224.280 - Repeater Inteference Update

The club's 1.25-meter repeater, the 224.280 repeater, negative offset, is also located at Cox South Hospital in Springfield. At almost the same time as we started receiving interference on the 442.275 repeater, we also started having problems with receive sensitivity on the 224.280. The interference appeared to be there all the time. This coincidence has hindered progress getting the problem resolved.

Club leadership felt it was more important to tackle interference on the 442.275 repeater since it had more use.

While at the site, with the UHF repeater interference resolved, James looked into the interference to this repeater as well and confirmed that it is the 72.720 MHz GPS Synchronization transmitter used by the hospital to synchronize all the clocks inside the hospital patient rooms!

This interference, which also began about 6 months ago, has been reported to the hospital. We found the unit not operating properly several months ago - it was transmitting 24/7. Normally, it transmits from 39 minutes after the hour to 9 minutes after the hour. Cox engineering contacted the company, who found several problems with the unit, and they replaced it completely. This did not fix the amplifier, however. The unit, manufactured by Primex out of Canada, uses a TPL amplifier for use in the 70 MHz band. The amplifier itself is not type accepted by the FCC and is produced for export use only. It is a very dirty amplifier with no output filtering. Primex gets the unit and their exciter type accepted as a single unit and imports it back to the US.

Today, James measured 45-watts of RF on the 3rd harmonic of 218.160 MHz. With a third harmonic that powerful, it is desensing our receiver and we are hearing that transmitters data on the input to our repeater.

You may have heard it - it sounds like desense noise like when your repeater's duplexers are not tuned properly. James checked our duplexers today and found them to be in tune properly, ruling out desense from our transmitter. However, when the GPS transmitter was unplugged during the middle of an interference burst, the problem went away immediately.

Jeff Kerr, KC0VGC, is working with a representative at Cox to help us get the problem resolved.

We hope this problem will be resolved soon as well so the repeater can be turned back on and used by everyone once again.

If you have a tri-band HT, or other 220 rig, maybe you have a mobile that's been shelved for awhile and not used, or maybe you haven't programmed your 220 frequencies into your new rig because you didn't know we had a 220 repeater in Springfield, make sure and program in the 224.280 repeater with a negative offset. The PL tone is 162.2 Hz. Watch this website and the Nixa Hams Yahoo! group for updates and details announcing when the problem is resolved and it will be back on the air.