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Local Hams Aid Rescue Squad to Solve Public Safety Interference Issue

07/29/2009

When you live on a remote island with numerous mountains and valleys, communications can be tricky. Add interference that blocks the main communications frequency used by the local emergency rescue squad and you've got a disaster waiting to happen. That's what responders and residents on St John in the US Virgin Islands recently found themselves facing.

On June 12, the primary repeater output frequency for St John Rescue was completely blocked by a 2-tone AFSK signal that continued for more than a week. Because St John Rescue uses the frequency to dispatch, monitor and provide two-way communications during emergency calls, it was vital that the cause of the problem be detected and corrected.

According to Phyllis Benton, NP2MZ, a Public Information Officer in the ARRL's US Virgin Islands Section, some members of St John Rescue are also members of ARES®. With some additional help from the FCC, three hams -- Paul Jordan, NP2JF, Mal Preston, NP2L, and George Cline, KP2G -- set out to find the source of the interference.

The interference was not directly affecting operation of a second rescue repeater, Benton told the ARRL. "St John Rescue Chief Gilly Grimes and Paul Jordan, NP2JF, used handheld Yagi antennas to 'fox hunt' for the source of interference," she said. "To their surprise, the signal was being received off the back of the antennas and coming in at a very strong 120 W."

The source of the interference turned out to be 32 miles away from a tower on Mount St Georges on the island of St Croix. "The carrier frequency was just 7.5 kHz above the rescue frequency of 158.7525 MHz," she explained. "Upon closer inspection, the problem was isolated to a repeater that is part of the new US Virgin Islands territory-wide MPT 1327 trunking system. This transmitter was licensed for and was putting out 120 W with a pass band of 50 kHz and was being tested as the control channel."

Benton said that the second, unaffected repeater operates at an output frequency of 159.660 MHz, far enough away from the trunking frequency being tested to avoid being affected: "This second repeater serves areas not covered by the primary repeater. So, until the problem was resolved, a large part of St John was left without reliable rescue emergency radio communications. Once the source of the problem was identified, the interference was turned off on June 19."

To head off any future interference problems, the trunking system promoters have asked St John Rescue to change its current repeater frequencies to frequencies that theoretically would not receive interference from the trunking system. Benton said that St John Rescue is considering this request.  -- Information provided by PIO Phyllis Benton, NP2MZ



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