood Houston, TX 77079
Transcribed from original scans by AI. Minor OCR errors may remain.
