GB2113922A

GB2113922A - Antennae

Info

Publication number: GB2113922A
Application number: GB08236807A
Authority: GB
Inventors: Peter Geoffrey Rummery
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1982-01-09
Filing date: 1982-12-24
Publication date: 1983-08-10
Also published as: GB2113922B

Abstract

An antenna for a helicopter includes an elongate tubular electrical conductor 11 maintained in spaced relationship from an electrically conducting surface region 12 of the helicopter by means of an electrically conducting first support member 13 and a second support member 14 which incorporates an antenna feed unit 15. A third support member 16 incorporates a switch 17 operable to switch out a major portion of electrical conductor 11. <IMAGE>

Description

SPECIFICATION Antennae This invention relates to antennae for use in the H.F. waveband (2 to 30 MHz) and in particular to antennae for use on helicopters or aircraft.

One particular form of antenna used in the past on aircraft has been the notch type of antenna.

This comprises a notch cut out of an electrically conducting part of the structure of the aircraft body, the radio frequency signal being fed across the outer corners of the notch. In this type of antenna, the radiation resistance of the antenna is proportional to the square of area enclosed by the notch. The power radiated by the antenna is equal to the square of the current flowing multiplied by the radiation resistance. Hence as the area enclosed by the notch increases, so the radiation resistance increases as the square thereof, and thus the power radiated increases.

However, the size of notch and hence the enclosed area which may be cut into the aircraft structure is severely limited by the structural loading of the aircraft and is usually relatively small and thus the efficiency of this type of antenna may be relatively low as compared to other conventional forms of antenna such as a top-loaded monopole. The efficiency of the antenna is particularly important in the case of a helicopter, which is electrically small - that is the electrical paths extending from the antenna around the electrically conducting portions of the surface of the helicopter are relatively small - as compared to many forms of aircraft.

According to one aspect of this invention, there is provided an antenna arrangement for an airborne vehicle the arrangement including an elongate electrical conductor maintained in spaced relationship from an electrically conducting surface by means of an electrically conducting support member interconnecting a first end region of the elongate electrical conductor with an adjacent part of the electrically conducting surface, the second end region of the elongate electrical conductor being arranged for connection to an antenna feed unit.

Preferably the arrangement includes switch means arranged between a region of the elongate conductor towards said second end region and an adjacent part of the electrically conducting surface and operable to shorten the effective electrical length of the elongate conductor to allow the antenna to be used for relatively high frequencies in the H.F. band.

When the antenna is to be used on a helicopter,, the electrically conducting surface preferably comprises the external surface of the helicopter.

Preferably, the elongate electrical conductor is in the form of a pipe.

When the antenna is to be used on an aircraft, the electrically conducting surface preferably comprises part of the internal structure of the aircraft and the arrangement is enclosed by a radome.

The elongate conductor is preferably approximately 3 metres in length and the switch means, when provided, reduce the effective electrical length of the elongate conductor to approximately 0.45 m.

The arrangement preferably includes control means adapted to close the switch means when the antenna is to operate at frequencies above 10 MHz.

In order to reduce the electrical resistance of the elongate electrical conductor, it may be provided with a corrugated surface.

By way of example only, two specific embodiments of this invention will now be described in detail, reference being made to the accompanying drawings, in which: Figure 1 is a view of an antenna arrangement on a helicopter; Figure 2 is a view of an antenna arrangement on an aircraft, and Figure 3 is a circuit diagram of an equivalent electrical circuit for the antenna of Figure 1.

Referring to Figure 1 , there is illustrated the tail region 10 of a helicopter. The antenna arrangement comprises an elongate tubular electrical conductor 11 of aluminium which is maintained in spaced relationship from an electrically conducting surface region 12 of the helicopter by means of an electrically conducting first support member 13 connected between a first end region of the elongate conductor 11 and an adjacent part of the electrically conducting surface region 12. At its second end the elongate conductor is supported by a second support member 14 which incorporates an antenna feed unit 1 5 which is described in detail below.The length of the elongate conductor is approximately 3 m., and at a position approximately 0.45 m, away from the second support member there is provided a third support member 1 6 which incorporates a switch 1 7 operable to effect connection between the adjacent portions of the elongate conductor 11 and the electrically conducting surface 12. It is'important to note that the surface of the helicopter tail is prepared so that a return path of low electrical resistance exists between the bases of the first and second support members.

Figure 2 shows a similar antenna arrangement positioned forwardly of a rudder of an aircraft. In this arrangement, the antenna is suppressed within a radome 20. An elongate conductor 21 is supported at a distance away from an electrically conducting surface by means of an electrically conducting first support member 13 and a second support member 14 which is electrically insulating. An antenna feed unit is provided adjacent second support member 14 to feed the antenna.

Referring now to Figure 3, there is shown an equivalent circuit of the antenna arrangement of Figure 1. The elongate conductor 11 may be thought of as a Resistance R in series with an inductance L. Switch 17, which acts to shorten the effective length of the elongate conductor may be thought of as shorting out part of the resistance R and inductance L. The antenna feed unit 1 5 includes a matching variable capacitor C1 and a timing variable capacitor C2. These variable capacitors handle high currents during use and are therefore preferably of the vacuum type.

On operation of the antenna, the capacitors C, and C2 are varied dependent on the frequency supplied so that the impedance of the circuit is approximately 50 ohms, which is the usual impedance of the transmitter T. Switch 1 7 is closed automatically when the frequency supplied is above 10 MHz., so that the effective length of the electrical conductor is 0.45 metres.

In order to increase the efficiency of the antenna, the circuit resistance should be as low as possible. To this end, the electrical resistance offered by the connections in the antenna arrangement should be as low as possible. The electrical resistance offered by the elongate member 11 may be reduced by increasing its surface area. This can be achieved by corrugating the surface thereof.

Claims

1. An antenna arrangement for an airborne vehicle, the arrangement including an elongate electrical conductor maintained in spaced relationship from an electrically conducting surface by means of an electrically conducting support member interconnecting a first end region of the elongate electrical conductor with an adjacent part of the electrically conducting surface, the second end region of the elongate electrical conductor being arranged for connection to an antenna feed unit.

2. An antenna arrangement according to Claim 1 , wherein switch means are arranged between a region of the elongate conductor towards said second end region and an adjacent part of the electrically conducting surface and operable to shorten the effective electrical length of the elongate electrical conductor.

3. An antenna arrangement according to Claim 1 or Claim 2, wherein the elongate electrical conductor is of hollow tubular form.

4. An antenna arrangement according to Claim 3, wherein the surface of the elongate electrical conductor is corrugated.

5. A helicopter provided with an antenna arrangement as claimed in any of the preceding Claims, wherein the elongate electrical conductor is maintained in spaced relationship from the tail region thereof and extends generally longitudinally with respect thereto.

6. An antenna arrangement substantially as hereinbefore described with reference to, and as illustrated in, any of the accompanying drawings.