HF COMMUNICATION

Wave type: Sky Wave

Sky waves are refracted by the ionosphere to such an extent that they are bent sufficiently to return to earth.

The H.F. ground wave suffers quite rapid attenuation with distance from the transmitter. Ionospheric attenuation also takes place, being greatest at the lower frequencies.

A significant feature of H.F. is its long range communication.

Transmission is that it is subjected to selective fading over narrow bandwidths (tens of cycles).The current and future norm is to use single sideband( s.s.b.) mode of operation for H.F. communications, although sets in service may have provision for compatible or normal A.M., i.e. carrier and one or two sidebands being transmitted respectively.

A feature of aircraft H.F. systems is that coverage of a wide band of R.F. and use of a resonant antenna requires efficient antenna tuning arrangement which must operate automatically on changing channel in order to reduce the VSWR to an acceptable level.

INSTALLATION

A typical large aircraft H.F. installation consists of two systems each of which comprises a

1. Transceiver

2. Controller

3. Antenna tuning unit (ATU)

4. Antenna

Each of the transceivers are connected to the AIS for microphone telephony and P.T.T provision. In addition outputs to SELCAL decoders are provided.

The transceivers contain the receiver, transmitter, power amplifier and power supply circuitry.

They are mounted on the radio rack and provided with a flow of cooling air, possibly augmented by a fan.

Telephone and microphone jacks may be provided on the front panel, as might a meter and associated switch which will provide a means of monitoring various voltages and currents. Coupling to the antenna is achieved via the antenna tuning unit (ATU). Some systems may employ an antenna coupler and a separate antenna coupler control unit.

The ATU provides, automatically, a match from the antenna to the 50 ohm transmission line.

Closed-loop control of matching elements reduces the standing wave ratio of 1.3: 1 or less.

Since the match must be achieved between line and antenna the ATU is invariably mounted adjacent to the antenna lead-in, in an unpressurized part of the airframe.

For high-flying aircraft (most jets) the ATU is pressurized possibly with nitrogen. Some units may contain a pressure switch which will be closed whenever the pressurization within the tuner is adequate. The pressures which may be used for ohmmeter checks or, may be connected in series with the key line thus preventing transmission in the event of a leak. Alternatively an attenuator may be switched in to reduce power.

Light aircraft HF systems in service are likely to have a fixed antenna coupler. Such a system operates on a restricted number of channels (say twenty). As a particular channel is selected, appropriate switching takes place in the coupler to ensure the R.F. feed to the antenna is via previously adjusted, reactive components, which make the effective antenna length equal to a quarter of a wavelength thus presenting an impedance of approximately 50 Ohm.

The required final manual adjustment must be carried out by maintenance personnel on the aircraft.