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1. COMMUNICATION’S EQUIPMENT AND CHARACTERISTICS

2. VHF Communication An aircraft VHF communication transceiver is comprised of either a single or double conversion superheterodyne receiver and an A.M transmitter. A modern set provides 720 channels at 25 kHz spacing between 118 MHz and 136 MHz; until recently the spacing was 50 kHz giving only 360 channels. Communication by VHF is essentially 'line of sight' by direct (space) wave. Superheterodyne receiver - radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency which can be more conveniently processed than the original carrier frequency.

3. VHF Communication Line of Sight Calculation: Where, h = height of the antenna R = radius of the earth d l  = total line of sight d r  = radio horizon (actual service range) for  h  in metres and  d  in kilometres for  h  in feet and  d  in miles`

4. VHF Communication Let  R  be the radius of the Earth and  h  be the altitude of a telecommunication station. Since the altitude of the station is much less than the radius of the Earth, If the height is given in metres , and distance in kilometres , If the height is given in feet, and the distance in miles,

5. VHF Communication

6. Controls and Operation Volume Control Squelch Control - acts as a noise gate to cut the ever-present background radio noise generated by atmospheric activity and man-made sources. Mode Selector Control On-Off Switch Receiver Selectivity Switch

7. TRANSRECEIVER A frequency synthesizer is an electronic circuit that uses an oscillator to generate a preprogrammed set of stable frequencies with minimal phase noise.

8. RECEIVER Components Varactor Tuned RF Filter and Amplifier Mixer Filter IF Amplifier AM Detector Squelch Detector

9. RECEIVER

10. TRANSMITTER

11. CHARACTERISTICS Receiver Sensitivity 3 μV , 30 per cent modulation at 1000 Hz to give S + N/N > 6 dB. Selectivity Minimum 6 dB points at± 15 kHz(± 8 kHz sharp). Maximum 60 dB points at ± 31-5 kHz ( ± 15 kHz sharp). Maximum 100 dB points at± 40 kHz(± 18·5 kHz sharp). Cross Modulation With simultaneous receiver input of 30 per cent modulated off-resonant signal and an unmodulated desired signal, the resultant audio output shall not exceed -10 dB with reference to the output produced by a desired signal only when modulated 30 per cent (under specified signal level/off resonance conditions). Undesired Responses All spurious responses in band 108-135 MHz shall be down at least 100 dB otherwise, including image, at least 80 dB down.

12. CHARACTERISTICS Audio Output Gain A 3 μV a.m. signal with 30 per cent modulation at 1000 Hz will produce 100 mW in a 200-500 Ω load. Frequency Response Audio power output level shall not vary more than 6 dB over frequency range 300-2500 Hz. Frequencies>5750 Hz must be attenuated by at least 20 dB. Harmonic Distortion Less than 7·5 per cent with 30 per cent modulation. Less than 20 per cent with 90 per cent modulation. AGC No more than 3 dB variation with input signals from 5 μV to 100 mV.

13. CHARACTERISTICS Transmitter Stability Carrier frequency within ± 0·005 per cent under prescribed conditions. Power Output 25-40 W into a 52 Ω load at the end of a 5 ft transmission line. Sidetone With 90 per cent a.m. at 1000 Hz the sidetone output shall be at least 100 mW into either a 200 or 500 Ω load. Mic. Input Mic. audio input circuit to have an impedance of 150 Ω for use with a carbon mic .. or a transistor mic . operating from the (approx.) 20 V d.c . carbon mic . supply. Antenna Vertically polarized and omnidirectional. To match 52Ω with VSWR< l.5 : 1.