AirCraft Radio System (Communication System)

Communication System

There is a fundamental need for communicationbetween aircrew and ground controllers, among the aircrew and between aircrew and passengers. External communicationi s achievedb y means of radio-telephon(eR /T) link while internal communication'(intercom or audio integrating system) is by wire

as opposed to wireless. Although intercom' is not a radio system, it is included in this chapter because of its intimate relationship with the aircraft radio systems. Voice recorders and in-flight entertainment systems are also considered since they are usually the responsibility of the aircraft radio technician/engineer- The first items of radio equipment to appear on aircraft were low-frequency (l'f.) communications sets in the World War I days of spark gap transmitters. lntercom was by means of a Gosport (speaking) tube' By the 1930s the early keyed continuous wave (c'w') (radio-telegraphyw) as beginning to be replaced by R/T although' key-bashing'hadit s place as long as aircraft carried radio operators. Early R/T was within the l.f. and h.f. bands, the sets operating on only one or very few frequencies. With airfield swidely spaced and low-powered transmission there was little interference and so the need for many channels did not arise.

The situation has drastically changed since World War II; air traffic and facilities have increased with the consequend temand for extra channel swhich can not be provided in the Lf., m.f. or h.f. bands. Fortunately v.h.f. equipment has been successfully developed from early beginnings in World War II

figtrter control. The current situation is the v.h.f. is used for tort-range communication while h'f. is used for long-range. A large airliner, such as aBoeing74T , carriest hree v.h.f.sa nd dual h'f. In addition, in such aircraft, selective calling (Selcal) facilities are provided by a dual installation such that a ground station can call aircraft either singly or in groups without the need for constant monitoring by the crew. Provision for satellite communication (Satcom) on v.h.f. lrequencies is often found; unfortunately aeronauticaliommunicationss atellitesa re not to be found (1979). The audio integrating system (AIS) complexity depends on the type of aircraft. A light aircraft may provide two transmit/receive channels for dual u.h.f. iomms and receive only for dual v'h'f' nav., ADF, DME and marker. Each receive channel has a speakeroff'phone switch while the microphone can be switched bitween v'h.f. comms I and v'h'f' comms 2. A multi-crew large airliner has very many more facilities,as described later.

V.H.F. Gommunications

Basic Principles An aircraft u.h.f. comrns transceiver is comprised of either a single or double conversion superhet receiver and an a.m. transmitter' A modern set provides 720 channels at 25 kHrzs pacing between I l8 MHz and 135'975M Hz; until recently the spacingw as 50 kHz giving only 360 channels. The mode of operation is single chanel simplex( s,c.s.),i .e. one frequency and on J antenna for both receiver and transmitter' If provision for satellite communication is included inaccordance with ARINC 566 then in addition to a.m. s.c.s.w e will havef .m' double channel            simplex (d.c.s.),i .e. different frequencies for transmit and receive. Communication by v.h.f. is essentially 'line of sight'by direct (space)w ave. The rangea vailablec an be approximated by I '23 (\/.\ + y'ft1)nm where ftt is are trilght, in feet, above sea level of the receiver while ftl.is the same for the transmitter' Thus, with the ground station at sea level, the approximate ,nr*I*urn range for aircraft at l0 000 and 1000 ft (30 000 and 3000 m) would be 123 and 40 nm respectively.

installation
A single v.h.f. installation consists of three parts' namjy control unit, transceivera nd antenna' In
addition crew phones are connected to the v'h'f' via selections witches in the AIS. Light aircraft v.h.f.s
usually have a panel-mounted combined transceiver and control unit, an example being the King KY 196 illustrated in Fig. 2.1. The current trend is for combined COM/NAV/RNAV; Fig. 2.2 illustrates the BendixC N-2011a, general viation panel-mounted unit comprising two comms transceivers two nav. receiver slide path receiver marker receiver,frequency control for internal circuits and d.m.e. and
last but not least,a udio selections witches. Such equipment will be considered in Chapter1 2. Figure 2.3 shows one of a triple v.h.f. comms installation as might be fitted to a large passenger transport aircraft: 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 connect mic and tels directed disable the squelch, and measure VSWR. These provisions for testing are by no means universal but if the system conforms to ARINC 566 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 or suppressed In a triple vl.f. comms installation
these may be two top-mounted blade antennas and one bottom-mounted: an altemative would be two
bladea nd one suppressewd ithin the fincap dielectric. The whip antennas 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 cornplex. It will be self-resonannte are the centre of the band with 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.