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APPENDIX H. RF SHIELDED CONTAINERS
The COSPAS-SARSAT system is designed to receive very weak signals radiating from the surface
of the Earth. With distress devices generating approximately 5 watts of RF energy, if this
energy is not properly contained, it will be received by the satellite system and trigger an alarm.
Therefore when testing these devices it is important to ensure that the device being tested
does not radiate a normal signal strong enough to be received by the system. There are several
means to accomplish this. Some are costly while others outright inexpensive but still very
effective. Additionally, consideration needs to be made in order to couple enough signal from
the device under test to the tester. The signal level required is insufficient for the satellite
system to pick up. The following examples have been used and tested and have proven to be
successful.
THE METAL MAIL BOX
Using a standard metal mail box install, a BNC bulkhead connector on any suitable and
convenient surface. Remove any paint from the flange on the door and the mating surface on
the box allowing a good contact to be made around the entire door edge and the box. Also
remove any paint around the area where the bulkhead connector is installed to ensure a good
contact between the connector and the box. When testing ELTs that have an external antenna
connection, connect it to the bulkhead connector and, using a good quality cable connect the
other end of the bulkhead connector to the ANTENNA LOAD on the tester to make power
measurements. When testing devices that do not have a provision for an external antenna
such as a PLB, use a short coax cable that has had its outer shielded braid removed for at least 7
inches and which has been connected to the bulkhead connector inside the mail box. Place the
exposed center conductor of the pick-up cable adjacent to the device being tested and secure
with a piece of tape. Connect the Tester’s ANTENNA LOAD using a good quality cable to the
other side of the connector. Use the FIELD STRENGTH mode and compare the readings to
those from a known good device that has been set up with the same configuration since there
is no direct means of measuring the actual power being produced by the device.
NOTE: A GOOD QUALITY CABLE IS MADE UP USING DOUBLE SHIELDED 50 OHM WIRE SUCH AS
RG 223/U, CABLES MADE UP WITH THE POPULAR RG58/U HAVE ENOUGH LEAKAGE TO RADIATE
ENOUGH SIGNAL TO SET OFF AN ALARM.
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