The problem of installing a V H F communications equipment in a helicopter is the fitting into the aircraft of a transmitting and receiving unit and associated parts The installation should be planned with three considerations in mind, namely, operating efficiency, crew comfort and maintenance I intend to amplify these three considerations but before doing so let us look at two examples of modern V H F equipment on display here tonight

Outwardly they are of similar pattern and in both cases consist of a rack-mounted, quickly detachable, transmitter/receiver unit, which incorporates the frequency selector switch, an aerial and various types of cable connector The Murphy MR 80, MR 100 equipment has a backplate or junction box attached to the rear of the rack into which the main unit slides All control unit, aerial, and battery cable connections terminate in the backplate

The Standard Telephones STR9X is of similar form except that the cable connectors terminate on the front of the main unit

Without a good aerial system an Aircraft Radio Equipment is about as useful as a helicopter without a rotor and yet it still happens that the proper provision and siting of aerials is neglected The intention of this paper is to outline the basic requirements of aerials for the radio systems in general used on helicopters and to show how the difficulties peculiar to this class of aircraft can be overcome

CLASSIFICATION

For the purposes of this discussion three broad categories of radio equipment can be distinguished These are

1. 1 Low frequency—i e, up to 2 Mc/s or wavelengths greater than 150 meters In this range the major aircraft dimensions are much less than the wavelength

2. 2 High frequency (H F) 2 to 30 Mc/s or wavelengths of 150 to 10 meters in which the aircraft size is becoming comparable with the wavelength

3. 3 Very High frequency (VHF), 100 to 300 Mc/s wavelength to 1 meter where the major aircraft dimensions are appreciably greater than the wavelength but where cross-sectional dimensions may be of the same order

The Automatic Direction Finder is a primary radio navigational aid, providing a pilot or operator with a direct indication of the relative bearing of any radio transmitter to which the equipment is tuned, normally in the band 150 to 1,800 Kc/s

Using a single transmitter the co-ordinates of which are known, a simple homing can be obtained and with the use of two or more transmitters it is possible to obtain a position fix

To this end small beacon transmitters are often installed at airfields Most homing procedures terminate with the passage of the aircraft over the top of the beacon and this is indicated by a reversal of the bearing indicator needle

The Decca Company has great faith in the future of the helicopter and we firmly believe that co-operation between our Company and the helicopter industry—both the manufacturing and operating sides of the industry is of the utmost importance We welcome therefore, the opportunity to discuss in the most general, non-technical terms, the problems associated with the installation of Decca equipment on helicopters

I shall refer to the equipment under three headings (1) The aerial, (2) The black boxes, (3) The controls and display, and shall consider the problems associated with each section separately

Before proceeding, however, I must point out that there are a vast number of different combinations of Decca Navigator equipment ranging from a simple single chain receiver with flight log display only at a total installed weight of 40 lb to a nine chain receiver with decometers, lane identification and flight log weighing 100 lb