Care & Maintenance Of Transmitter
When first installed, when changes are made to it and at least one a year, every transmitter must be tested. The FCC required tests are
- Power output or input
- Modulation deviation or percentage
1. Power output or input
Power output may be tested into the antenna only if the channel is not busy and can be done in a few seconds; otherwise a dummy load is used. An RF wattmeter — SWR meter is added between transmitter and dummy load. When the transmitter is switched on, the wattmeter indicates the power output. The output power may also be determined by
Po = la2 Ra
Where Ia is the value read on a thermocouple antenna ammeter calibrated for VHF or UHF and R is the resistance of the dummy load, or impedance of the antenna at the measured point. When a dummy load is used, The SWR should be nearly 1 : 1. Into an antenna it may be higher. If SWR is above 1.3 : 1, there may be something wrong with the antenna system. Power input is determined by
Pin = VdcIdc
of the final RF amplifier, where Idc, is the output circuit dc current and VdC is the output circuits dc power supply voltage. Power output may also be determined by
Pin X efficiency of the final stage
Transmitters should not be operated at levels greater than the manufacturers ratings.
2. Modulation deviation or percentage
Modulation tests for an FM transmitter require a calibrated modulation monitor. A sinusoidal 1000-Hz tone or (whistle in an emergency) should produce a maximum of about 4.5 KHz of deviation at the beginning of limiting, and it should be equal in both the positive and negative directions. Voice modulation should then be set to produce 5-KHz deviation. If positive and negative deviations are dissimilar (non-symmetrical), the cause may be due to mistuning of one or more of the RF stages, detuned antenna or to improper functioning of the modulator stage.
For AM transmitters a sinusoidal 1000-Hz input should be able to produce at least 70% modulation, as shown on a modulation monitor or oscilloscope. Speech should never drive the negative peaks to zero on the scope. Distortion, audible or rioted on a scope, can be checked by observing waveforms as the VF progresses from the microphone to the modulator. A distortion analyzer may be used to check suspected VF distortion. In ability to produce the required modulation levels can often be pinpointed by use of a scope. A spectrum analyzer provides a measurement of the side bands or spurious signals (“spurs”) generated by modulation The peak envelope power, or PEP, of SSB, FM, AM, or CW transmission can be determined with a calibrated oscilloscope connected across the known antenna transmission line impedance by
P = V2 /Z
3. Frequency Checks
Frequency checks are made by special frequency monitors or by using a frequency counter set at a I-S gating time. The frequency tolerance must be known to determine whether the transmitter is operating within limits. The frequency should be checked when the transmitter is cold and again after it has been turned on for at least 20 min. direction of frequency drift is thus established. The transmitter frequency should be set so that when warm it as close to that assigned frequency as possible without allowing it to be out of tolerance when starting cold. No modulation, voice, tone, CTCSS, etc. should be on the carrier when frequency is being checked.
Equipment’s that may be required for these tests includes a dummy antenna, calibrated RF and AF signal generators, deviation monitor, distortion analyzers, RF wattmeter, RF ammeter, AF voltmeter, calibrated oscilloscope, counter, and spectrum analyzer.