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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
• dl = total line of sight
• dr = 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.