Abstract:
An apparatus, method, and system are disclosed that can be used to reduce the peak radiated flux density of a horn antenna or for testing the electronics associated with a horn antenna. A horn antenna with narrow and wide ends can have disposed within it a surrogate waveguide. The surrogate waveguide has a wide end smaller than the wide end of the horn antenna, and the wide end of surrogate waveguide extending to or beyond the wide end of the horn antenna. A mounting plate or face plate covers a portion of the wide end of the horn antenna.

Description:
FIELD 
       [0001]    This disclosure is directed toward the field of satellite microwave payload test equipment. 
       BACKGROUND 
       [0002]    High powered horn antennas, such as those used in satellite communications, can produce a focused high flux density that creates challenges for testing the communication electronics attached to a horn antenna. To test attached electronics, one typical configuration includes a Radio Frequency (RF) absorber backed by an actively cooled aluminum plate which is positioned in front of the horn antenna. A thin aluminum shroud surrounding the space between the absorber and the horn antenna can be added to create a Field Aperture Load (FAL) configuration. Limits on the absorption and cooling rate of the FAL limit the maximum allowable flux density at the absorber pad. To reduce the flux density at the absorber pad, the absorber pad can be moved farther from the horn antenna such that the energy emitted from the horn is more diffuse and a sufficiently low maximum flux density is found. As the absorber pad moves further from the horn, the entire FAL, including the aluminum shroud, must grow. 
         [0003]    Large test configurations can create increase costs and create other challenges, especially during Spacecraft Thermal Vacuum (SCTV) testing. SCTV testing is often required for satellite communication systems and it requires an entire test configuration to fit within a vacuum chamber. Large vacuum chamber testing facilities are generally expensive with limited schedule availability. 
       SUMMARY 
       [0004]    Examples disclosed here include an apparatus, system and method for testing the electronics associated with a horn antenna. Some examples reduce the maximum flux density emitted and hence allow test equipment such as an RF absorbent pad to be closer to the antenna being tested and allow the entire FAL structure to be smaller. This simplifies the test configuration, and in particular allows use of smaller SCTV test chambers. In one example, a surrogate waveguide is slip-fit into the horn antenna being tested, with a smaller aperture horn attached to the end of the surrogate waveguide. This causes the energy emitted from the antenna to be more diffused and less focused, thereby reducing the maximum flux density at any particular distance from the antenna. An electric field probe (E-Probe) can be incorporated into the surrogate waveguide to detect or insert signals into the energy fields being transmitted or received by the antenna. 
         [0005]    The features, functions, and advantages that have been discussed can be achieved independently in various examples or may be combined in yet other examples, further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0006]    The following detailed description may be better understood when read in conjunction with the appended drawings. For the purposes of illustration, various examples of aspects of the disclosure are shown in the drawings; however, the invention is not limited to the specific methods and instrumentalities disclosed. 
           [0007]      FIG. 1  illustrates a surrogate waveguide mounted in a horn antenna with an external E-probe. 
           [0008]      FIG. 2  illustrates a mounting plate used to mount a surrogate waveguide in a horn antenna. 
           [0009]      FIG. 3  illustrates a surrogate waveguide mounted in a horn antenna with an internal waveguide coupler. 
           [0010]      FIG. 4  illustrates a cross-sectional linear polarization orientation with respect to the waveguide coupler of  FIG. 3 . 
           [0011]      FIG. 5  illustrates a schematic of an E-probe directional coupler for  FIG. 1 . 
           [0012]      FIG. 6  illustrates a single-sided E-probe directional coupler. 
           [0013]      FIG. 7  illustrates a dual-sided E-probe directional coupler. 
           [0014]      FIG. 8  illustrates a collection flight horns mounted on a satellite panel. 
           [0015]      FIG. 9  illustrates an FAL test configuration for the flight horns of  FIG. 8 . 
           [0016]      FIG. 10  illustrates the flux density difference between a 5″ aperture flight horn and a 2″ aperture surrogate waveguide. 
           [0017]      FIG. 11  illustrates the 3 dB beam width difference between a 5″ aperture flight horn and a 2″ aperture surrogate waveguide. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  depicts an example apparatus used to test a microwave frequency payload. Flight horn  102  has a microwave frequency payload attached (but not depicted) and is the antenna connected to the payload being tested. If the payload being tested is for a satellite, flight horn  102  and its attached electronics will be sent into orbit on a satellite, while the other components depicted are used only during testing on the ground. The payload being tested, however, need not be part of a satellite for the disclosed examples to be a useful aid in testing; any payload with a horn antenna can be tested using the disclosed principles. 
         [0019]    A surrogate waveguide  104  can be disposed in the flight horn  102 , and is composed of several portions. The slip-fit portion  110  of a surrogate waveguide  104  can have one end slip-fit into the narrow end of the flight horn  102 , and the other end of the slip-fit portion attached to a face plate or mounting plate  106  that is mounted to the aperture end (or wide end) of the flight horn  102 . The mounting plate  106  has a hole in its center to form a thin portion of the surrogate waveguide  104 . In this example, an E-probe housing  112  forms the next portion of the surrogate waveguide  104  and is attached directly to the outside of the mounting plate  106 . Note, however, that an E-probe housing is not necessary for all test configurations that benefit from the reduced peak flux density. However, many modern satellites have many horn antennas and do not have internal test couplers designed into the spacecraft. An E-probe integrated into a surrogate waveguide is useful for testing such modern satellite systems. Details of the E-probe are provided below. The last portion of the surrogate waveguide  104  is the aperture portion  108  which at one end is attached to the E-probe housing in this example, and the other end is a flared aperture. The final aperture flare size is smaller than the aperture size of the flight horn  102 . For example, a flight horn  102  designed for Ku-Band satellite communication may have a 5 inch aperture, while the aperture end of the surrogate waveguide may be 2 inches, leading to a reduction in peak flux density of approximately one-fifth the peak flux density without the surrogate waveguide. 
         [0020]    While the interior surfaces of the Field Aperture Probe Coupler&#39;s transmission path (the portion that propagates microwave frequency energy) are bare metal such as aluminum, any or all of the exterior surface of the surrogate waveguide  104 , mounting plate  106 , coupler housing  112 , aperture portion  108  may be coated with a highly thermally emissive material such as anodization or high temperature paint. The materials used to make many of the test apparatus elements can be thermally emissive, and many neighboring elements can be thermally coupled. For example, all portions of the surrogate waveguide  104  and the mounting plate  106  can be made from materials with high thermal emissivity and can all be thermally coupled to each. Anodized aluminum can be used to provide both thermal emissivity and electrical decoupling to prevent spurious electrical currents at contact points between the surrogate waveguide and the flight horn. In addition, an anodized layer can also preventing galling where neighboring parts rub against each other, such as where the surrogate waveguide  104  is slip-fit into the narrow end of the flight horn  102 . 
         [0021]    While a surrogate waveguide can have many shapes, such as circular or rectangular, only circular surrogate waveguides are depicted. The shape of a surrogate waveguide may be similar to the narrow end of the flight horn, and interior cross-sectional dimension of the surrogate waveguide may often be within a few percent of the interior cross-sectional dimension of the horn input waveguide where the slip-fit portion is mated to the flight horn in order to maintain a sufficient electrical match. The end of the slip-fit portion can be machined down until it is be very thin to help minimize difference in the dimensions, and matching rings or other elements can also be added to reduce coupling mismatch. 
         [0022]    The benefits of a surrogate waveguide are demonstrated in the simulated test results in  FIGS. 10 and 11 . These figures are graphs showing differences between a 5″ flight horn antenna and a 2″ surrogate waveguide inserted into a 5″ flight horn antenna. Benefits are shown at 10″ from the flight horn aperture with a reference 1 watt transmitted at 12.7 GHz.  FIG. 10  shows a 5″ flight horn produces a simulated flux density of 0.126 watts/in̂2, while the 2″ surrogate waveguide produces only 0.026 watts/in̂2. This is a reduction in flux density of almost 5 times at 10″ from the horn by inserting the surrogate waveguide.  FIG. 11  shows a related increase in beam width. At 10″ from the flight horn aperture, the 3 dB beam width is 2.3″ for the flight horn alone and 5.85″ when the surrogate waveguide is inserted. This results in a 3 dB beam width spread over nearly 6.5 times the area with the surrogate waveguide as compared to the flight horn alone. 
         [0023]      FIG. 2  details an example of the mounting plate  106  from  FIG. 1 . The mounting plate  206  is mounted to flight horn  202  with three pins  220 , each pin  220  in a pin mount  208 . The pins insert into matched receiving holes in the flight horn  202  that are right next to the horn&#39;s aperture end. The three pin mounts  208  can be distributed unevenly around the circumference of the mounting plate (at positions somewhat greater and lesser than  120  degrees from each other) to ensure the mounting plate will mount in only one orientation. The pin mounts  208  can be affixed to the mounting plate  206  with bolts  224 . Swiveling pins  220  can be disengaged or engaged with a matching receiving hole in the flight horn  202  by sliding the pin handle  222 . It will be understood that many mechanisms for attaching a mounting plate  206  to a flight horn  202  exist, and these pins  220  and pin mounts  208  are just one example. 
         [0024]    The center of the mounting plate  206  can have a hole with shape and dimension of the surrogate waveguide  104  and this forms a portion of the surrogate waveguide  104  along with other portions attached to either side of the mounting plate  206 . The mounting plate  206  can have one or more additional openings  210  to allow any energy to leak out that might build up between the flight horn  202  and the outside of the surrogate waveguide. The diameter of the openings  210  may be greater than or equal to half the wavelength of signals being tested (large enough to be above cutoff) to allow the energy in those signals to leak out, but the number and size of the holes  210  should be small enough to prevent most of the energy reflected back at the horn from re-entering the horn. The openings  210  also allow for venting and cable routing, for example if the surrogate waveguide has a coupler inside the flight horn  202  (as depicted in  FIG. 3 ). 
         [0025]      FIG. 3  illustrates a surrogate waveguide  304  with a directional coupler that fits inside the flight horn  302  and mounting plate  306 . In this example, the slip-fit portion  310  of the surrogate waveguide  304  is again slip-fit into the narrow waveguide end of the flight horn  302 , but the other end of the slip-fit portion is attached to a directional coupler  312  portion of the surrogate waveguide, which then connects to the mounting plate  306 , and then finally connects to the aperture portion  308  of the surrogate waveguide  304 . Alignment pins  314  can ensure the separate portions of the surrogate waveguide are attached to each other in only one alignment, enabling careful alignment to, for example, certain polarizations of the electric field. The directional coupler  312  may be comprised of three aligned air spaces at a  45 -degree angle relative to the electric and the magnetic field, as depicted in  FIG. 4 . The horn mounting flange  320  can be used for mounting the flight horn  302  to a satellite, and it will be understood that flange  320  may not exist in other examples, or may be of different shape or position as is appropriate for any particular application of a flight horn  302 . 
         [0026]      FIG. 4  shows a cross sectional view of the airspaces inside the directional coupler of  FIG. 3 . The central airspace  410  forms the portion of the surrogate waveguide  304  that runs through the middle of the directional coupler. As depicted in the examples of  FIGS. 3 and 4 , the surrogate waveguide  304  and corresponding airspace  410  may have a circular cross-section. Adjacent to, and aligned with, the central circular airspace  410  are two rectangular airspaces  412  that run along the directional coupler  312 . The rectangular airspaces  412  can be on opposite sides of the central airspace  410 , and aligned at a  45  degree angle from both the electric (E) and magnetic (H) fields of a single linear polarization, dual linear polarization, or circular polarization (CP) such that both electric and magnetic fields can be sampled, and test signals can be inserted into both fields. 
         [0027]    Continuing the discussion of the directional coupler  312  in  FIG. 3 , coaxial connectors  316  allow for the sampling and inserting of signals. Cover  326  may form the outside wall of a rectangular airspace  412  and is attached to directional coupler  312  with cover screws  318 . The other three sides of the rectangular airspaces  412  and the circular airspace  410  can be made by machining a single piece of aluminum. Energy holes  324  allow signal energy inside the surrogate waveguide  304  to leak into and out of the rectangular airspaces  412  for sampling and insertion with coaxial connector  316  field probes. Filter fins  322  can be tuned, for example, to couple in energy of the received signal while rejecting the energy of the transmitted signal. 
         [0028]      FIGS. 5 ,  6 , and  7  provide details of some examples of an E-probe directional coupler attached to E-probe housing  112  from  FIG. 1 .  FIG. 5  is a schematic of an E-probe directional coupler. The surrogate waveguide  502  has an incident wave  506  propagating from right to left while a reflected wave  504  propagates from left to right. These waves are sampled by probe one  510  and probe two  508  which are spaced at a distance  512  apart of one quarter of the circular waveguide wavelength at, for example, approximately 11 GHz. A delay equivalent to one quarter of a wavelength is added to probe one  510  through port A  516  by the internal phase shift of 90° of a 3 dB hybrid coupler  522  (such as a Krytar® 90° Hybrid Coupler). Probe two  508  through delay  512  equivalent to one quarter of a wavelength is directly connected to 0° port B  514  of the hybrid coupler  522 . Incident wave  506  coupled by probe one  510  and probe two  508  both being delayed 90° arrive in phase and at the port for sum incident power  520 . Reflected wave  504  couples directly to 0° port B  514  through probe two  508  and couples to 90° port A  516  through delay  512  equivalent to one quarter of a wavelength and arrive at the sum power port  520  180° out of phase with each other thereby canceling and providing directivity between incident and reflected power at sum port  520 . Reflected and incident power sum and cancel respectively at port  518  in the same manner that the incident and reflected power sum and cancel respectively at sum port  520 . Hybrid coupler  522  provides outputs for reflected power  518  and incident power  520 . 
         [0029]      FIG. 6  depicts a single-sided E-probe coupler configuration that can be effective with a 2 inch surrogate waveguide aperture and Ku band signals in the 10-15 GHz range. The surrogate waveguide  604  is formed from the pipe  602  through aperture  608 . The pipe attaches to attachment plate  606  (or face plate or mounting plate), followed by attaching to the E-probe housing, and then the aperture  608 . The E-probe housing has two pairs of SubMiniature version A (SMA) probes  610  on opposite sides of the surrogate waveguide  604 . Each pair of SMA probes  610  can correspond to probe one  510  and probe two  508  in  FIG. 5 . This single-sided configuration has a 3 dB hybrid coupler  612  attached to one pair of SMA probes, with only loads  620  attached to the other pair for mode balancing. One port of the  3  dB hybrid coupler has a load attached, while the other port has a 3 dB attenuator to improve match, connected to a cable  618  with 10-15 GHz signals that leads to a transmit (TX) combiner/receive (RX) splitter. Alignment pins  622  enable rotational alignment of the E-probe housing and E-probe with, for example, with certain electric field polarizations. 
         [0030]      FIG. 7  depicts a double-sided E-probe configuration that can be useful for both 10-15 GHz and 17-18 GHz Ku band signals. In this example, the surrogate waveguide  704  is formed from the same pipe  702 , attachment plate  706 , E-probe housing, and aperture  708  as in single sided configuration of  FIG. 6 . Here, however, there are two hybrid couplers attached to the two pairs of SMA probes  710  of the E-probe housing. The upper hybrid coupler is just like that of  FIG. 6 , and is a 3 dB hybrid coupler  712  with a load  714  attached to port B  713  and a 3 dB attenuator  716  attached to port A  715 . Cable  718  is attached to 3 dB attenuator  716  and may again carry signals in the 10-15 GHz range for both transmit and receive. The lower hybrid coupler  726  is attached to the lower pair of SMA probes  710 . Attached to port C  721  of the lower hybrid coupler  726  is a load  724 , and attached to port D  719  is a 10 dB attenuator  720  with cable  728  containing the 17-18 GHz received signal. Alignment pins  722  enable rotational alignment of the E-probe housing and E-probes, for example, with certain electric field polarizations. 
         [0031]    Alternate E-probe configurations are possible. For example, starting with the configuration of  FIG. 7 , and then removing load  714  and 3 dB attenuator  716 , and assuming matched devices (with little reflection) attached to port A  715  and port B  713 , then the  10 - 15  GHz frequency range will couple to port A  715  while the 15-18 GHz frequency range will couple to Port B  713 . Such a dual-band mode for 3 dB hybrid coupler  712  may work because signals to and from the hybrid ports in the E-probe coupler begin to reverse direction at approximately 15 GHz where the spacing between the probes is approximately one half circular waveguide wavelength. The hybrid coupler port C  721  and port D  719  have reversed direction at receive frequency 17-18 GHz in this configuration due to the phase delay between probe one  508  and probe two  510  of  FIG. 5 , which is now approaching the three quarter circular waveguide wavelength at approximately 20 GHz. Port D  719  now sums incident power  506  and cancels reflected power  504  near 20 GHz, thus providing directivity at receive frequency 17-18 GHz. A similar alternate configuration can also be applied to the lower hybrid coupler  726  for a dual-band mode at Port C  721  and Port D  719 . 
         [0032]      FIG. 8  depicts a group of flight horns as they might be arranged for a panel of a satellite. A satellite may have several such panels. In this panel, fourteen flight horns  802  are mounted together. Each of the horns can be fitted with a surrogate waveguide, for example as depicted in  FIG. 1 . Each surrogate waveguide may include an E-probe coupler, for example as depicted in  FIG. 6 . A 16:1 RX power divider/TX power combiner  804  can be temporarily mounted on the panel, and the cables from the E-probes can then be connected to power divider  804 . Finally, as depicted in  FIG. 9 , all fourteen flight horns  802  with surrogate waveguides can be covered by a single a FAL test configuration (surrogate waveguides not shown). Such a test configuration may include an actively cooled RF absorbent pad  806  placed in front of flight horns  802  at approximately a 10° tilt from the direction of energy from the horns, and a shroud  808  surrounding all four sides and all fourteen flight horns  802 . 
         [0033]    While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosures herein. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of certain of the disclosures herein.