Field of the invention
[0001] The present invention relates to devices for handling microwave signals and, more
particularly, to an improved waveguide-based phase shifter electronically controlled.
Background of the invention
[0002] Nowadays microwave phase shifters are essential components in advanced communication
systems such as Satellite-On-The-Move systems and phased arrays antennas
[0003] In the field of the advanced communications, it is common to employ techniques to
perform the task of continuously adapting the polarization of the signal according
to the requirements of the satellite or base station involved in the datalink. By
means of the combination of two independent and linearly polarized microwave signals,
it is possible to coherently tilt the plane in which the resulting electric field
is confined. It is then mandatory to perfectly control the relative amplitude and
phase of both components to obtain as a result, an antenna of variable linear polarization.
[0004] There exist other alternatives available in the market as the electronically controlled
Vane Phase Shifter, from Cernexwave. The working principle of this kind device is
a movable dielectric layer that invades the inside of the waveguide and alters the
propagation characteristics of the travelling mode. However, the phase versus frequency
response of the dielectric vane is not constant for all the insertion distances. If
the dielectric vane is totally removed from the waveguide, the phase versus frequency
characteristic is exactly the inherent value of the waveguide itself. Once the dielectric
is inserted into the waveguide, the slope of the phase shift versus frequency characteristic
is noticeably increased. In addition to this, the employment of dielectric as the
main actuator within the waveguide always brings a raise in losses and a limitation
in the power handling.
[0005] AFT Microwave and similar companies offer ferrite-based waveguide phase shifters.
Ferrite based phase shifters can be divided in reciprocal or non-reciprocal depending
upon the phase difference induced is a function of the direction of propagation of
the field. Further, ferrite phase shifters may be latching (twin-toroid and dual-mode)
or non-latching (rotary-field), depending upon continuous holding current must be
applied to sustain the magnetic bias field. The latching phase shifters show high
insertion loss and it exponentially rises with frequency. The peak RF capability of
the device is limited by the fast increase in insertion loss, especially when the
RF power exceeds a specific value. Special attention must be given to mode control
in these devices since the bias wire in enclosed by the waveguide, thus allowing a
Transverse Electro-Magnetic (TEM) mode to propagate as well as higher order LSE and
LSM modes.
[0006] The rotary-field phase shifter exhibits an upper bound of the operation frequency
since the diameter of the ferrite rod decreases in direct proportion to the frequency,
which in turn varies the plane of polarization of the linearly polarized wave. The
main drawback of the ferrite-based phase shifters deals with the dependency of their
electrical and magnetic properties with the environmental agents. The temperature
forces a change in the permeability of the ferrite which causes an undesired tilt
of the polarization plane of the linearly polarized signal.
[0007] Patent document
US3,001,153, entitled "Microwave Phase Shifter", claims the employment of a rotatable section
of circular waveguide, means to continuosly rotating that section and devices at both
ends of the rotatable part in order to insert and extract simultaneously two microwave
signals having opposite senses of circular polarization. The main goal of the invention
disclosed in this patent document is to generate an amplitude modulated signal having
a modulation frequency fourfold the frequency of rotation of the circular waveguide.
The microwave phase shifter disclosed in
US3,001,153 has several differences with regard to the present invention:
- 1- Its working principle needs two circularly polarized microwave signals with opposite
sense and same frequency.
- 2- The short-circuited orthomode transducers employed to convert the incoming linearly
polarized signal into circularly polarized wave are narrowband.
- 3- A rotatable part with circular cross section.
- 4- The rotatable part needs to continuously turn to achieve the needed modulation.
- 5- For producing the advance in phase of one of the components it uses a half-wave
dielectric plate.
[0008] A prior art variable polarization antenna is described in patent document
US3,287,730, entitled "Variable Polarization Antenna". The bulky feeding subsystem of this antenna
possesses two rotatable waveguides with elliptical and rectangular cross sections
and the polarization of the radiated energy depends on the orientation of the waveguides
with respect to each other.
Summary of the invention
[0009] The main object of the present invention is to provide a microwave phase shifter
with continuous phase shifting capabilities, highly accurate and stable against the
environmental agents. The microwave phase shifter is particularly suitable for, but
not limited to, antennas of variable linear polarization.
[0010] The microwave phase shifter comprises:
- A non-rotating linear-to-circular polarization converter with a squared cross section,
located at one end of the phase shifter.
- A non-rotating circular-to-linear polarization converter with a squared cross section,
located at the opposite end of the phase shifter.
- A rotatable squared waveguide with a circular-to-linear-to-circular polarization converter,
positioned at a middle section of the phase shifter.
- A pair of rotary joints with a circular cross section for rotating the rotatable squared
waveguide. The rotary joints are located at both ends of the rotatable squared waveguide.
- A plurality of mode launchers for adapting the squared waveguides to the circular
cross section of the rotary joints.
[0011] In a preferred embodiment, the linear-to-circular polarization converter comprises
a first septum polarizer and the circular-to-linear polarization converter comprises
a second septum polarizer parallel to the first septum polarizer. The circular-to-linear-to-circular
polarization converter of the rotatable squared waveguide preferably comprises a double
symmetric septum polarizer, such that the angular difference between the plane that
holds the first and second septum polarizers and the plane that contains the double
symmetric septum polarizer specifies the shifting in phase of the microwave signal.
[0012] The microwave phase shifter may also comprise a control unit and means for rotating
the rotary joints a determined angle. The means for rotating the rotary joints may
comprise a motor and a belt driven gear.
[0013] The advantages of the present invention compared to the prior art are the following:
- 1- The way in which the delay or advance in phase is produced does not require the
employment of ferrites or any other ferromagnetic material, which makes the system
to be more stable and robust against environmental agents such as temperature and
humidity.
- 2- The phase versus frequency response is constant for all the rotation angles.
- 3- Due to the properties of the different components of the present invention, it
has a wide bandwidth of operation.
- 4- Since within the waveguide there is no need to include a coaxial device to tilt
the field, the present invention is durable against acceleration and vibrations which
makes it especially suitable for rough scenarios.
- 5- Compared to latched ferrites, the present invention does not need a continuous
current to sustain the magnetic bias field, which reduces the power consumption. In
addition to this, electromagnetic incompatibility issues between the biasing cables
and the propagating waveguide are avoided.
- 6- The accuracy of the present invention is 0.1° which is much more precise that the
existing alternatives of the market.
- 7- Since neither substrate-based plates nor ferromagnetic components are employed,
the present invention shows insertion losses lower than other techniques and higher
maximum deliverable power.
- 8- Compared to the rotary-field phase shifter, it can be designed to work at much
higher frequencies.
Brief description of the drawings
[0014]
Figure 1 depicts a microwave phase shifter according to the present invention.
Figure 2 shows a longitudinal cross-section of the microwave phase shifter of Figure
1.
Description of a preferred embodiment of the invention
[0015] Referring to
Figure 1, the microwave phase shifter (10) of the present invention comprises three different
main parts:
- The first component is a fixed (not rotating) linear-to-circular polarization converter
(1) with a squared cross section.
- The second component is a rotatable squared waveguide (2) with a circular-to-linear-to-circular
polarization converter, which ends are connected to two mode launchers (3, 4) that
adapt the squared waveguide to the circular cross section of the rotary joints (5,
6). Between the rotary joints (5,6) and septum polarizers (13,15) there are also two
mode launchers (7,8) for the same purpose.
- The third component is a fixed circular-to-linear polarization converter (9) similar
to the first component (1) and hold in the same plane but with the opposite conversion
of polarization rotation.
[0017] In a septum polarizer, the fundamental mode within the rectangular waveguide TE10
is converted into a circularly polarized (CP) microwave signal in the squared waveguide
and vice versa. Whether right-hand circular polarization (RHCP) or left hand circular
polarization (LHCP) is produced depends upon which of the two rectangular ports (11,12)
is excited. In the original model presented by Chen and Tsandoulas the septum divides
the input signal in two orthogonal components with the same amplitude, but a dielectric
slab is needed to adjust the phase of one of the components and thus improve the existing
orthogonality between signals. The septum of the present invention is optimized to
get the desired power splitting, input port isolation, input matching and output orthogonality.
[0018] The circular-to-linear-to-circular polarization converter of the rotatable squared
waveguide (2) is implemented by a double symmetric septum polarizer (14). The double
symmetric septum polarizer (14) is the cornerstone of the present invention. The angular
difference between the plane that holds the first (13) and second (15) septum polarizers
and the plane that contains the double symmetric septum polarizer (14) specifies the
shifting in phase that is forced to the travelling microwave signal. Independently
of the rotation angle, the travelling wave always follows the same path within the
whole device due to the septa disposition. When the rotatable squared waveguide (2)
turns a certain angle θ, the circularly polarized signal faces the first stage of
the double symmetric septum (14) which converts the incoming signal into a linearly
polarized component. At this time the electric field is forced to turn the same angle
θ to get adapted to the new boundary conditions imposed by the presence of the stepped
first septum polarizer (13). The linearly polarized wave reaches the second half of
the double symmetric septum (14) and it is converted to a circularly polarized signal
again. Since the second septum polarizer (15) is fixed and parallel to the first septum
polarizer (13), the field is tilted again an angle θ. As a conclusion, when the microwave
signal reaches the output of the microwave phase shifter (10), it has suffered a 2·θ
phase shifting compared to the situation when the three septa are contained in the
same plane.
[0019] Regarding the control subsystem, a belt driven gear makes the rotatable part turn
a desired angle. The dimensional relation between the radio of the motor gear and
the rotary joint gear permits to refine the accuracy and torque four times. The whole
control unit is separated from the RF path so that coexistence issues are avoided.
1. A microwave phase shifter, comprising:
a non-rotating linear-to-circular polarization converter (1) with a squared cross
section, located at one end of the phase shifter (10);
a non-rotating circular-to-linear polarization converter (9) with a squared cross
section, located at the opposite end of the phase shifter (10);
a rotatable squared waveguide (2) with a circular-to-linear-to-circular polarization
converter, positioned at a middle section of the phase shifter (10);
rotary joints (5, 6) with a circular cross section for rotating the rotatable squared
waveguide (2), the rotary joints (5, 6) being located at both ends of the rotatable
squared waveguide (2);
mode launchers (3, 4, 7, 8) for adapting the squared waveguides (1, 2, 9) to the circular
cross section of the rotary joints (5, 6);
2. The microwave phase shifter of claim 1, wherein the linear-to-circular polarization
converter (1) comprises a first septum polarizer (13) and the circular-to-linear polarization
converter (9) comprises a second septum polarizer (15), parallel to the first septum
polarizer (13).
3. The microwave phase shifter of claim 2, wherein the circular-to-linear-to-circular
polarization converter of the rotatable squared waveguide (2) comprises a double symmetric
septum polarizer (14), such that the angular difference between the plane that holds
the first (13) and second (15) septum polarizers and the plane that contains the double
symmetric septum polarizer (14) specifies the shifting in phase of the microwave signal.
4. The microwave phase shifter of any preceding claim, comprising a control unit and
means for rotating the rotary joints (5, 6) a determined angle.
5. The microwave phase shifter of claim 4, wherein the means for rotating the rotary
joints (5, 6) comprises a motor and a belt driven gear.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only.
It does not form part of the European patent document. Even though great care has
been taken in compiling the references, errors or omissions cannot be excluded and
the EPO disclaims all liability in this regard.
Patent documents cited in the description
Non-patent literature cited in the description
- CHENTSANDOULASA wide-band Square-waveguide Array PolarizerIEEE Transactions on Antennas and Propagation,
1973, 389-391 [0016]