§ 2.1511 Measurements of radiated emissions.  


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  • The Commission's Rules require that the peak efficetive radiated power (PERP) of a Class A, B or S EPIRB not be less than 75 mW under certain specified conditions. The PERP of an EPIRB transmitter is determined by comparing its level to a reference PERP generated by a standard quarter-wave monopole antenna located on a one wavelength minimum diameter metal ground plane. The Rules also require that all spurious and harmonic emissions be attenuated by a specified amount with respect to the reference PERP. In addition, there is a limit on the PERP of radiated emissions with the switch in the test mode. These measurements are to be made in accordance with the following procedure.

    (a) General set-up instructions. Measurements of radiated electromagnetic emissions (EME) are to be performed on the 30 meter open field test site described in §2.1503(a) of this part and on one of the pair of frequencies listed in §2.1507 of this part. A receiver, tuned dipole antennas and a calibrated signal generator as described in §2.1505 of this part are required. The EPIRB should be powered by its own internal battery with its standard antenna attached and deployed.

    (b) Set-up for radiated EME tests.

    Step (1) Place a 121.5 MHz quarter-wave vertical antenna element at the center of the ground plane and connect the output of the calibrated signal generator to the antenna.

    Step (2) Mount the tuned dipole antenna on the antenna mast, tune the elements to 121.5 MHz and connect the antenna to the receiver.

    Step (3) After an appropriate warm up, turn the receiver to the frequency of the test unit, set the detector to peak mode and the bandwidth to 100 kHz.

    (Note: It is sometimes helpful to monitor the receiver audio output with a speaker. The EPIRB signal may be identified by its distinctive modulation.)

    (c) Radiated EME tests.

    Fundamental emissions-peak effective radiated power

    Step (1) Turn on the signal generator and adjust the output to 75 mW at 121.5 MHz.

    Step (2) Vary the antenna height from one to four meters in both vertical and horizontal polarization. Record the highest receiver reading in dBm as the reference level.

    Step (3) Disconnect the signal generator and replace the quarter-wave vertical element on the ground plane with the EPIRB under test. The EPIRB is to be positioned directly on the surface of and in the center of the metal ground plane.

    Step (4) Activate the EPIRB.

    Step (5) Vary the receive antenna height from one to four meters in both vertical and horizontal polarization. Record the highest receiver reading in dBm and the instrument settings, antenna height and direction for maximum radiation, antenna polarization and conversion factors, if any, associated with that reading.

    Step (6) Repeat Step 5 with the EPIRB switch in the test position. Return the switch to the normal operation position.

    Step (7) Rotate the EPIRB 30 degrees and repeat Steps 5 and 6. Repeat this step for all successive 30 degrees segments of a full, 360 degree rotation of the EPIRB.

    Step (8) Repeat §2.1511(b) and Steps 1 through 7 for 243 MHz.

    Step (9) Compute the peak effective radiated power for the maximum level of each measured emission using the following formula:



    where:

    dBmmeasis the measured receiver reading in dBm, and

    dBmrefis the reference receiver reading found in step 2 of §2.1511(c).

    Step (10) Record the PERP in mW. The FCC limit for minimum power in the normal operation mode (i.e., with the EPIRB switch in the normal operating position) is 75 mW. The FCC limit for maximum power in the test mode is 0.0001 mW.

    Spurious emissions

    Step (11) Reset the signal generator to operate at 121.5 MHz.

    Step (12) For each spurious and harmonic emission to be measured, retune the receive antenna to the appropriate frequency and repeat Steps 5 and 7.

    Step (13) Determine the FCC limit on power for spurious emissions on the frequency of each measured emission as follows:

    The rules require that spurious emissions be attenuated at least 30 decibels below the transmit power level. Therefore, the maximum received power limit for a spurious emission can be calculated from the formula:

    dBmspur= dBmmeas+AF121.5−AFspurfreq−30

    where:

    dBmmeas= measured receiver reading (Section 2.1511(c), step 5).

    AF121.5= tuned dipole antenna factor at 121.5 MHz.

    AFspurfreq= tuned dipole antenna factor at spurious freq.

    Step (14) Record in dB below the fundamental emissions the level of all spurious and harmonic emissions within 10 dB of the FCC limits.