A well known problem in radiofrequency (RF) astronomy is how to separately investigate both polarization modes from a RF source. Orthomode transducers (OMTs) are used to de-diplex incident electromagnetic (EM) radiation. In many other applications there is need for polarization diplexers and de-diplexers. Generally low return loss, high isolation and low cross-polarization OMTs are desired. While it is also generally desirable to reduce insertion loss, this loss is mostly attributed to conduction losses within the waveguide structure, and so is principally determined by the materials, and less so by the design, and are rather small. As return loss, cross-polarization and isolation depend on wavelength, there is a need for OMTs that provide acceptable quality in each respect over an intended operating range. In some cases the operating range is narrow band, but in many cases the broader the band the better. All OMTs are trade-offs in these features along with costs of production and assembly, and reliable operation.
A chief component of OMTs that can be used to classify them is the (principal) junction, which connects a polarization diplexed waveguide with at least 2 paths. Some OMT designs use a turnstile junction while other OMT designs use Boifot junctions or double ridge junctions.
The Boifot junction is a relatively complex device that requires two pins and a septum (three matching elements) or an iris to be positioned within the waveguide. Features such as the matching pins, septum, or iris, increase return loss, increase an expense of the device, complicate assembly, limit the bandwidth over which the OMT operates, and limits the smallest size the OMT can obtain (and/or the fabrication techniques that can be employed to produce them), limiting low cost production of higher frequency band OMTs. Higher frequency OMTs require smaller devices, and greater accuracy of the definition of the matching elements, which is increasingly difficult to produce. Moreover, the septum is a mobile piece and the return losses of the OMTs are therefore prone to change when the septum is moved out of alignment.
There are several commonalties between the Boifot and double ridge OMT structures. In both cases, two arms are fundamentally different: one arm is provided by a pair of (initially oppositely directed) waveguides that are subsequently reunited, and the other arm is never divided. The double ridge design also requires intricate features (on both sides of the waveguide) that are also more difficult and expensive to produce and assemble, and increasingly so at smaller scales (higher frequencies). As matching features have to be provided on two or more parts and as there are very low-tolerances for the alignment of these features, it is unsurprising that these designs fail to produce high quality OMTs with good repeatability, especially at higher frequencies (i.e. above 30 GHz).
For example, Double-Ridged OMT (Shin'iciro Asayama National Astronomical Observatory of Japan) according to which ridges are provided from above and below an input waveguide shows a design that is, in principle, scalable to smaller dimensions and higher frequencies. While 7 examples were produced and all have apparently the similar reflection losses, cross-polarization over the band from 110-170 GHz varies from −24 to −40 dB depending on the example, and over ALMA band 4, varies from −28 to −42 dB. Having matching elements produced on multiple parts complicates production and assembly and leads to small errors that can affect reproducibility and/or quality of the OMT. The problems with repeatability may be caused by the fact that the matching elements are provided in multiple parts.
A turnstile junction is a waveguide network with a diplexed waveguide port (+z axis) and two paired perpendicular waveguide paths (+,−x axis, and +,−y axis). A matching (or tuning) element (or feature having one or more elements) is provided at the origin where these waveguide paths meet, opposite the diplexed waveguide port. To produce an OMT, the oppositely directed pairs of waveguide paths are made to recombine after traveling equal electrical path lengths, and the recombined paths communicate with respective ports. Thus there are three ports, one for s-polarized signal, one for p-polarized signal, and one for the diplexed signal.
Various matching features are known for turnstile junctions, including a trunked pyramid (as used by Navarrini et al. described below), and concentric cylinder matching stubs (e.g. M. A. Meyer et al. entitled “Applications of the Turnstile Junction” IRE-Transactions on Microwave Theory and Technique pp. 40-45, December 1955).
Among the OMT literature surveyed by the Applicant, there was only one device that was able to offer very good cross-polarization and isolation. It was taught by Navarrini et al. in “A Turnstile Junction Waveguide Orthomode Transducer” IEEE Transactions on Microwave Theory and Technique, v.54, Not 2006. The OMT was designed for the 18-26 GHz frequency range, and across this range, the insertion loss was 0.15 dB, and the cross-polarization was less than −47 dB for both polarizations. While it may be desirable to improve on the return loss of (−19 dB), it represents a major improvement over the available alternatives. The model of this OMT includes one 180° bend for each of the 4 waveguide paths from the turnstile so that the paired waveguide paths are convergent, and the pairs are then coupled by E-plane Y power combiners. The OMT is divided into 4 parts that are assembled as 4 quarters, each having an edge that meet along a center axis that passes through the center of the diplexed waveguide.
When Navarrini (Test of 1 mm Band Turnstile Junction Waveguide Orthormode Transducer, 17th Int. Symp. on Space Terahertz Technology P1-21) attempted to miniaturize the same design, to produce an OMT for use in the 200-270 GHz frequency range, the transmission loss was 0.8 dB, the return loss was −12 dB and the cross-polarization was lower than −25 dB for both polarizations. This device is not nearly as successful as the 18-26 GHz frequency range device.
There are other OMT designs known in the art that use a turnstile junction with matching feature as the principle divider. Some are appreciated for their compactness, and low part count, but deliver (or even fail to deliver) marginal quality polarization diplexing or de-diplexing, i.e. return loss, cross-polarization and isolation from −20 to −25 dB, even over lower bandwidths. The low quality of the many known turnstile-based OMT designs, Navarinni's first design excluded, and the non-scalability of Navarrini's device would lead research away from this design. Some examples of low quality turnstile-type OMT designs and their noted features include U.S. Pat. No. 7,397,323 to Hozouri and a paper to Aramaki et al. entitled “Ultra-Thin Broadband OMT with Turnstile Junction” (also patented U.S. Pat. No. 7,330,088 and U.S. Pat. No. 7,019,603).
U.S. Pat. No. 7,397,323 to Hozouri teaches a waveguide orthomode transducer having at a first layer, a turnstile junction having a main waveguide and four waveguide ports, each coupled to a respective magic-T with an E-port, two opposed side-ports, and an H-port. The magic-Ts (called therein hybrid Ts) are ring-arranged around the turnstile junction so the waveguide ports each communicate with one H-port, so adjacent magic-Ts inter communicate with their respective side-ports, and so the E-ports form two sets of opposed E-ports. In a second layer two H-plane power dividers/combiners each have an axial-port and two opposed side-ports. The H-plane power dividers/combiners are arranged so their respective side-ports communicate with different ones of the two sets of opposed E-ports and so their axial-ports are polarization ports. This permits a single signal with two fundamental orthogonally polarized modes to enter the main waveguide and exit separated at the polarization ports, and vice versa.
This design is stated to be advantageous in that it provides a compact and thin waveguide OMT, and that it is easy to manufacture, however no explanation as to how it would be manufactured is taught or suggested. Furthermore, no example is provided, and no data regarding signal power, isolation, mode purity, bandwidth, voltage standing wave ratio, or any other feature (except profile height, which is not supported by any simulation or other data).
In any case, the ring coupling of the 4 turnstile arms is expected not to provide low return loss, isolation, or low cross-polarization, because of the use of magic-T junctions. Magic-T couplers typically have theoretical return losses of about −20 dB over a 22.4% bandwidth (−30 dB minimum at a point), if the magic-T is properly matched (in this case with an inductive post), and is never better than about −5 dB without the matching element. FIG. 1 is a graph showing simulated return losses for a magic-T coupler, with and without an inductive post matching feature. It will be noted that unmatched magic-Ts have high return losses that constitute impermissible losses in many applications. Furthermore these losses can result in standing waves that lead to internal arcing, which must be avoided, for example, by limiting a power the OMT can handle safely. Matching features (inductive post, iris, reflectors or matching screws) can be added to the magic-T to reduce reflection losses. Such matching features are typically expensive to manufacture or position, and, while they may significantly reduce return losses (reflection), this improved loss is typically over a narrow bandwidth. The inclusion of these elements must be precisely aligned. As they are formed on different planes of different parts, this complicates alignment within required precision. Furthermore these features also limit the power the OMT can safely diplex/de-diplex. Hozouri does not mention any matching element, without which the device has a theoretically optimal return loss of about −5 dB.
In Hozouri's example, each magic-T junction is used twice. The magic-T is first used as a (polarization neutral) H-plane divider, to divide the output of the turnstile into R and L signals, and to couple these R and L signals to the ring (in opposite directions). Then the magic-T is used as an E-plane combiner for combining +L with −R (or vice versa) from the other two adjacent turnstile outputs, and sending the combination up to the next level. Thus, twice the return loss of the magic-T is imparted to the incoming signals. Finally, the +L-R signal is combined with the −L+R signal in the second plane using a third T or magic-T junction. These losses, over the Ku band (12.4-18 GHz), are substantial.
The paper to Aramaki et al. shows a turnstile junction-based OMT that can advantageously be defined in 3 pieces. Like Hozouri, Aramaki et al. have illustrated the E-plane power combiners for each arm as a T coupler. T couplers are very poor quality junctions, and are never better than the magic-T couplers described above. Unfortunately, it is impossible to replace such couplers and have the desired parts count. For example, in both cases, replacing these T couplers with more complex structures would require at least 2 more pieces, or will not permit low cost fabrication equipment to be used. T couplers are typically higher reflection than magic-T couplers, and result in the same standing wave problems that limit power handling of the OMT.
The best example from Navarrini et al. is operable in the 18-26 GHz range, and the other (200-270 GHz) does not provide acceptable signal quality for some applications. The double ridge and Boifot type OMTs are not scalable to higher frequencies and cannot be produced with low cost forming techniques.
Thus there remains a need in the art for an OMT capable of high quality (low return loss, cross-polarization and isolation) without multiple matching elements that complicate manufacture, limit operating range, and increase cost. Especially desirable is an OMT that operates over a broad frequency range, a frequency range that includes frequencies above about 30 GHz, or one that can be machined and assembled with high accuracy with relative ease.
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