Note: Descriptions are shown in the official language in which they were submitted.
~" UK9-92-045 2 ~ D ~
DIRECTIONAL HF 1~ FOR A HELI~
Eield of the Invention
The inVention relates to transmission and reception of radio
waves in the HF spectrum and more specifically to use of
rotor blades on a helicopter (or rotary winged aircraft) as
an efficient directional antenna.
R~Ck.J.~l~ of the Invention
Conventionally antennas for helicopters have been mounted
clo~e 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 away from the
helicopter body. Insulators join the wire onto the spacers.
The spacers are usually relatively short which result in the
antenna being placed close to the body of the helicopter.
Both of these alternative antennas can be made directional,
but result in a shorter effective length of antenna. In
addition any directionality is fixed relative to the
orientation 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 o~ dipoles flattened along the
centrel~ine 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 MHz and has a number
.'~ ;. ' ~: ~ ':
~ .~
'~ ~';.'..,
: '' '~ '
UK9-92-045 2
21~1903
of technical and tactical advantages over the higher
frequencies that are available. In a typical modern
installation, military VHF (30-170 MHz) and UHF (225-400
MHz) are used alongside the HF band for communication
between the helicopter and ship~ or other helicopters and
aircraft.
~, . .
Some advantages of the use of HF band frequencies are that
HF band frequencies are the highest 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 and propagation
attenuation increases with frequency by a factor o~ 20 log
frequency. The natural phenomena of range, antenna
efficiency and atmospheric noise are all functions of
frequency and the best compromise of the ~actors 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 ln Hertz. The velocity of
propagation of radio waves is constant and is approximately
equal to 3 x 108 metresjsecond.
~ .
For UHF communications (typically 300 MHz), the wavelength
calculated from the above equation i~ 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 hellcopter.
This can be overcome by using a submultiple of the ideal
length obtained from the wavelength calculation. However
' ~''
~ UK9-92-045 3
2~01~03
the antenna efficiency falls as the length of the antenna is
reduced.
Another factor that limits the HF band performance on
aircraft and helicopters is the difficulty of providing a
directional antenna. A directional antenna has an increased ~ -
gain in a direction or directions relative to the antenna.
It also has a decreased gain in other directions relative to
the antenna. The lack of directionality of an antenna
results in a loss of communication range compared with an
antenna having directionality and can also result in the
signal being received by other than the receiver for which
it was intended. In order to provide optimal communication
between the directional antenna and another antenna, the
directional antenna needs to be oriented so that a direction
of increased gain is oriented toward the desired receiving
antenna. ~-~
. ~, .
This orientation can be achieved by physical rotation of a ~;
directional antenna to point towards the other antenna,
however this further limits its length and hence e~ficiency, :
thus offsetting any benefit from the increased gain due to
directionality.
'~'"~"'''
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 constructions. An -
example is the Aerospatiale Ecureuil which has a Starflex
rotor blade that is made mainly of glass fiber. Other
helicopters have blades made of carbon and gla~s fibers with
internal foams. -~
Di~closure of the Invention
Accordingly the invention provides a directlonal antenna for
use with appa~atus for transmission or reception of radio
;~:
UK9-92-045 4
:
2101~Q3
waves in a rotary winged aircraft having a body provided
with rotor b~ades, the antenna comprising three or more
electrically non-conductive rotor blades, capable of being
rotated with respect to the body of the rotary winged
aircraft around an axis parpendicular to the blades; two
first electrical conductors, each conductor being positioned
parallel to the major axis of a respective rotor blade and
being in contact therewith; one or more second electrical
conductors, each conductor being positioned parallel to the
major axis of a respective rotor blade, the second
electrical conductors not being electrically connected to
the first electrical conductors; means for providing a
connection from the plurality of first electrical conductors
to the apparatus for transmission or reception of radio
waves; means for dynamically selecting the first electrical
conductors from all of the electrical conductors as those
having an angular position nearer to a first predetermined
angular position relative to the body of the rotary winged
aircraft than others of the conductors, and for dynamically
selecting the others of the conductors as second electrical
conductors.
In a first embodiment the second electrical conductors are
electrically connected to the body of the rotary winged
aircraft.
In a second embodiment one or more of the second electrical
conductors are provided with a signal that is out of phase
with the signal provided to the first electrical conductors.
Preferably one or more of the second electrical conductors
not provided with an out of phase signal are electrically
connected to the body of the rotary winged aircraft.
: ~;
Preferably the connection means comprises slip rings and
contact brushes. The slip rings and contact brushes are
preferably also the means for dynamically selecting
conductors as first conductors or as second conductors. In
another embodiment the selecting means comprises electrical
diodes and means for controlling the direct current biasing
of the electrical diodes.
.:..
;,. .,''''
'';
UK9-92-045 5
2~019Q3
Preferably the directional antenna further comprises a
control means for maintaining said first predetermined
angular position with respect to a known geographic point.
The control means preferably comprises a stepping motor.
Preferably the directional antenna further comprises means
for maintaining the first predetermined angular position
constant with respect to a remote apparatus for transmitting
or receiving radio waves.
In a preferred embodiment the directional antenna the length
of each electrical conductor is substantially similar to
that of the rotor blade and the radio waves transmitted or
received have wavelengths in the range from 10 metres to 150
metres.
Also provided is 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 apparatus for transmission or reception of radio
waves; three or more electrically non-conductive rotor
blades, capable of being rotated with respect to the body of
the rotary winged aircraft around an axis perpendicular to
the blades; two first electrical conductors, each conductor
being positioned parallel to the major axis of a respective
rotor blade and being in contact therewith; one or more
second electrical conductors, each conductor being
positioned parallel to the major axis of a respective rotor
blade, the second electrical conductors not being
electrically connected to the first electrical conductors;
means for providing a connection from the plurality of first
electrical conductors to the apparatus for transmission or
reception of radio waves; and means for dynamically
selecting the first electrical conductors ~rom all of the
, ~
electrical conductors as those having an angular position
nearer to a first predetermined angular position relative to
the body of the rotary winged aircraft than others of the
conductors, and for dynamically selecting the others of the
conductors as second electrical conductors. ~ ~;
.
. ~
. ~
UK9-92-045 6
Brief Description of the Dr~bQng~ -
Embodiments of the invention will now be described, by way
of example, with reference to the accompanying drawings, in
which: ~.
Figure l 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 l
incorporating an element of an HF antenna;
Figure 3 is a polar diagram of radiation from or to an ~ : .
omnidirectional antenna such as one formed from the elements
of figure 2; ;~
Figure 4 is a view of a prior art system using
omnidirectional antennas such as that of figure 2; .t:
'
Figure 5 is a polar diagram of radiation from or to a
directional antenna such as is used in the present
invention;
Figure 6 is a view of a communications system using at least ;.
one directional antennas such as that of the present
invention; ;i~
Figure 7 is a perspective view of a commutator used in the
present invention;
Figure 8 is a diagram used to illustrate the connection via .
the commutator of figure 7 of the electrical conductors in
the invention; . . ;
Figure 9 is a schematic diagram ~howing the selection of ~.'''i
individual coils used as an alternative to the commutator of '~
figure 7 in the invention.
Detailed Description of the Invention .. .
''', :'. ~,
~ .:
UK9-92-045 7
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
of the fuselage 102 that is not a ground plane and does not
already have other equipment 103 attached to it, HF antennas
are usually short antennas with resultant low efficiency.
This gives a reduction in system performance that is
difficult to overcome without increasing the transmitter
power. In addition, because of the difficulty of providing a
rotatable antenna, HF antennas are usually omnidirectional.
:
The efficiency of an antenna can be shown to approximate as
follows in an isotropic radiator;
Efficiency = 2.5
_______________________ :::
1.5
2.5 -~ ---------------
(2 * PI * h)3
wavelength
where h = the length of the antenna in metres
wavelength = the wavelength of the transmitted or
received i~
. " ~
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 fre~uency, could ~ ;
have an efficiency of only 0.0033. This degradation of ; i
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 and
the directionality of the antenna as well as other factors. ~'~
::
UK9-92-045 8
The relationship between an~ ~nQ~g9aQn and the overall system
performance can be represented by the following equation:
Pr = Pt + Gt ~ Gr - 20 loq 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 minimized in any good
design. If a full wavelength omnidirectional antenna is used
from, for example, a ship then a gain of 1 might be
po~sible. Conventional helicopters are forced to use
inefficient omnidirectional 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 helicopter 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 a radiating element included in the
blade, the structure of the rotor blade 200 must be i ~.'
substantially electrically non-conducting. The use of the
rotor blades as an antenna improve~ the system performance,
particularly for helicopter to helicopter communication, '~
~ ''" ''''.'~':.''
; ~ , ,.'.,
UK9-92-045 9
2 ~ Q '~ ~:
where the antenna efficiency has an effect both on the
transmit antenna gain and the receive antenna gain.
Figure 2 shows a rotor blade 200 which 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.
~ ~.
An alternative to using the erosion shield is to use a
heater filament 212 that is already present at the front
edge of the rotor blade 200 as an antenna. The heater
filament 212 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
transferiof power to the heater filament 212. Use of the
heater filament also requires a means to combine the
transmitter signal with the power for the heater filament
212 as well as for separating the receiver signa} 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 automobiles as both
demisting elements and receiving aerials. As with the usei of
the erosion shield 210, the length of the antenna will be
substantially similar to the length of the rotor blade 200.
!
- ' UK9-92-045 10
2 ~ Q 3
Electrical connections are made from the erosion shields 210
to the apparatus in the fuselage 10~ of the helicopter 100
by brushes and slip rings or the like. Typically, when the
present invention is not implemented, 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, when the present
invention is implemented, 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 wic~s, for
example).
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 rotor blade
antenna is much more mechanically robust and less liable to
damage in the ground handling process. For a helicopter that
' UK9-92-045 11
2 ~ Q 3
has a folding tail for storage in restricted spaces the
rotor blade antenna 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 reducing the transmitter power also reduces
exposure to electromagnetic fields.
Directional antennas are used to concentrate the radiated
field strength either from a transmitting antenna, or to a
receiving antenna. Figure 3 shows the polar diagram of
radiation from or to an omnidirectional antenna 300. The
radial position represents the relative field strength
transmitted or received in that angular direction. The polar
plot of an omnidirectional antenna using conductors affixed
to all of the rotor blades of a helicopter is of this form.
~::
,
Figure 4 shows a possible situation using omnidirectional
transmissions. Helicopter 411 transmits using an
omnidirectional aerial so that the signal strength received
by ships 421 and 422 is dependent only on their radial
distance from helicopter 411 and not on their angular
position. The vessel 421 with which it is desired to
communicate actually receives a lower signal strength than
vessel 422. Electromagnetic radiation emissions from the
transmitting helicopter can be monitored in such a
situation. Signals are often encoded by an encryption
device, however the fact that radio signals are detected at
all may be useful information.
Figure 5 shows the polar plot of a typical directional
antenna 500, in this case a Yagi type antenna. Angular
portions enclosed by secondary lobes 531, 532 have a higher
gain than adjacent areas 521, 522 but this gain is much
lower than the gain in angular positions enclosed by the
main lobe 510 or the gain from an omnidirectional antenna
300 having a polar diagram such as that of figure 3. The
gain of the antenna 500 is increasèd within the angular
'~ UK9-92-045 12
2~ als~3
positions enclosed by the main lobe 510 at the expense of
radiation in the unwanted areas 521, 522. The increased gain
can be shown to equate as follows;
Gain (ratio) = (4 * pie * Ae) / (Wl * Wl)
Where Ae = effective area of the lobes, ~:
and Wl = wave length of the transmitted or ~" ;
received signal.
Increased range is achieved in the desired direction (within
the angular positions enclosed by lobe 510) during both
transmission and reception. Also, as described below with ;'~
reference to figures 4 and 6, increased security is achieved ~ ;~
against a signal being received by other than the receiver
for which it was intended. ;
Figure 6 shows the same situation as figure 4 but helicopter -
611 has a directional antenna having a polar diagram such as ~
that of figure 5. The directional antenna may be used to '-
communicate with, for example, a ship, whilst minimizing the -~
risk of detection.
A coupling method is necessary to link the rotating antenna
to the radio equipment. A slip ring and brushes may be ~i
employed for this purpose. If a slip ring in the form of a
commutator is used, then only during a portion of thé arc
described by the rotating antenna as it rotates, is the
antenna selectively connected to the apparatus for receiving i ~
and transmitting radio signals. Electromagnetic radiation ~ -
from the antenna is only received from or transmitted to
another antenna located in that portion when the antenna is
connected to the apparatus for receiving and transmitting
radio waves.
Figure 7 is a perspective view of a commutator 700, where
some of the elements of a rotating antenna/blade assembly ~ ;
are used for transmission whilst other element~ of the
antenna/blade assembly are grounded to the aircraft -~:
structure, or fed with a phase shifted signal during
~ UK9-92-045 13
21019~'3
selected segments of its sweep, in order to produce a
controllable directional antenna.
The commutator 700 consists of a shell 711, which does not
rotate with the rotor blades, and a part of the rotor blade
shaft 712 which rotates with the rotor blades. The shell 711
contains a number of slip rings, such as 721 and 722,
preferably one slip ring per rotor blade. The slip rings are
connected to the apparatus for transmitting and receiving
radio signals or to the helicopter body as described later.
The rotor shaft has brushes, such as 741 and 742, which are
connected to one or more of the electrical conductors on the
rotor blade. The brushes 741, 742 provide a connection to a
corresponding slip ring 721, 722.
Each slip ring is divided into angular portions, each
portion being connected to either the helicopter body or to
a zero phase shift signal or to a phase shifted signal. In
this way the signal provided to any one electrical conductor
on the rotor blades can be made dependent on the conductors
physical position relative to the helicopter body. Normally
there will be a gap 731, 732 between breaking of a
connection from the electrical conductor to the making of
another connection to the electrical conductor.
The use of a commutator assumes and takes advantage of the
capability of achieving directionality in the antenna
pattern. It al~o further offers the advantage of using the
commutation in various configurations to control the
direction of the directed energy lobe 510. By proper
selection of the connections to the electrical conductors,
the main beam of the antenna can be made more directional
and its direction of maximum radiation can be controlled
relative to the aircraft longitudinal axis.
The directionality is achieved by feeding two or more blades
with the transmitter power and utilizing one or more o~ the
remaining blades either with phase shifted energy or by
grounding them to reduce the radiated field intensity of the
back lobe.
UK9-92-045 14
.. , ,' ~ ~
Grounded configuration 2 10 1 9 Q 3
In a first embodiment, if the antenna is grounded as it
rotates in the portion of the arc located at 180 degrees
from the active portion, then the directivity is further
enhanced. Figure 8 shows a typical composite rotor having
five rotor blades. This gives an angle between the rotor
blades of 72 degrees. If a commutator is arranged to connect
the apparatus to the antenna conductor of two rotor blades
801, 805 at any one time, as they rotate, then this will
form a 'V' configuration antenna that has directional
properties without any contribution to directionality from
the remaining three rotor blades. The antenna conductors of
rotor blades 802, 804 are preferably grounded to act as a
reflector and further add to the directivity of the antenna.
Blade 803 ma~ either be grounded or may be open circuit. As
the rotor blade assembly rotates in an anticlockwise
. . .
direction the rotor blade which was connected as blade 802
rotates to the position of rotor blade 801. In a preferred
embodiment there is a gap in the commutator where the blade
is open circuit between the times when it is connected to
the apparatus and when it is grounded. ~;
A vertically staggered system is used for the commutator, as
shown in figure 7. This technique ensures that the antenna
conductors of two blades are always connected to the
apparatus and optimum coupling is achieved.
The electrical coupling can also be achieved by inductive
.
coupling or capacitor coupling using multiple coils or
capacitors, one for each blade. The individual coils or
capacitors can be selected at the appropriate time by means
of a reversed biased diode control, for example.
Figure 9 shows such an arrangement using inductive coupling
for the connection of rotor blades 805 and 801 when these
rotor blades are connected to the apparatu~ for transmission
and reception of radio waves. A connection 901, preferably a
coaxial connection, is made from the apparatus for
., ~ :..
UK9-92-045 15
2~0~0~
transmitting and receiving radio waves to the apparatus
shown in figure 9.
The following description will assume that a signal is to be
transmitted by rotor blades 805 and 801. For a signal to be
received the path would be reversed. The signal is
inductively coupled through transformer 902 for d.c.
isolation. A signal is applied via connection 906 in order
to control the bias of diode 904. This signal is a pulse
synchronized with the rotor shaft position and present when
it is desired for the transmitted signal to be passed on to
rotor blades 805 and 801. The signal is applied through an
r.f. choke 905 to prevent short circuiting of the
transmitted signal through the bias supply. When the pulse
is present the diode is forward biased and allows the
transmitter signal to pass through to the inductive coupling
mechanism 910 to the rotor blades 805 and 801. Coil 912 is
on the rotor shaft and coil 911 is mounted concentric with
coil 912, but is fixed to the fuselage 102. When the pulse
is not present the diode is reverse biased and the
transmitter signal cannot pass. The transmitted signal
returns to the isolating transformer through d.c. blocking
capacitor 903. The diodes must be capable of handling the
radiated r.f. power and the blocking capacitor must be
capable of conducting the r.f. current.
In a preferred embodiment with five such rotor blades (801,
802, 803, 804, 805), a single connection 901, isolating
transformer 902 and d.c. blocking capacitor 903 are used.
For each blade there are separate diodes 904 and inductive
coupling mechanisms 910. The series combinations of the
diodes and inductive coupling mechanisms are connected in
parallel between the isolating transformer and the d.c.
blocking capacitor. The number of series combinations is the
same as the number of blades, with the coils 912 being
connected between respective pairs of rotor blades.
The r.f. chokes 905 connect from each diode to a respective
source of timing pulses. The timing pulses are obtained from
the rotor shaft by using a magnet ~ixed to the rotor shaft
~ UK9-92-045 16
21~19Q3
and a pickup coil for each pulse required. Alternatively, an
arrangement of photo-optical coupling may be used. The
pulses are shaped and the amplitude processed as re~uired.
They are then passed through a delay means, the amount of
the delay being capable of being controlled external to the
delay means. The control of the delay may be used to effect
control of the angular position of the radiating lobe with
respect to a fixed geographic position. All of the pulses
are delayed by the same amount.
Alternatively the pulses could be generated by a pulse
generator, synchronized by one shaft position pulse. Each
control pulse is displaced from the preceding one by 72
degrees in the time domain in the case of a 5 bladed rotor
assembly.
Phase ~hift configuration
In a second embodiment two adjacent rotor blades (801, 805)
are fed in a 'V' configuration with the primary (that is,
zero phase shift) radio frequency feed, which in itself
results in a directional pattern in bipolar form (that is
with a forwards lobe and a backwards lobe). However there is
still significant radiation directed in a direction opposite
to the desired direction. To reduce this rotor blades (802,
804) are driven with a phase shifted (that is, displaced
from zero degrees) signal to reinforce the forward lobe and
provide for some cancellation effect for the back lobe. The
amount of phase shift required may be established by
mathematical ~odelling, as is known to those skilled in the
art of antenna theory. Rotor blade 803 can also be utilized
in the same fashion (that is, fed with an out of phase
signal or grounded to act as a reflector).
As the blades rotate approximately 72 degrees
counterclockwise, the commutator wipers fed from the
apparatus for receiving and transmitting radio waves
disengage rotor blades 805 and 801 and instead engage blades
801 and 802 as the primary radiators. Blades 801 and 802 are
similarly disengaged and blades 802 and 803 now engage the
. ~
~'~ UK9-92-045 17
211~19Q3 ; ~
wipers with the phase controlled feed to provide
reinforcement of the main lobe and a cancellation effect on
the back lobe. The rotation of the commutator causes the
connection of the blades, both primary feed and phase
controlled feed, to change every 72 degrees of rotation.
In order to provide optimal communication between the
directional antenna and another antenna, the directional
antenna needs to be oriented so that a direction of
increased gain is oriented toward the desired receiving
antenna.
This can be achieved by rotating the commutator shell 711
that connects to the transmitting and receiving apparatus so
that the electrical conductors connect to the apparatus over
a dif~erent portion of the arc swept by the conductors. The
rotation of the commutator can be achieved by the use of a
s~epper mo~or.
All modern aircraft eguipment communicates with other
equipment on the aircraft via a digital data bus. This data
bus carries instructions from a central computer to the
avionic equipment. The stepper motor is controlled via an
suitable interface from such a data bus. The construction of
sbch an inter~ace is well known to those skilled in the art
of design of avionic equipment. A flight computer is given
the bearing of the receiver or transmitter with which it is
desired to communicate. It also has access to the present
course of the aircraft. From these it is able to determine
the required position of the commutator and control the
stepper motor and therefore the directionality of the
antenna. As the aircraft changes course, the antenna remains
orlented in the appropriate direction.
In all of the variations of the invention described above,
as well as in prior art antennas 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 frequen~ies.
The design and construction of antenna tuning units is well
.'~
: ':
~- UK9-92-045 18
21019Q3
known to those skilled in the art and will not be discussed
further.
. ;~ ''
';' ~', ~'''' '
;'~ ~'''', ' ','
: : .:.:
.. ~" .,
,:
. :.. ::~,.,:
? .,., ~
,""~
: " " ;''~
~"
,.',
' ~;' '';
