GB2269482A

GB2269482A - HF antenna for a helicopter.

Info

Publication number: GB2269482A
Application number: GB9216585A
Authority: GB
Inventors: Luther E Brown; Graham Luck; Terence Keith Gibbs
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1992-08-05
Filing date: 1992-08-05
Publication date: 1994-02-09
Also published as: GB9216585D0; CA2101867C; TW217470B; CA2101867A1; ZA934080B; MX9304727A

Abstract

An antenna arrangement for a helicopter 100 comprises one or more electrically non-conductive rotor blades 101, each having an electrical conductor (210, 310, Fig 2) positioned parallel to the major axis of a respective rotor blade 101. Means are provided to connect the antenna to apparatus for the reception or transmission of radio waves which is located in the fuselage 102. The electrical conductor may be embedded within or affixed to the surface of the rotor blade or it may be a surface coating. The electrical conductor may also be used as a heater filament. <IMAGE>

Description

HF ANTENNA FOP. A HELICOPTER Field of the Invention The invention relates to transmission and reception of radio waves in the HF spectrum and more specifically to use of a rotor blade or rotor blades on a helicopter (or rotary winged aircraft) as an efficient antenna.

Background of the Invention Conventionally antennas for helicopt:ers have been mounted close to the body of the helicopter. Typically an antenna has consisted of a rigid member parallel to and spaced from the helicopter body by spacers.

An alternative that has been used is where the antenna consists of a wire stretched between two spacers used to space the antenna awa.y from the helicopter body. Insulators join the wire onto the spacers. The spacers are usually relatively short whi.ch result in the antenna being placed close to the body of the helicopter.

United States patent 4,042,929 shows a navigation system in which antennas are used at the tips of each of the blades of a helicopter rotor. The received signals are processed on the rotor blade and introduced into the body of the helicopter by means of slip rings and contact brushes. The antennas consist of a series of dipoles flatt.exled along the centreline of each blade, positioned proximate to the tips of the blades.

A popular band of frequencies for operation. of military and commercial communications is the HF band of frequencies. This band extends between 2 MHz and 30 MII? and has a number of technical and tactical advantages over the higher frequencies that are availa'ble. In a typical modern installation, military VHF (30 " 170 tEz) and 01W (225 - 400 MHz) are used alongside the HF band for communication between the helicopter and ships or other helicopters and aircraft.

Some advantages of the use of HF band frequencies are: HF band frequencies are the hi.ghest frequencies that will reflect from the ionosphere to provide long range skip communication; Higher frequencies offer only line of site communication and cannot go over the horizon; Propagation attenuation increases with frequency by a factor of 20 log frequency; The natural phenomena of range, antenna efficiency and atmospheric noise are all functions of frequency and the best compromise of the factors is achieved between 2 and 30 MHz; More efficient power amplifiers are available at the HF band of frequencies.

One factor that limits the HF band performance on aircraft and helicopters is the length of the antenna. For maximum efficiency, the antenna should be equal in length to the wavelength. The wavelength in metres can be calculated as the velocity of propagation of the radio waves in metres per second divided by the frequency in Hertz. The velocity of propagation of radi.o waves is constant and is approximately equal to 3 x 108 metres/second.

For UHF communications (typically 300 MHz), the wavelength calculated from the above equation is 1.0 metres. This is a practical length for an antenna, such as those types described earlier, to be mounted on a helicopter, despite intense competition for space from electronic equipment and in military helicopters also from heavy armament.

In the HF band, at a frequency of 3 MHz, the wavelength calculation shows that an antenna of length 100 metres is required. This is an impractical length for a helicopter. This can be overcome by using a submultiple of the ideal length obtained from the wavelength calculation. However the antenna efficiency falls as the length of the antenna is reduced.

Conventional helicopters have rotor blades made primarily of metal.

These rotor blades are fixed to the gearbox and engines via a substantial conductive path of metallic parts making the blades difficult to employ as an antenna.

The new generation of helicopters are moving away from metallic rotor blades to using composite construction.s. An example is the Aerospatiale Ecureuil which has a Starflex rotor blade that is made mainly of glass fibre. Other helicopters have blades made of carbon and glass fibres with internal foams.

Disclosure of the Invention Accordingly the invention provides an antenna for use with apparatus for transmission or reception of radio waves in a rotary winged aircraft having a body provided with rotor blades, the antenna comprising one or more electrically non-conductive rotor blades, one or more electrical conductors, each conductor being positioned parallel to the major axis of a respective rotor blade, and means for providing a connection from the electrical conductor to the apparatus for transmission or reception of radio waves.

Preferably the length of each electrical conductor is substantially similar to that of the rotor blade. The radio waves transmitted or received preferably have wavelengths in the range from 10 metres to 150 metres.

In a first embodiment the electrical conductor is positioned along the leading edge surface of the rotor blade. In a second embodiment the electrical conductor is also used as n heater filament and the antenna further comprises means for combining or separating the radio signal being transmitted or received from the power being supplied to the heater filament.

In a third embodiment the electrical conductor is embedded within or affixed to the surface of the rotor blade. In a fourth embodiment the electrical conductor is a conductive coating applied to the surface of the rotor blade.

Preferably the connection means comprises a first and second surface, the first surface rotating with the rotor blades, the second surface being fixed to the fuselage and the first and second surfaces being connected capacitively. Another means of connection comprises a first and second transformer winding, the first winding rotating with the rotor blades, the second winding being fixed to the fuselage and the first and second windings being connected inductively. Yet another means of connection comprises slip rings and contact brushes.

The invention also provides a communications system for transmission and reception of radio waves in a rotary winged aircraft having a body provided with rotor blades, the system comprising one or more electrically non-conductive rotor blades, one or more electrical conductors, each conductor being positioned parallel to the major axis of a respective rotor blade, apparatus for transmission or reception of radio waves and means for providing a connection from the electrical conductor to the apparatus for transmi.ssion or reception of radio waves.

Brief Description of the Drawings Embodiments of the invention will now be descri.bed, by way of example, with reference to the accompanying drawings, in which: Figure 1 is schematic view of a. helicopter showing the rotor blades positioned above the helicopter; Figure 2 is a section of one of the blades shown in figure 1 incorporating a first embodiment of the invention; Figure 3 is a section of one of the blades shown in figure 1 incorporating a second embodiment of the invention; Figure 4 is a cross-sectional diagram of a means for providing an electrical connection between blades, such as those of figure 2, and apparatus within the helicopter of figure 1; Figure 5 is a cross-sectional diagram of another means for providing an electrical connection; ; Figure 6 is a cross-sectional diagram of yet another means for providing an electrical connection.

Detailed Description of the Invention Figure 1 shows a helicopter 100 with rotor blades 101 positioned above the fuselage 102 of the helicopter 100. The helicopter 100 is approximately 15 metres long and carries various equipment 103 fixed to the exterior of the fuselage 102. The fuselage 102 of the helicopter 100 is a ground plane for the frequencies used for radio reception and transmission. Because of the difficulty of finding an area cf the fuselage 102 that is not a ground plane and does not already have other equipment 103 attached to it, HE antennas are llsually short antennas with resultant low efficiency. This gives a reduction in system performance that is difficult to overcome wi.thout increasing the transmitter power.

The efficiency of an antenna can be shown to approximate as follows in an isotropic radiator; Efficiency = 2.5 1.5 2.5 + --------------- (2 * PT * (----------? (wavelength) where h = the length of the antenna in metres wavelength = the wavelength of the transmitted or received signal in metres PI = the constant 3.1415926 approx.

From the above equation, it can be seen that at 3 MHz a 100 metre long antenna could have an efficiency of 1, whilst a 2 metre long antenna, working at the same frequency could have an efficiency of only 0.0033. This degradation of efficiency due to the reduced length exists when the antenna is used both in the transmit mode and in the receive mode. The receive antenna gain is dependent on the efficiency as well as other factors such as the directionality of the antenna.

The relationship between antenna gain and the overall system performance can be represented by the following equation: Pr = Pt + Gt + Gr - 20 log f - 20 log R - 32.4 - La where Pr = Received power (dBm) Pt = Peak transmitter power (dBm) Gt = Transmit antenna gain (dB) Gr = Receive antenna gain (dB) f = Frequency (MHz) R = Range (Nautical miles) La = Additional losses (dB! The factor of 32.4 is a constant which reflects the units used for frequency and range.

The frequency and required range are fixed for any given scenario.

Additional losses are all minimised in any good design. If a full wavelength antenna is used from, for example, a ship then a gain of 1 might be possible. Conventional helicopters are forced to use inefficient antennas, as measured by the equation for the antenna efficiency given above, so that the only option to achieve the required performance is to increase the transmitter power.

If the communication link is from hlicopter to helicopter, then from the same equation we can see that the link performance will suffer both at the transmission and reception antennas.

Use of a rotor blade 200 or rotor blades of the helicopter, such as is shown in figure 2, as an antenna allows the antenna to be considerably longer and hence more efficient. With the exception of the radiating element included in the blade and described later, the structure of the rotor blade 200 must be substantially electrically non-conducting. The use of the rotor blades as an antenna improves the system performance, particularly for helicopter to helicopter communication, where the antenna efficiency has an effect both on the transmit antenna gain and the receive antenna gain.

Figure 2 shows a first embodiment of the invention where the rotor blade 200 has an erosion shield 210 fitted to the leading edge. The erosion shield 210 is intended to protect the leading edge of the rotor blade 200 from damage by particles in the air such as dust particles.

It also provides some protection from such things as striking: foliage, during hovering or landing, when close to the ground. The erosion shield 210 will typically be fabricated from a metal. such as titanium.

The erosion shield 210 extends for the length of the rotor blade and so provides an antenna which is substantially equal to the length of the rotor blade. By making continuous simultaneous connection to more than one rotor blade, an effective antenna length of about twice the length of the rotor blade can be obtained. At 3 MHz this will provide an efficiency of around 0.7 (depending on the length of the rotor blades), compared with 1 for the ideal antenna length of 100 metres and 0.0033 for a 2 metre antenna. In the preferred embodiment all of the rotor blades of the helicopter are used.

Electrical connections are made from the erosion shields 210 to the apparatus in the fuselage 102 of the helicopter 100 by one of three methods which are described later with reference to figures 4 to 6.

Typically bonding jumpers are used to ground the erosion shields 210 to provide protection against lightning strikes and electromagnetic pulse.

This protection can be retained, if required, by using spark gaps of a suitable breakdown voltage as is well known to those skilled in the art.

Similarly, methods known to those skilled in the art to prevent static build up caused by the motion of the rotor blades can be used (in the form of discharge wicks, for example).

Figure 3 shows a second embodiment of the invention which uses a heater filament 310 that is already present in the rotor blade 300 as an antenna.

At the front edge of the rotor blade 300 is a heater filament 310 which is used for deicing the leading edge of the rotor blade. The transmitted and received signals are provided to the radio apparatus in the fuselage 102 of the helicopter 100 by use of the same means used for the transfer of power to the heater filament 310. This will be described later with reference to figures 4 to 6. This embodiment will also require a means to combine the transmitter signal with the power for the heater filament 310 and also for separating the receiver signal from the power. The means for achieving this are well known to those skilled in the art, and are widely applied to such areas as the dual use of windscreen elements in automohi.les as both demisting elements and receiving aerials.

As with the erosion shield 210 used in the first embodiment, the length of the antenna will be substantially similar to the length of the rotor blade 300, or twice this value if multiple rotor blades are used.

The overall system performance is improved for a negligible increase in system cost.

If the transmitter power is kept constant, then a greater signal power is radiated, giving the transmitted signal greater immunity against radio jamming.

The antenna is positioned above the helicopter fuselage so that the radiation from the antenna becomes omnidirectional with no shading of the antenna due to the fuselage itself.

When the helicopter is hovering at low altitudes the antenna is placed higher relative to the ground giving improved communications over an antenna placed on the fuselage of the helicopter.

The losses shown in the equation for system performance as additional losses include capacitive losses from the antenna to the airframe. These losses are reduced because of the greater distance from the fuselage to the antenna.

When compared with a conventional antenna of the type described earlier which consists of a wire antenna with insulators spaced from the helicopter, the antenna of the present invention is much more mechanically robust and less liable to damage in the ground handling process. For a helicopter that has a folding tail for storage in restricted spaces the antenna of the present invention is less liable to damage during this process also.

The safety of personnel is improved because the transmitting antenna is placed further away from the occupants, with the resultant decreased exposure to electromagnetic fields. The possibility of reduci.ng the transmitter power also reduces exposure to electromagnetic fields.

Figure 4 shows a method of making a connection from the radio apparatus in the fuselage 102 of the helicopter 100 to the antenna. It uses capacitive coupling between a rotating plate, which can be in the form of a first cylinder 410, attached to the antenna (such as erosion shield 210 or heater filament 310) on one or more rotor blades and another fixed plate, which can also be in the form of a second cylinder 412, concentric with the first cylinder. An air space 411, which acts as a dielectric, exists between the two cylinders. An insulator 422 is used to insulate the rotor blade shaft 421 from the capacitor formed by the two cylinders (410, 412).

Figure 5 shows a means of making a connection to the antenna using inductive coupling. One rotating winding 510 of an air spaced transformer is connected to the rotor blades via cable 420 and the rotor shaft via connection 511. Another fixed winding 521 of the air spaced transformer is connected to the radio apparatus in the fuselage 102 of the helicopter 100 via cable 520.

Figure 6 shows another means of making a connection to the radiating element. The radiating element may be an erosion shield 210, a deicing element 310, an embedded wire or a conductive coating. A cable 420 is connected to the element and follows a path to the rotor blade shaft 421. Here contact is made to the apparatus in the fuselage 102 of the helicopter 100 by means of slip rings 620 and contact brushes 623. The cable 420 is connected to a conducting slip ring 620 rotating with a shaft 421 carrying the helicopter rotor blade 101. Brushes 623 cooperate with slip rings 620 and are tied to a stationary conductor.

Whether the heater filament 310 or the erosion shield 210 (or even a conventional HF antenna) are used as an antenna and whatever means of connection to the antenna is used, it is necessary to include in the system an antenna tuning unit to allow the impedance of the antenna to be matched to the impedance of the transmitter and receiver over a wide range of frequencies. The design and construction of antenna tuning units is well known to those skilled in the art and will not be discussed further.

A third embodiment of the antenna involves a wire embedded into the blade. This wire is preferably placed within a cavity in the rotor blade or is made by including a layer of metallic foil on the surface of the rear section of the blade 200. Connection to the antenna is achieved by any one of the three methods described above, that is by slip rings 620 and brushes 623 or by capacitive or inductive coupling.

A fourth embodiment of the antenna utilises a nickel spray that is used to provide protection against corrosion on composite blades. A connection is made to the conductive coating as described above for the wire embedded in the blade 200.

Claims

1. An antenna for use with apparatus for transmission or reception of radio waves in a rotary winged aircraft (100) having a body (102) provided with rotor blades (101), the antenna comprising: one or more electrically non-conductive rotor blades (200, 300); one or more electrical conductors (210, 310), each conductor being positioned parallel to the major axis of a respective rotor blade (200, 300); and means (410, 412, 510, 521, 620, 623) for providing a connection from the electrical conductor to the apparatus for transmission or reception of radio waves.

2. An antenna as claimed in claim 1 wherein the length of each electrical conductor (210, 310) is substantially similar to that of the rotor blade (200, 300).

3. An antenna as claimed in any preceding claim wherein the radio waves transmitted or received have wavelengths in the range from 10 metres to 150 metres.

4. An antenna as claimed in any preceding claim wherein the electrical conductor (210) is positioned along t:he leading edge surface of the rotor blade (200).

5. An antenna as claimed in claims 1 to 3 wherein the electrical conductor is also used as a heater filament (310) and further comprising means for combining or separating the radio signal being transmitted or received from the power being supplied to the heater filament (310).

6. An antenna as claimed in claims 1 to 3 wherein the electrical conductor is embedded within or affixed to the surface of the rotor blade (101).

7. An antenna as claimed in claims 1 to 3 wherein the electrical conductor is a conductive coating applied to the surface of the rotor blade (101).

8. An antenna as claimed in any preceding claim wherei.n the connection means comprises a first (410) and second (412) surface, the first surface (410) rotating with the rotor blades (101), the second surface (412) being fixed to the fuselage (102), and the first (410) and second (412) surfaces being connected capacitively.

9. An antenna as claimed in claims 1 to 7 wherein the connection means comprises a first (510) and second (521) transformer winding, the first winding (510) rotating with the rotor blades (101), the second winding (521) being fixed to the fuselage (102) and the first (510) and second (521) windings being connected inductively.

10. An antenna as claimed in claims 3 to 7 wherein the connection means comprises slip rings (620) and contact brushes (623).

11. A communications system for transmission and reception of radio waves in a rotary winged aircraft (100) having a body (102) provided with rotor blades (101), the system comprising: one or more electrically non-condnctive rotor blades (200, 300); one or more electrical conductors (210, 310), each conductor being positioned parallel to the major axis of a respective rotor blade (200, 300); apparatus for transmission or reception of radio waves; and means (410, 412, 510, 521, 620, 623) for providing a connection from the electrical conductor to the apparatus for transmission or reception of radio waves.