41
APPENDIX I FIELD STRENGTH MEASUREMENTS
While standards and procedures have been established for making precise measurements of
the field strength of a radiated signal, the intent of field strength measurements with the ELT
1091A tester is to evaluate whether or not a distress beacon is sufficiently radiating a signal
that can be received by the COSPAS-SARSAT system. Therefore, it is neither practical nor
necessary to utilize the sterile environment utilized in making measurements used to certify
various radiating systems, however, a basic knowledge of the variables that can effect such
measurements can be helpful.
DISTANCE BETWEEN THE SOURCE AND SENSOR
The amount of measured energy for a given amount of radiated signal remains the same for a
given amount of area that the signal is being radiated. Stated another way, if an object in the
radiating field reflects some of the energy in another direction, that energy will appear as a
reduction behind the reflecting surface, and an addition in the opposite direction. The
reflecting surface can be of any conductive material or shape and will have the optimum effect
if it its size and location is related to the frequency being radiated. Simply stated, an object that
significantly affects 121.5 Mhz. will have a different effect on a 406 Mhz. signal. In any event
the most reliable results will be obtained by making measurements where there are no
intervening objects or objects located opposite from the radiating antenna. If such objects
exist, a point 90 degrees from a line between those objects and positioned abeam the antenna
should be used. A distance between the antenna and the tester should be established that will
give a baseline to use in determining if an antenna and transmitter are functioning properly and
is referred to as the calibrated distance which once determined should be used for all like
measurements. A good starting distance is no less than 5 feet and no more than 15 feet.
THE EFFECTS OF DIFFERENT POWER LEVELS
The amount of measured signal on 121.5 will be substantially less at a given distance than that
on 406 since the 406 signal can be as much as 20 times stronger than a valid signal on 121.5.
While a specific reading on 121.5 Mhz. will be acceptable, it will not be so for 406 Mhz. Put
another way, the 406 Mhz signal should be significantly stronger at the same distance than a
121.5 Mhz signal.
TESTER AND ANTENNA ORIENTATIONS
Most fixed installation antennas are vertically polarized and so when making field strength
measurements, the tester should be orientated with the long axis also in a vertical orientation.
To Next Page
Loading...