AN ANTENNA SYSTEM FOR POLARIZATION DIVERSITY
TECHNICAL FIELD
The present disclosure relates t o an antenna system for transmission and reception of
polarized signals, and in particular to transmission and reception of polarized radar signals.
BACKGROUND
Low frequency radar systems, i.e., involving wavelengths on the order of meters, and airborne
low frequency radar systems in particular, can be used for finding targets buried under ground
or hidden below camouflage or trees. Low frequency radar systems can also be applied in
establishing environmental parameters such as biomass.
The physics governing how low frequency radar signals interact with the ground depend t o a
high degree on the polarization state of the signal. Therefore, collecting data for both vertical
and horizontal polarization is often valuable and sometimes even necessary.
Due t o the comparably low frequencies involved, and the dependency on polarization state of
the radar signal, the design of radar antennas in the area of airborne meter wavelength radar
contains several significant challenges.
For instance, antennas must be physically quite large - the smallest high efficiency antenna is
half a wavelength dipole, meaning that such dipoles will be of meter size. A dipole is not
directive in contrast t o conventional radar antennas, which extends for many wavelengths not
only in one dimension, as a dipole, but in two dimensions. Just scaling such antennas is clearly
not feasible for low frequency radar. Also, dipoles are often required t o be wideband in the
sense that the radar often needs t o function and keep a reasonably constant antenna diagram
or antenna pattern across a bandwidth of at least octave order.
Also, the polarization state of transmitted and received radar signals must often be
controllable or selectable. In some applications, the polarization state also needs t o be
alternated with kHz order switching frequency, or used in parallel for horizontal and vertical
polarization, so that radar response for both polarizations can be collected.
Hence, there is a need for an antenna system for use with low frequency radar systems wh
is comparably compact in terms of size, and where the polarization state of the transmi
and received radar signals can be controlled.
SUMMARY
An object of the present disclosure is to provide antenna systems, vehicles, and methods,
which seek to mitigate, alleviate, or eliminate one or more of the above-identified deficiencies
in the art and disadvantages singly or in any combination.
This object is obtained by an antenna system comprising an antenna structure consisting of a
single antenna element having first and second antenna ports arranged to pass a respective
first and second antenna signal. The first and second antenna signals arranged to be derived
from a first common antenna signal and also arranged to be essentially equal in envelope. The
antenna structure is arranged to have an antenna pattern which is selectable between a first
antenna pattern having a first polarization and a second antenna pattern having a second
polarization substantially orthogonal to the first polarization. The first antenna pattern is
selected by setting the first and second antenna signal to have the same polarity on first and
second antenna ports. The second antenna pattern is selected by setting the first and second
antenna signal to have substantially opposite polarities on first and second antenna ports.
Thus, there is provided an antenna system particularly suitable for use with low frequency
radar systems in that the antenna system is compact in terms of size due to the single antenna
element, which is an advantage.
The provided antenna system brings an additional advantage in that the polarization state of
transmitted and received signals can be controlled or selected in a straightforward way by
setting the polarities of the first and second antenna signal.
According to one aspect, the antenna system further comprises a first antenna interface unit
comprising a 180 degree hybrid coupler, a switching unit, and a first common port for passing
the first common antenna signal. The 180 degree hybrid coupler has first and second coupler
ports connected to the first and to the second antenna port, respectively, as well as a
summation and a difference port connected to the switching unit. The switching unit is
arranged to connect the first common port of the antenna interface unit to either of the
summation port or the difference port of the 180 degree hybrid coupler, thus selecting
between the first and the second antenna pattern of the antenna system.
Thus, by the feature of the antenna interface unit, connecting a radar transceiver to the
antenna system is facilitated, which is an advantage. Further, switching between polarization
states, i.e., selecting the polarization of transmitted and received signals, is simplified due to
the feature of the switching unit, which is also an advantage.
According to another aspect, the first and second antenna ports are also arranged to pass a
third and a fourth antenna signal, respectively. The third and fourth antenna signals have
substantially identical envelopes and are derived from a second common antenna signal which
is substantially orthogonal to the first common antenna signal. The antenna system, according
to said aspect, comprises a second antenna interface unit. The second antenna interface unit
comprises a second and a third common port for passing the first and the second common
antenna signal, respectively, as well as a 180 degree hybrid coupler. The 180 degree hybrid
coupler has first and second coupler ports connected to the first and to the second antenna
port, respectively, as well as a summation and a difference port connected to the second and
third common ports, respectively, thus selecting the first polarization for one of the first and
the second common signal, and selecting the second polarization for the other of the first and
second common signal.
Thus, advantageously, the antenna system can be used simultaneously in both polarization
states. The first common antenna signal mainly resides in one polarization, the second
common antenna signal mainly resides in the other polarization.
According to a further aspect, the antenna structure comprises an elongated conductive
bridge having a length between first and second ends smaller than half of the wavelength
corresponding to the highest frequency of the first common antenna signal. The antenna
structure further comprises two elongated conductive legs arranged in parallel and having
respective lengths between first and second ends smaller than half of the wavelength
corresponding to the highest frequency of the first common antenna signal. Said legs are
attached at first leg ends to either end of the elongated conductive bridge in right angles with
respect to the conductive bridge, thus substantially forming a U-shape. The sum of lengths of
the elongated conductive bridge and the two elongated conductive legs is substantially equal
to half of the wavelength corresponding to the center frequency of the first common antenna
signa l . The f irst antenna port is connected to the second end of one leg, the second antenna
port is con nected to the second end of the other leg. The antenna structure is arra nged to
have a ground pla ne orthogona l to both legs and located approximately at the second ends of
the legs. The antenna structure has a tota l length, including elongated conductive bridge and
both legs, less tha n the wavelength corresponding to the highest frequency of the f irst
com mon antenna signa l .
According to an aspect, the elongated conductive bridge is extended by f irst and second
conductive extension units con nected at either end of the elongated conductive bridge, thus
substa ntial ly forming a P -sha pe, the tota l length of the elongated conductive bridge with
extension units being sma ller tha n the wavelength corresponding to the highest frequency of
the com mon anten na signa l .
Thus, adva ntageously, by any of the U-sha pe or P -sha ped antenna structu res disclosed herei n,
there is provided a wideba nd anten na of com para bly sma ll size which faci litates attai ning
com plia ncy with, e.g., aeromecha nica l requi rements and where the pola rization state of the
tra nsmitted and received signa ls can be control led and also alternated or even used in parallel
for horizontal and vertical pola rization so that, e.g., rada r response for both pola rizations can
be collected. This wil l be further discussed in the detailed descri ption below.
The feature of the extension units being sma ller tha n the wavelength corresponding to the
highest frequency of the com mon anten na signa l adva ntageously contributes to preventing
excitation of the bridge due to the length of the bridge bei ng on the order of a wavelength of
the f irst com mon signa l in size.
According to one aspect, the antenna system is ada pted to be mounted on an airborne
vehicle.
According to another aspect, the antenna system is ada pted to be mounted on a su rface
based vehicle.
The object is also obtai ned by an airborne vehicle arra nged to carry the anten na system of the
present disclosu re.
The object is further obtained by a method for selecting an anten na pattern of an anten na
system . The antenna system com prising an antenna structure consisting of a single antenna
element having f irst and second antenna ports arra nged to pass a respective first and second
antenna signal. The antenna pattern of the antenna structure is arranged t o be selectable
between a first antenna pattern having a first polarization and a second antenna pattern
having a second polarization substantially orthogonal t o the first polarization. The method
comprising the steps of receiving a first common signal, and deriving the first and second
antenna signals from the first common signal, as well as setting the first and second antenna
signal t o have the same polarity on first and second antenna ports in case the first antenna
pattern is selected or setting the first and second antenna signal t o have substantially opposite
polarities on first and second antenna ports in case the second antenna pattern is selected.
The vehicles and the method all display advantages corresponding t o the advantages already
described in relation to the disclosed antenna system.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects, features, and advantages of the present disclosure will appear from the
following detailed description, wherein some aspects of the disclosure will be described in
more detail with reference to the accompanying drawings, in which:
Figures 1-4 show block diagrams of antenna systems of the disclosure, and
Figures 5-6 schematically show radiation patterns of disclosed antenna structures, and
Figures 7-8 schematically show airborne vehicles comprising an antenna system of the
disclosure, and
Figure 9 shows a flowchart illustrating methods of the disclosure.
DETAILED DESCRIPTION
Aspects of the present disclosure will be described more fully hereinafter with reference t o
the accompanying drawings. The apparatus, vehicles, and method disclosed herein can,
however, be realized in many different forms and should not be construed as being limited t o
the aspects set forth herein. Like numbers in the drawings refer t o like elements throughout.
The terminology used herein is for the purpose of describing particular aspects of the
disclosure only, and is not intended t o limit the invention. As used herein, the singular forms
"a", "an" and "the" are intended to include the plural forms as well, unless the context clearly
indicates otherwise.
In the following, we will mainly discuss antenna behavior during transmission - reception
behavior is assumed substantially the same by reciprocity.
Fig 1 shows an antenna system 100 comprising an antenna structure 110 consisting of a single
antenna element. The single antenna element has first 111 and second 112 antenna ports
arranged to pass a respective first and second antenna signal, which are derived from a first
common antenna signal J i and arranged to be essentially equal in envelope. The antenna
pattern of the antenna structure 110 is arranged to be selectable between a first antenna
pattern having a first polarization and a second antenna pattern having a second polarization
substantially orthogonal to the first polarization. The first antenna pattern is arranged to be
selected by setting the first and second antenna signal to have the same polarity on first 111
and second 112 antenna ports, while the second antenna pattern is selected by setting the
first and second antenna signal to have substantially opposite polarities on first 111 and
second 112 antenna ports.
Herein, 'essentially equal in envelope' means that the first antenna signal is essentially equal
to the second antenna signal except for a possible difference in phase between the two
signals. In other words, assuming that the first antenna signal is given by Si (t) and the second
antenna signal is given by s2(t), then | s i (t) |º |s2(t) | . If the first and second antenna signals
have the same polarity on first 111 and second 112 antenna ports, then Si (t)º s2(t), while if the
first and second antenna signal have opposite polarities on first 111 and second 112 antenna
ports, then Si (t)º -s2(t).
As will be further exemplified below in connection with Fig 5a, 5b, 6a, and 6b, the antenna
structure 110 is arranged according to a geometry such that the polarity of antenna signals
determines the polarization state of signals transmitted or received by the antenna system
100. The antenna signals are taken from the same common signal, and simply altered in
polarity. Herein, opposite polarity is to be construed as two signals having substantially
opposite phases, i.e., are out-of-phase, while same polarity is to be construed as signals having
the same phase, i.e., are in-phase.
The antenna system shown in Fig 1 is arranged in either of a receiver mode, a transmitter
mode, or a transceiver mode. The antenna structure 110, when in receiver mode, is arranged
t o output antenna signals on first 111 and second 112 antenna ports received via the antenna
structure 110. The antenna structure 110, when in transmitter mode, is arranged t o receive
antenna signals on first 111 and second 112 antenna ports t o be emitted via the antenna
structure 120. The antenna structure 110, when in transceiver mode, is arranged t o
simultaneously output and receive antenna signals on first 111 and second 112 antenna ports.
Fig 2a shows an antenna system 200 which further comprises a first antenna interface unit
210. The first antenna interface unit comprises a 180 degree hybrid coupler 220, a switching
unit 230, and a first common port 213 for passing the first common antenna signal Ji. The 180
degree hybrid coupler 220 has first 211 and second 212 coupler ports connected t o the first
111 and t o the second 112 antenna port, respectively, as well as a summation 221 and a
difference 222 port connected t o the switching unit 230. The switching unit 230 is arranged t o
connect the first common port 213 of the antenna interface unit 210 t o either of the
summation port 221 or the difference port 222 of the 180 degree hybrid coupler 220. Thus
selecting between the first and the second antenna pattern of the antenna system 200 is
facilitated by means of the switching unit 230.
The switching unit 230 can be configured t o alternate between states in a pre-determined
schedule, or be configured t o be manually controlled by an external control signal which
determines the state of the switch. As a general rule of thumb, the switching of the switching
unit 230 should be done as seldom as possible, e.g., with switching frequency kHz range, in
order t o obtain best performance of an attached low frequency radar system used in a
synthetic aperture radar, SA , application.
Fig 2b shows an antenna system 250 where the first 111 and second 112 antenna ports further
are arranged t o pass a third and a fourth antenna signal, respectively, in addition t o the first
and the second antenna signal. The third and fourth antenna signals have substantially
identical envelopes and are derived from a second common antenna signal J2 which is
configured t o be substantially orthogonal t o the first common antenna signal Ji. The antenna
system 250 also comprises a second antenna interface unit 260. The second antenna interface
unit 260 comprises second 261 and third 262 common ports for passing the first Ji and also
the second J2 common antenna signal. There is also comprised a 180 degree hybrid coupler
220. The 180 degree hybrid coupler 220, as in Fig 2a above, has first 211 and second 212
coupler ports connected to the first 111 and to the second 112 antenna port, respectively, as
well as a summation 221 and a difference 222 port connected to the second 261 and third 262
common ports, respectively, thus selecting the first polarization for one of the first Ji and the
second J2 common signal, and selecting the second polarization for the other of the first Ji and
second J2 common signal.
According to different aspects, the first and second common antenna signals are configured to
be orthogonal in different ways. Here, orthogonal means that the first and second common
antenna signals are separable and do not interfere significantly with each other during
operation.
According to one such aspect, the first Ji and the second J2 common signal are orthogonal by
separation in frequency, e.g., by means of frequency division duplex, FDD.
According to one such aspect, the first Ji and the second J2 common signal are orthogonal by
separation in time, e.g., by means of time division duplex, TDD.
According to one such aspect, the first Ji and the second J2 common signal are orthogonal by
separation in code, e.g., by means of band spreading by orthogonal codes.
Fig 3 shows an antenna structure 110' comprising an elongated conductive bridge 310 having
a length LH between first and second ends smaller than half of the wavelength corresponding
to the highest frequency of the first common antenna signal Ji. The antenna structure 110'
further comprises two elongated conductive legs arranged in parallel and having respective
lengths L i and L 2 between first and second ends smaller than half of the wavelength
corresponding to the highest frequency of the first common antenna signal Ji. The legs 320 are
attached at first ends to either end of the elongated conductive bridge 310 in right angles with
respect to the conductive bridge 310. Thus the legs and the bridge together substantially form
a conductive U-shape. The sum of lengths of the elongated conductive bridge 310 and the two
elongated conductive legs 320 is substantially equal to half of the wavelength corresponding
to the center frequency of the first common antenna signal Ji. The first 111 antenna port is
connected to the second end of one leg, the second 112 antenna port is connected to the
second end of the other leg.
The antenna structure 110' is further arra nged to have a ground pla ne 330 orthogona l to both
legs and located approximately at the second end of the legs. Prefera bly, there is arranged a
sma ll sepa ration between the legs 320 and said ground pla ne 300, as shown in Fig 3.
The anten na structure 110' has a tota l length including elongated conductive
bridge 310 and both legs 320 less tha n the wavelength correspondi ng to the highest freq uency
of the first com mon antenna signa l Ji.
It should be noted that any references to sizes, freq uencies, and wavelengths are to be
construed as approximate. Thus, at least partia l functiona lity of the antenna system is
obtained even if dimensioned slightly outside of given lengths.
References wil l be made herei n to a U-sha pe or U anten na, and to a Pi-sha pe or Pi anten na.
The U-sha pe antenna corresponds to the antenna system 300 shown, e.g., in Fig 3, while the
Pi-sha pe antenna corresponds to the antenna system 400 shown, e.g., in Fig 4. However, it is
also noted that, according to aspects, the two legs of the antenna structure need not
necessa rily have the same lengths. In fact, according to some aspects, one leg in the U-sha pe
can have zero length.
Also, the lengths of the two parallel legs wil l at times herein be referred to by the com mon
reference sym bol L , instead of L i and LV2. In such cases when the com mon reference sym bol
L is used, the two legs are assumed to be of equa l length, i.e., L i = L 2= L .
Basic to the present technique is an antenna element in the form of a conducting materia l
bent in U sha pe. This U-sha pe, as shown in Fig 3, consists of two vertica l legs each with one
open end and the opposite ends interconnected by a horizonta l bridge. The antenna element
wil l have the open ends meeting ground pla ne at right angles without actua lly bei ng in
electrica l contact with this pla ne, and the ends are connected to the rada r tra nsceiver and fed
from this transceiver with the same or with opposite pola rity.
When excited by, e.g., a rada r tra nsceiver, the U element will interact with its mirror image in
the ground pla ne. If the length L is chosen as a qua rter of the free space wavelength of the
antenna feeding signa l, i.e., the com mon signa l, and feeding the two legs of the U with the
same pola rity, each leg will behave as a monopole. This mea ns that each leg constructively
interacts with the grou nd pla ne mirror image to forma a half wavelength dipole. Keeping the
bridge length LH shorter tha n half a wavelength mea ns that the bridge wil l not provide any
significant radiation contribution. Thus the radiation produced will be that of the monopole
pair and will provide maximum radiation in the direction broadside to the U-shape 110'. There
will be no significant radiation in the vertical direction. The electric field will always be
substantially parallel to the legs of the U-shape 110'.
If on the other hand the two legs are fed with opposite polarity and 2LV + LH is half a
wavelength of the U-shape 110', the U-shape 110' constructively interacts with its mirror
image to form a magnetic dipole. For impedance matching reasons LH should not be too small
but say that both LH and L are on the order of a little less than quarter of a wavelength, then
both the L are close to quarter of a wavelength and 2LV + LH are close to half a wavelength
meaning that the antenna has two efficient modes of excitation. A magnetic excitation will
produce a radiation pattern where radiation is zero on the broadside of the U-shape 110' and
is maximum in the pane of the U. The magnetic field will always be parallel to the broadside
direction.
The U-shape antenna shown in Fig 3 is broad-band in the sense that radiation efficiency
diminishes when the antenna feed wavelength becomes larger than the resonant condition so
far discussed. However, the radiation pattern will not change. When frequencies increase
from the resonance condition described the radiation diagram will remain the same until L
approaches half a wavelength of the feed signal or 2LV + LH approaches a full wavelength. It
follows that the acceptable limit of top frequency is when L and LHare both smaller than half
of the smallest wavelength of the common antenna signal.
A typical selection of frequency and bandwidth center frequency fc is chosen equal to
bandwidth B and depending on application fc =55 MHz or fc =240 MHz. For these two choices
one gets half of the smallest wavelength of the common antenna signal equal to
approximately 1.8 meters and 0.4 meters, respectively. Typical choices of L and LH can be 1 m
and 0.25 m respectively.
Fig 4 shows an antenna system 400 where the elongated conductive bridge 310 has been
extended by first 410 and second 420 conductive extension units connected at either end of
the elongated conductive bridge 310, thus substantially forming a P -shape. The total length of
the elongated conductive bridge 310 including extension units 410, 420 is configured to be
smaller than the wavelength corresponding to the highest frequency of the common antenna
signal J.
The feature of the elongated conductive bridge 310 including extension units 410, 420 being
smaller than the wavelength corresponding to the highest frequency of the common antenna
signal advantageously prevents excitation of the bridge due to the bridge being on the order
of a wavelength in size.
Note also the ground plane 330 of the antenna structure 110", which ground plane 330 is
shown in Figure 4 as a dash-dotted line.
Fig 5a shows a "U-shape" antenna of the disclosure with electric excitation, i.e., where the first
and second antenna signal are set to have the same polarity on first 111 and second 112
antenna ports.
Fig 5b shows a "U-shape" antenna of the disclosure with magnetic excitation, i.e., where the
first and second antenna signal are set to have opposite polarity on first 111 and second 112
antenna ports.
The antenna structures in Figures 5a/5b are arranged to be fed by a first common signal J
connected to the summation port and the difference port of a hybrid coupler, respectively.
The ground plane 330 of each antenna structure 110' is also shown in Figures 5a and 5b.
Fig 6a shows a "Pi-shape" antenna with magnetic excitation, i.e., where the first and second
antenna signal are set to have opposite polarity on first 111 and second 112 antenna ports.
Note that it is only shown the contribution to the radiation pattern from the bide extension,
which is added to the diagram of the original U-shape antenna of Figure 5a/5b.
Fig 6b shows a "Pi-shape" antenna with electric excitation, i.e., where the first and second
antenna signal are set to have the same polarity on first 111 and second 112 antenna ports.
The antenna structures in Figures 6a/6b are also arranged to be fed by a first common signal J
connected to the summation port and to the difference port of a hybrid coupler. The ground
plane 330 of each antenna structure 110' is also shown in Figures 6a and 6b.
Shown in Figures 5 and 6 are also electric and magnetic field vectors Eand H, respectively, of
the antenna systems indicating field directions.
The present antenna system design comprises two modes of vehicle integration, one that
exploits the U antenna as is, and the other a modification of the U antenna. Before discussing
actual aircraft integration, this modification will be described, with reference to Fig 4. As seen
in Fig 4 the basic U shape has been altered t o a Pi-shape by extending the bridge of the U to a
given distance outside either leg. In this case, when the total length of the bridge including
extension units is half a wavelength and when feeding the legs with opposite polarity, the
current running through the bridge between the legs will excite a resonance in the extended
bridge by which it will start radiating as a horizontal half wavelength dipole. Doing so it will
interact with its mirror image in the ground plane which (because the net electric field in the
ground plane must be zero) will create an electric field of opposite polarity. As a result the
radiation created in the broadside direction will again be zero. If the distance L is half a
wavelength, then maximum radiation will be in the vertical direction. Gain will be increased in
directions between the vertical and broadside directions (though going eventually to zero as
the broadside direction is approached), in which both electric dipole radiation of the extended
bridge and the magnetic dipole radiation combines. In these directions, it is understood that
the electric field is parallel to the direction of the bridge.
When the legs of the Pi antenna are fed in phase, only the legs - behaving as monopoles -
should be excited. No excitation of the bridge occurs. For this reason the extension units must
not extend beyond a quarter of a wavelength. Applying the discussion above regarding
acceptable bandwidth one finds that L , LH and extension units should all be smaller than half
of the smallest wavelength of the common antenna signal. Thus the total length of the bridge
shall be less than the smallest wavelength. In the example above a total bridge length
(including extension units) of 3 m for the low band and 0.75 m for the high band is
conceivable.
By exciting the horizontal bridge at two separated points the full wavelength excitations of the
resulting dipole are pushed to higher frequencies compared to a centrally fed dipole. Thus the
separated feeds enable the antenna to be longer and thus more efficient without
compromising the radiation pattern, which is an advantage of the present technique.
We finally mention certain preferred modes of aircraft integration of the Pi-shape and Ushape
antenna types described above.
The ideal antenna illumination pattern for low frequency radar mapping should provide
maximum radiation at approximately 30° depressed direction (with respect to the horizon) at
right angles to the flight axis and either to the left or right. The direction should preferably be
selectable. For the essentially dipole type of antennas considered here the beams are very
wide and diffuse meaning that even though the maximum direction may point in another
direction it may well be sufficient also at this ideal direction.
Figures 7 and 8 illustrate two examples of modes of integration for the U-shape and Pi-shape
antennas of the present teaching.
When integrating the antennas described here on an aircraft, the aircraft body is considered
the ground plane. The conditions for this assumption to be valid is that the aircraft has
dimension of many wavelengths and has good conductivity (if this should turn out not to be
the case it must be painted with conductive paint). However, it may not be required that the
antennas are installed on a continuous or flat area of the aircraft - also very irregular
structures may serve the purpose of a ground plane (a typical case is that the underside of the
landing skids of a helicopter may behave as a ground plane).
Figures 7 and 8 each show an example of an airborne vehicle 710, 810 arranged to carry an
antenna system 100, 200, 300, 400 according to the present disclosure.
The airborne vehicles 710, 810 can, as shown in Fig 7 and Fig 8, further be arranged to carry a
first and a second antenna system 100, 200, 300, 400, i.e., double antenna structures.
Fig 7 shows a "U-shape" antenna installation on a Hermes 450 un-manned aerial vehicle, UAV.
In flight right and left U-shape antennas swing down to a 45° depression with respect to the
horizontal plane. With electric excitation of right hand antenna and magnetic of left hand
antenna, the right antenna provides vertical polarization to the left while the left antenna
provides horizontal polarization to the left. With these excitations both antennas have
radiation nodes to the right, so the requited one-sidedness of the radiation pattern is
achieved.
Thus, according to an aspect, the length of the elongated bridge is approximately 1.2 meters,
and the length of both legs is approximately 1 meter.
According to another aspect, the length of one of the legs is approximately 0 meters, thus
substantially forming a single leg antenna system (not shown in Fig 7).
As seen in Figures 5a/5b discussed above, then radiation patter of the U-shape antenna can be
chosen to obtain horizontal polarization to one side by magnetic excitation of the U-shape
antenna at the same side. For vertical polarization the opposite side antenna is used with
electric excitation.
Fig 8 shows a "Pi" antenna installation on a Bell 212 Huey helicopter. The landing skids prevent
use of the antenna concept shown in Figure 7. Instead, an arrangement is used where the
antenna unit unfolds in air from a collapsed position between the landing skids when the
helicopter is on ground. The right and left antenna are fed with a 90° phase shift and are
separated by a quarter of the center frequency wavelength to obtain a one-sided radiation
diagram. The radiation intensity maximum is directed downwards from the helicopter, i.e.,
directed towards ground, but since the radiation pattern is very diffuse a significant amount of
radiation hits the ground also at relatively shallow depression angles of approximately 30°.
As is exemplified in Fig 8, certain platform designs, in particular helicopters equipped with
landing skids, preclude the preferred 45° depressed position of the U-shape antenna
arrangement. Use of the Pi antenna arrangement can then be suitable. Note however that the
U-shape antenna arrangement can be allowed to swing clear of the ground thus increasing the
field of application for the arrangement. Figure 7 is an example of where such a mechanical
movement is allowed (this case is actually concept where the downward movement of the
antennas is actuate by wind forces when the UAV is settling to its normal cruise attitudethese
forces works against a spring load keeping the antennas horizontal when on ground).
Also, note that the Pi-shape antenna like the U-shape antenna with contra-polar feed has
maximum radiation normal to the ground plane, i.e. in the downward direction in Figure 8.
However, the U antenna radiates equally strong to the front and to the rear of the platform
resulting in low gain outside the plane of the antenna. The Pi antenna radiates less strongly to
the front and rear resulting in stronger radiation outside the plane of the antenna and in
particular in the required radiation directions between down and broadside.
Some radar applications derive resolution from the aircraft motion (so called synthetic
aperture). Thus in contrast to radar in its simplest form the antenna directivity is not required
for getting the angular resolution of the radar. However, a prerequisite for synthetic aperture
principle to be applicable is that the radar responses only stem from one side of the aircraft.
Thus, the antenna arrangement must allow any responses coming from the other side of the
aircraft to be effectively suppressed.
Further, when the antenna is non-directive, the radar signal transmitted though the antenna
will strongly couple to the metallic structure of the aircraft itself. This interaction cannot be
efficiently handled unless antennas are allowed to geometrically extend from the aircraft by a
distance of at least quarter of a wavelength order (herein meter order).
Thus, according to an aspect, the length of the elongated conductive bridge 310 with
extension units 410, 420 is approximately 3 meters, and the length of each leg is
approximately 1.2 meters.
According to one aspect, the antenna system of the present disclosure is adapted to be
mounted on an airborne vehicle 710, 810.
According to another aspect, the antenna system 100, 200, 300, 400 is adapted to be mounted
on a surface based vehicle.
According to an aspect, the airborne vehicle 710, 810 is arranged as a ground plane of the
antenna system 100, 200, 300, 400.
According to an aspect, the antenna system 100, 200, 300, 400 is adapted to be mounted on
an airborne vehicle 810 comprising first and second landing skids 820, the antenna system
being arranged between the first and second landing skids 820.
Fig 9 shows a flowchart illustrating a method for selecting an antenna pattern of an antenna
system 100. The antenna system 100 comprises an antenna structure 110 consisting of a
single antenna element having first 111 and second 112 antenna ports arranged to pass a
respective first and second antenna signal, the antenna pattern of the antenna structure 110
is arranged to be selectable between a first antenna pattern having a first polarization and a
second antenna pattern having a second polarization substantially orthogonal to the first
polarization, the method comprising the steps of receiving SO a first common signal, and
deriving SI the first and second antenna signals from the first common signal, as well as
setting S2 the first and second antenna signal to have the same polarity on first 111 and
second 112 antenna ports in case the first antenna pattern is selected, and setting S3 the first
and second antenna signal to have substantially opposite polarities on first 111 and second
112 antenna ports in case the second antenna pattern is selected.
Aspects of the disclosure are described with reference to the drawings, e.g., block diagrams
and/or flowcharts. It is understood that several entities in the drawings, e.g., blocks of the
block diagrams, and also combinations of entities in the drawings, can be implemented by
computer program instructions, which instructions can be stored in a computer-readable
memory, and also loaded onto a computer or other programmable data processing apparatus.
Such computer program instructions can be provided to a processor of a general purpose
computer, a special purpose computer and/or other programmable data processing apparatus
to produce a machine, such that the instructions, which execute via the processor of the
computer and/or other programmable data processing apparatus, create means for
implementing the functions/acts specified in the block diagrams and/or flowchart block or
blocks.
In some implementations and according to some aspects of the disclosure, the functions or
steps noted in the blocks can occur out of the order noted in the operational illustrations. For
example, two blocks shown in succession can in fact be executed substantially concurrently or
the blocks can sometimes be executed in the reverse order, depending upon the
functionality/acts involved. Also, the functions or steps noted in the blocks can according to
some aspects of the disclosure be executed continuously in a loop.
In the drawings and specification, there have been disclosed exemplary aspects of the
disclosure. However, many variations and modifications can be made to these aspects without
substantially departing from the principles of the present disclosure. Thus, the disclosure
should be regarded as illustrative rather than restrictive, and not as being limited to the
particular aspects discussed above. Accordingly, although specific terms are employed, they
are used in a generic and descriptive sense only and not for purposes of limitation.

CLAIMS
1. An antenna system (100) com prisi ng an antenna structure (110) consisting of a single
antenna element having f irst (111) and second (112) anten na ports arranged t o pass a
respective f irst and second anten na signa l, the first and second anten na signa ls arra nged
t o be derived from a f irst com mon antenna signa l (Ji) and arra nged to be essentia lly equa l
in envelope, the antenna structure (110) bei ng arranged t o have an antenna pattern
which is selectable between a f irst antenna pattern having a f irst pola rization and a
second antenna pattern having a second pola rization substa ntia lly orthogona l t o the f irst
pola rization, the f irst antenna pattern being selected by setti ng the f irst and second
antenna signa l t o have the same pola rity on f irst (111) and second (112) antenna ports,
the second antenna pattern being selected by setting the f irst and second antenna signa l
t o have su bsta ntia lly opposite pola rities on first (111) and second (112) antenna ports.
2. The antenna system (100) according t o claim 1 arra nged in either of a receiver mode, a
tra nsmitter mode, or a tra nsceiver mode, the anten na structure (110) when in receiver
mode being arranged t o output antenna signa ls on f irst (111) and second (112) anten na
ports received via the anten na structure (110), the antenna structure (110) whe n in
tra nsmitter mode being arranged t o receive antenna signa ls on f irst (111) and second
(112) antenna ports t o be emitted via the antenna structure (120), the antenna structure
(110) when in tra nsceiver mode being arra nged t o simulta neously output and receive
antenna signa ls on f irst (111) and second (112) antenna ports.
3. The antenna system (200) according t o any preceding claim, further com prisi ng a first
antenna interface unit (210) comprisi ng a 180 degree hybrid coupler (220), a switchi ng
unit (230), and a f irst com mon port (213) for passi ng the f irst com mon anten na signa l (Ji),
the 180 degree hybrid coupler (220) having first (211) and second (212) coupler ports
connected t o the first (111) and t o the second (112) anten na port, respectively, as wel l as
a summation (221) and a difference (222) port con nected t o the switching unit (230), the
switching unit (230) bei ng arranged t o connect the f irst com mon port (213) of the
antenna interface unit (210) t o either of the sum mation port (221) or the difference port
(222) of the 180 degree hybrid coupler (220), t hus selecting between the f irst and the
second antenna pattern of the antenna system (200).
4. The antenna system (250) according to any of claims 1-2, the first (111) and second (112)
antenna ports further being arranged to pass a third and a fourth antenna signal,
respectively, the third and fourth antenna signals having substantially identical envelopes
and are derived from a second common antenna signal (J2) substantially orthogonal to the
first common antenna signal (Ji), the antenna system (250) further comprising a second
antenna interface unit (260), the second antenna interface unit (260) comprising second
(261) and third (262) common ports for passing the first (Ji) and the second (J2) common
antenna signal, respectively, as well as a 180 degree hybrid coupler (220), the 180 degree
hybrid coupler (220) having first (211) and second (212) coupler ports connected to the
first (111) and to the second (112) antenna port, respectively, as well as a summation
(221) and a difference (222) port connected to the second (261) and third (262) common
ports, respectively, thus selecting the first polarization for one of the first (Ji) and the
second (J2) common signal, and selecting the second polarization for the other of the first
(Ji) and second (J2) common signal.
5. The antenna system (250) according to claim 4, wherein the first (Ji) and the second (J2)
common signal are orthogonal by separation in frequency.
6. The antenna system (250) according to claim 4 or 5, wherein the first (Ji) and the second
(J2) common signal are orthogonal by separation in time.
7. The antenna system (250) according to any of claims 4-6, wherein the first (Ji) and the
second (J2) common signal are orthogonal by separation in code.
8. The antenna system (300) according to any preceding claim, the antenna structure (110')
comprising an elongated conductive bridge (310) having a length (LH) between first and
second ends smaller than half of the wavelength corresponding to the highest frequency
of the first common antenna signal (Ji), the antenna structure (110') further comprising
two elongated conductive legs arranged in parallel and having respective lengths (L i , L 2)
between first and second ends smaller than half of the wavelength corresponding to the
highest frequency of the first common antenna signal (Ji), said legs (320) being attached
at first leg ends to either end of the elongated conductive bridge (310) in right angles with
respect to the conductive bridge (310), thus substantially forming a U-shape, the sum of
lengths of the elongated conductive bridge (310) and the two elongated conductive legs
(320) being substantially equal to half of the wavelength corresponding to the center
frequency of the f irst com mon anten na signa l (Ji), the f irst (111) anten na port bei ng
connected to the second end of one leg, the second (112) antenna port bei ng con nected
to the second end of the other leg, the anten na structu re (110') bei ng arra nged to have a
ground pla ne (330) orthogona l to both legs and located approximately at the second ends
of the legs, the antenna structure (110') having a tota l length (L OT), including elongated
conductive bridge (310) and both legs (320), less tha n the wavelength corresponding to
the highest freq uency of the f irst com mon anten na signa l (J l).
9. The antenna system (400) according to claim 8, wherein the elongated conductive bridge
(310) is extended by first (410) and second (420) conductive extension units connected at
either end of the elongated conductive bridge (310), thus substa ntia lly formi ng a P -
sha pe, the tota l length of the elongated conductive bridge with extension units (410, 420)
being sma ller tha n the wavelength corresponding to the highest frequency of the
com mon antenna signa l (J).
10. The antenna system (100, 200, 300) accordi ng to any of claims 1-8, whe rein the le ngth of
the elongated bridge is approximately 1.2 meters, and the length of both legs is
approximately 1 meter.
11. The antenna system (100, 200, 300) according to any of claims 1-8, wherei n the length of
one of the legs is approxi mately 0 meters, thus substa ntia lly forming a single leg antenna
system .
12. The antenna system (100, 200, 400) according to claim 9, wherein the length of the
elongated conductive bridge (310) with extension units (410, 420) is approximately 3
meters, and the length of each leg is approxi mately 1.2 meters.
13. The antenna system (100, 200, 300, 400) according to any preceding claim ada pted to be
mounted on an airborne vehicle (710, 810).
14. The antenna system accordi ng to claim 13, wherei n the airborne vehicle (710, 810) is
arranged as a ground pla ne of the antenna system (100, 200, 300, 400).
15. The anten na system (100, 200, 300, 400) according to any of claims 13-14 ada pted to be
mounted on an airborne vehicle (810) com prisi ng first and second la ndi ng skids (820), the
antenna system being arra nged between the f irst and second la ndi ng skids (820).
16. The antenna system (100, 200, 300, 400) according to any preceding claim adapted to be
mounted on a surface based vehicle.
17. An airborne vehicle (710, 810) arranged to carry the antenna system (100, 200, 300, 400)
according to any of claims 1-15.
18. An airborne vehicle (710, 810) arranged to carry a first and a second antenna system (100,
200, 300, 400) according to any of claims 1-15.
19. A method for selecting an antenna pattern of an antenna system (100), the antenna
system (100) comprising an antenna structure (110) consisting of a single antenna
element having first (111) and second (112) antenna ports arranged to pass a respective
first and second antenna signal, the antenna pattern of the antenna structure (110)
arranged to be selectable between a first antenna pattern having a first polarization and a
second antenna pattern having a second polarization substantially orthogonal to the first
polarization, the method comprising the steps of
• Receiving (SO) a first common signal,
• Deriving (SI) the first and second antenna signals from the first common signal,
• Setting (S2) the first and second antenna signal to have the same polarity on
first (111) and second (112) antenna ports in case the first antenna pattern is
selected,
• Setting (S3) the first and second antenna signal to have substantially opposite
polarities on first (111) and second (112) antenna ports in case the second
antenna pattern is selected.