1. Objective of the 4-Port Feed- Network with switchable Polarization
The plurality of Earth Stations Antennas operating in C-Band is equipped with feed systems providing access to orthogonal circular polarization. Some satellites operate in the same frequency bands, however with orthogonal linear polarization. In order to increase the flexibility of Earth Station Sites it will be highly desirable to have at least one antenna (for occasional use) which is capable to be switched from circular to linear polarization and vice versa. This feature could be realised either with a new antenna or more economically as an upgrade of an existing antenna. In both cases our 4-Port Feed Network with switchable polarization is the right choice to be implemented in the antenna. The quality of the antenna is not affected by this implementation. In past years a number of earth station antennas in the size of 8 m to 32 m has been equipped with this Network and are working successfully for their operators.
2. Special Features of this unique 4-Port switchable Feed Network
This Network is characterised by seven outstanding features which are summarised below:
- The electrical performance which is fully compliant with INTELSAT “STANDARD A” Feed Systemseither with orthogonal linear or circular polarization.
- The absence of any rebuilt procedure during the switchover process from linear to circularpolarisation and vice versa.
- The absence of feed horn rotation during the polarization adjustment process with linearpolarization.
- The absence of any waveguide switch and coupler in the polarization switching mechanism.Therefore all waveguide connections between Feed Network and the Tx- and Rx- Subsystemsremain unchanged with any polarization.
- The absence of any flexible waveguides between the Tx feeder lines and the inputs of the 4-PortFeed Network usually needed for the polarization alignment due to a specially designed and uniqueTwo- Channel Rotary Joint.
- The compact and very slender constructural shape offers the application to almost any antennasindependent of it’s design concept and original manufacturer.
- The seventh outstanding feature (optional) is the possibility of independent alignment of theCoordinate Systems for the Rx- and Tx- Signals. Different Faraday- Rotation in the Ionosphere canthus be compensated. The implementation of this feature doesn’t affect any of the other features.
3. Main Components and Operating Modes of the 4-Port switchable Feed Network
Fig.1 shows the architecture of the 4-Port switchable Feed Network. It is embedded between the fixed Corrugated Feed Horn and the RF-Subsystem (Tx- Feeder Lines; Rx- LNA´s).
The main components are:
- Polarization Switch 4 GHz embedded between Rotary Joint #1- and #2
- Ortho Mode Transducer 4 GHz with Tx-Reject Filters and the interfaces to the LNA-Subsystem.
- Polarisation Switch 6 GHz embedded between Rotary Joint #3- and #4.
- Ortho Mode Transducer 6 GHz with the Tx-Feeder Lines to the Rotary Joint #5.
- Rotary Joint #5 with the interfaces to the fixed Tx-Feeder Lines coming from the HPA-Subsystem.
The two operating modes are achieved by the two polarizers and the rotary joints:
When operating in circular Polarization the switchable Polarizers are activated by a 45 degree rotation. The high precision Polarizer converts the received circularly polarized waves (RHCP and LHCP) into linearly polarized waves. The Ortho Mode Transducer 4 GHz separates the received Polarizations. The waveguide branches comprise Tx- Reject Filters delivering a high Tx- Rx Isolation.
The transmit signals are fed via the fixed input flanges of the Rotary Joint #5 to the 6 GHz Ortho Mode Transducer. The high precision Polarizers convert the transmitted linearly polarized waves into circularly polarized waves (RHCP and LHCP). Please see the Electrical Characteristics below for axial ratio (polarization purity), isolation etc.
When operating in linear Polarization the switchable Polarizers are deactivated by a minus 45 degree rotation. The Ortho Mode Transducer 4 GHz separates the received Polarizations (linear X and -Y). The waveguide branches comprise Tx- Reject Filters delivering a high Tx- Rx Isolation. The transmit signals are fed into the fixed input flanges of the Rotary Joint #5 to the 6 GHz Ortho Mode Transducer.
The polarisation adjustment as well as polarization tracking is done by rotating the whole Feed Network plus/minus 50 degrees. Rotary Joint #1 and Rotary Joint #5 allow this rotation with the result, that the Feed Horn can remain fixed in the Antenna Structure on the one hand and on the other hand no flexible waveguides for the Tx lines are needed to bridge the polarization axis. The linear polarized waves are transmitted with the desired polarization alignment. Please see the Electrical Characteristics below for polarization purity, isolation etc.
The option Faraday Rotation requires the implementation of two additional microwave components (not shown in the picture).
4. Electrical Characteristics of the 4- Port switchable Feed Network
Frequency Bands Rx 3.625 - 4.2 GHz 3.625 - 4.2 GHz Frequency Bands Tx 5.85 - 6.425 GHz 5.85 - 6.425 GHz Axial Ratio 1.035:1 Polarisation Purity 40 dB 35dB Tx/Rx-Isolation in Tx- Band 85 dB 85 dB Insertion Loss 0.2 dB 0.2 dB Return Loss - 16 dB - 16 dB Power Handling Capability 3 KW (CW) 3 KW (CW) Polarisation Alignment plus/minus 50 degrees Faraday Angle Correction plus/minus 20 degrees
5. Mechanical Characteristics of the 4- Port switchable Feed Network
Dimensions: Length: app. 1.820 mm Width app. 420 x 420 mm Weight: app. 75 kg Waveguide Interfaces: Feedhorn: 64 mm (circular cross section) Rx Waveguide size: R 40 (WR 229) Rx Flange: PDR 40 Tx Waveguide size: R 70 (WR 137) Tx Flange: PDR 70 Pressurization: Sealed to 0.5 psi Material of RF Components: Aluminium or Copper, silver plated Activation of the Polarization switching: Manually by two small Levers; Remote control as an option Polarization adjustment: Plus/minus 50 degrees by using a worm gear activated by a hand crank; Remote control as an option Display of Polarization : By Scale
6. Installation, Integration and Commissioning
In general, any C-Band Earth Station Antenna independent of its design concept (beam waveguide, turning head or others) and its original manufacturer can be equipped with this Feed Network, even as an upgrade. Depending on the mechanical design of the antenna hub or the Feed Tube, there is some adaptation work needed to fix both the bearings (Rotary Joint #1 and #5) to the antenna structure (see the sketch below, showing a possible arrangement of the Network in the antenna, depending on the space available inside the hub or feed tube). In any case, the Feed Horn remains stationary connected to the Feed Tube and don’t need to rotate for Polarization adjustment.
Rotary Joint #1 (1. bearing) is connected to the Feed Horn, either directly or via a piece of circular waveguide, usually needed to achieve the correct reflector illumination. If needed, a small support structure should be implemented at this location, to avoid to much stress on this waveguide interface.
Rotary Joint #5 (2. bearing) is stationary connected to the Feed Tube or the Antenne Hub, depending on the space offered in the antenna. Constructural provisions are needed to align the Feed Network exactly in the bore sight axis of the Reflector System.
This adaptation can be done either by ESA microwave service GmbH or by technicans of the customer with the assistance of a supervisor from ESA microwave service GmbH. Adequate documentation will be provided for the adaptation work.
Option 1: Electrical Activation of Polarization Mode (linear/circular)
Instead of manual levers for moving the polarizers, step motors are installed. A switching unit with positioning controllers provides a switch for selecting linear or circular polarization and a start button which initiates the move to the selected polarization. Reaching the desired polarization is shown by a LED and the switching unit can be switched off. Brakes on the step motors ensure that the positions are kept even without electrical power. For remote control it's possible to use the opto coupler inputs (5V to 24 V) of the positioning controllers directly or via additional relays. As well the transistor outputs (open collector) can be used for remote signalling."
Option 2: Electrical Adjustment of Polarization (Radiation Plane at linear Polarization)
The manual hand wheel for linear polarization can be replaced by a motor drive with encoder and a closed loop control. A switching unit with a positioning controller provides control to move to a desired linear polarization position. This is done either by an analogue input signal (-10V to 10V) which directly controls the position or by using up to 15 stored drive profiles for relative or absolute positioning. After reaching the desired linear polarization position the switching unit can be switched off. For remote control it's possible to use the opto coupler inputs (5V to 24 V) of the positioning controller directly or via additional relays. As well the transistor outputs (open collector) can be used for remote signalling.
The electrical performance of the principles of this 4- Port Feed-Network has been successfully demonstrated with several antennas of different size and design, being upgraded with this concept to achieve "INTELSAT Standard A" transmission parameters. One of the demonstrations was with a 32m Beam Waveguide Antenna manufactured by NEC (see picture below).
9. Company Profile
ESA microwave service GmbH is a small to medium size enterprise, that develops and produces Passive Microwave Components and Microwave Systems used in earth station antennas for different applications (Satellite Communication, Microwave Links, Deep Space Missions, Radio Astronomy).
We also offer our customers a comprehensive, highly rated and competitive package of services for all stages of the project .
Our core business is the Design and Supply of Feedsystems for all relevant frequency bands (L-, S-, C-, Ku-, Ka- Band), including Multi-Band and Polarisation; but also the Retrofit, Upgrade, Repair and Maintenance of existing Feedsystems is an important branch of our business.
The scope of supplies and services for antenna systems comprises:
- All passive RF- Components in the Tx path, i.e. HPA Combiner network, Waveguidetransmission lines up to the feed input, Couplers for monitoring and control, Feednetwork withPolarization device, Feedhorn.
- All passive RF- Components in the Rx path, i.e. Feedhorn, Feednetwork with Polarizationdevice, Couplers for tracking, monitoring and control, Waveguide connections to LNA’s.
For the execution of projects most modern and approved methods (software) and the latest highfrequency measurement technology are applied. In addition, near- and far-field tests can be conducted in an anechoic chamber.
Please don't hesitate to contact us for further information under the following address:
ESA microwave service GmbH
Phone: +49 (0)7191 902957
Fax: +49 (0)7191 902958