This was a timely story from the ARRL newsletter about T hunting
==> LOCAL HAMS AID RESCUE SQUAD TO SOLVE PUBLIC SAFETY INTERFERENCE ISSUE
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
<http://www.stjohnrescue.org/> 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
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 very strong."
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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