VHF COMMUNICATION

An aircraft VHF communication transceivers comprised of either a single or double conversion superheterodyne receiver and an Amplitude Modulated transmitter.

The mode of operation is Single Channel Simplex (SCS), i.e. one frequency and one antenna for both receiver and transmitter. If provision for satellite communication is included then, F.M. Double Channels Simplex (DCS), i .e. different frequencies for transmitting and receiving would be added up.

Wave type: Space wave.

Freq: 118-136 MHz

No. Of Channels and spacing:

720 channels at 25 kHz spacing

Or

360 channels at 50 kHz spacing

Range formula: 1.23 (√ht + √hr ) nm

where,

ht is height of Transmitter above sea Level;

hr is height of Receiver above sea Level

INSTALLATION

A single v.h.f. installation consists of three parts

• Control unit

• Transceiver

• Antenna.

In addition crew phones are connected to the VHF via selection switches in the AIS (Audio Integrated System).

VHF2 and VHF3 are similar to VHFI but are supplied from a different 28 V d.c. bus bar and feed different selection switches in the AIS. The transceiver which is rack-mounted, contains all the electronic circuitry and has provision for the maintenance technician to perform maintenance tasks.

These provisions for testing are by no means universal but if unit is not in the workshop.

The system conforms to ARINC 566, where a plug is provided to which automatic test equipment (ATE) can be connected.

A protective cover for the ATE plug is fitted when the unit is not in the workshop.

The antenna can take various forms:

• whip

• blade

• suppressed

in a triple v.h.f. comms installation these may be two top-mounted blade antennas and one bottom-mounted.An alternative would be two blade and one suppressed within the fincap dielectric.

The whip antenna is to be found on smaller aircraft. All antennas are mounted so as to receive and transmit vertically polarized waves.

The blade antenna may be quite complex. It will be self-resonant near the centre of the bandwidth. Bandwidth improvement provided by a short circuited stub across the feed terminal or a more complicated reactive network built in which will permit height and hence drag reduction.

CONTROLS AND OPERATION FREQUENCY

FREQUENCY CONTROL

is achieved by concentric knobs, the outer one of which varies the tens and units while the inner one varies the tenths and hundredths. An alternative is where there is one frequency control and two displays’ On rotating the frequency knobs clockwise or anticlockwise the standby frequency only will increases or decreases respectively.

Standby may then become in-use by operation of the transfer switch.

VOLUME CONTROL

A potentiometer, which allows variable attenuation of audio, prior to feeding the AIS may be fitted as a separate control or as a concentric knob on the frequency selector(s). Such a volume control may have side-tone coupled through it on transmit.

SQUELCH CONTROL

A squelch circuit disables the receiver output when no signals are being received so preventing noise being fed to the crew headsets between ground transmissions.

The squelch control is a potentiometer which allows the pilot to set the level at which the squelch opens, so allowing audio output from the receiver. When the control is set to minimum squelch (fully clockwise) the Hi and Lo squelch-disable leads, brought to the control unit from the transceiver, should be shorted, so giving a definite squelch disable.

MODE SELECTOR CONTROL

This Provides selection of normal A.M.. extended range A.M., or Satcom. If the Satcom antenna has switchable lobes such switching may be included the in mode switch, could separate.

ON-OFF SWITCH

This Energizes master power relay in transceiver. The switch may be separate, incorporated in mode selector switch as an extra switch position, or ganged with the volume or squelch control.

RECEIVER SELECTIVITY SWITCH

This provides Normal or sharp selectivity. When Satcom is selected sharp selectivity automatically applies.

VHF TRANSMITTER-RAMP TESTING

After checking for condition and assembly and making available the appropriate power supplies the following checks should be made at each station using each V.H.F.

1. Disable squelch, check background noise and operation of volume control.

2. On an unused channel rotate squelch control until squelch just closes( no noise). Press PTT button, speak into mic. and check sidetone.

3. Establish two-way communication with a remote station using both sets of frequency control knobs, in conjunction with transfer switch, if appropriate.

Check strength and quality of signal.

Note :- Do not transmit on 121.5 MHz (Emergency).

Do not transmit if refueling in progress.

Do not interrupt ATC-aircraft communications