Dual Frequency Antenna Feeding With Coincident Phase Centers

This invention relates in general to antenna feeding and more particularly concerns a dual frequency, prime focus, remotely adjustable polarization, antenna feed assembly having the two phase center locations of the feed coincident and resulting in coincident secondary radiation pattern main beams.

It is an important object of this invention to provide improved apparatus and techniques for dual frequency antenna feeding.

According to the invention, there is a microwave resolver having first and second colinear microwave cavities comprising circular and coaxial waveguides respectively. A conducting partition between the circular and coaxial waveguides comprises a higher frequency waveguide. A plurality of coaxial lines arranged around the periphery of each of the colinear microwave cavities, parallel to the cavity axis and equally spaced about it comprise means for electromagnetically coupling the two cavities, each of the small coaxial lines being approximately 1/4 waveguide wavelength from the bottom of each cavity and having inner conductors terminating in electric field probe extensions arranged radially within each cavity. The microwave resolver device comprises means for transforming the microwave field from a TE.sub.11 circular waveguide mode in the circular waveguide to an identically polarized TE.sub.11 coaxial waveguide mode in the coaxial waveguide. According to a specific aspect of the invention, the higher frequency waveguide comprising the cavity partition is connected to the inner conductor of the coaxial cavity to define a higher frequency transmission path inside the tubular inner conductor. According to another aspect of the invention the conducting partition is rectangular and comprises a polarization rotator assembly means allowing rotation of the polarization within the tubular inner conductor. Preferably, the circular and coaxial waveguide outer cavity portions may be connected to a low frequency polarization rotator. The coaxial waveguide end may be connected to or form a radiating aperture including the tubular inner conductor comprising a high frequency aperture and the coaxial section forming a lower frequency aperture, both radiating appropriate TE.sub.11 waveguide modes. Preferably, the phase center of the aperture comprising the tubular inner conductor and the phase center of the aperture comprising the coaxial waveguide are both at the same axial location for providing an apparent focal point in the two respective high frequency bands. The radiating aperture is preferably surrounded by a set of concentric metal rings having a depth approximately 1/4 to 3/8 wavelength and a spacing in the radial direction less than 1/2 wavelength. Preferably the coaxial cavity inner conductor includes a single metal choke tube having an approximate depth of 1/4 wavelength at the high frequency band and a dimeter which is somewhat greater than the inner conductor diameter to comprise means for suppressing currents flowing into the coaxial waveguide cavity.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

FIG. 1 is an axial sectional view of a dual frequency band feed according to the invention;

FIG. 2 is a sectional view through section 2--2 of FIG. 1;

FIG. 3 is a view through section 3--3 of FIG. 1;

FIG. 4 is a front elevation view of the embodiment shown in FIG. 1 with part of the low-band polarization rotator subassembly removed.

With reference now to the drawing and more particularly FIGS. 1-4 thereof, there is shown various views of a feed according to the invention.

Referring to FIG. 4, the feed comprises three sub-assemblies:

(a) Low-band polarization rotator sub-assembly 1

(b) Microwave resolver sub-assembly 2

(c) Radiating aperture sub-assembly 3

In this invention, the low-band polarization rotator may be any available device, but may be the polarization rotator described in U.S. Pat. No. 4,504,836, for example.

Referring to FIG. 1 the radiating aperture assembly comprises a set of "scalar" metal rings 4; that is, a series of concentric grooves nominally 1/4 to 3/8 wavelength deep, whose function is to shape the primary radiation pattern and minimize feed spillover and maximize antenna efficiency. Such feed "scalar" rings are in common use and have been widely discussed in the literature.

The high-band radiating aperture is an open-ended circular waveguide surrounded by a 1/4-wavelength deep choke 5; this waveguide is located coaxially with the low band radiating aperture, which is a coaxial waveguide.

The electromagnetic fields propagating within the high band circular aperture are designated the circular TE.sub.11 mode. The mode of propagation within the low band aperture is the coaxial TE.sub.11 mode and the dimensions of the respective circular tubes are selected to ensure that these desired modes propagate with cutoff frequencies nominally about 20% below the lowest operating frequency within each respective frequency band. The uppermost operating frequency is limited by the presence of transverse magnetic propagation modes and generally will set a bandwidth limit of about 30% on the respective operating frequency bands.

The central microwave "resolver" sub-assembly 2 is an important feature of this invention. Its function is to inject the desired coaxial TE.sub.11 mode into the low band coaxial aperture waveguide and to provide a means for incorporating a high-band polarization rotator device 6 within the device. A feature of this device is that it performs these functions for all angles of linear polarization, since many applications of this feed involve Earth Station antenna use in which the polarization must be rotated remotely for alignment with that of the satellite signal.

It is convenient to define a polarization rotator as that device which converts a TE.sub.11 rectangular waveguide mode signal into a remotely adjustable linear polarized TE.sub.11 mode signal in a circular waveguide.

A resolver according to the invention comprises a set of two axially displaced co-linear metal cavities 7 and 8 separated by a relatively thick metal shorting plate. One of the cavities 7 comprises a circular cross-section waveguide; the opposite cavity 8 comprises a coaxial cross-section waveguide. The thick shorting plate 9 which separates the two cavities 7 and 8 contains a rectangular waveguide 10 for the high-band signal; this waveguide extends radially from the center of the device to a waveguide flange port 20 outside the device, as seen in FIG. 4.

There are four small coaxial transmission lines 11 situated 90 degrees from each other around the outside diameter of the circular cavities 7 and 8 and extending approximately halfway up (about 1/4 low-band waveguide length) from the bottom of each cavity. The inner conductors 12 of these four coaxial lines are connected to a set of four radially disposed metal probes 13 formed onto (for example) a plastic laminate printed circuit board 14 of a low dielectric constant material such as fiberglass or Teflon composite. Their function is to "resolve" the TE.sub.11 mode which exists in their respective cavity at an angle "A" with respect to the probe set into two components whose amplitudes are given by the following table:

These resolved signals then propagate through the four coaxial lines to the opposite sets of probes where they are summed as vector fields into a TE.sub.11 mode whose polarization is identical to the original "A" angle in the first cavity.

Thus, the low band signal within the resolver travels through the device without polarization rotation (independent of the incident polarization) and is transformed from a circular waveguide TE.sub.11 mode in cavity 7 to a coaxial waveguide TE.sub.11 mode in cavity 8.

The high band signal is injected into the central circular waveguide 15 (which forms the center "conductor" of the low band coaxial waveguide 8) by a polarization rotator similar in design (or the equal) to that of the low band device. For background on this device, reference is made to U.S. Pat. No. 4,504,836.

It is arranged for the polarization of the high and the low band signals to be remotely rotated by mechanically coupling shafts 16 of the two (low and high band) polarization rotators. This is accomplished by arranging the high band polarization rotator shaft so that it mechanically engages the probe dipole 17) element of the low band polarization rotator. Therefore, the actuator (motor or servo device) which rotates the low band polarization also rotates the high band polarization. In use, the two frequency band polarizations are usually aligned parallel to each other since most applications have common polarization alignment at the satellite or transmitting location. However, nothing prevents other low/high band alignments other than adjustment of the shaft coupling during assembly.

One of the principal uses of the invention is to receive signals from so-called "hybrid" geostationary communications satellites which emit signals in the 3.7-4.2 GHz (C-Band) and the 11.7-12.2 GHz (Ku-Band) frequency bands simultaneously. Other frequencies or combinations may, or course, be of interest as well.

These C- and Ku-Band signals may be received from a particular version of the subject invention which, as an example, will be described here for clarity and to illustrate a practical case.

The dimensions shown in FIG. 1 have been found to be preferred for this frequency band combination. The high and low band waveguide port flange 20 and 21 support a weather cover 19 over the radiating apertures.

Performance parameters for this particular feed which have been verified by actual measurement are as follows:

There has been described novel apparatus and techniques for dual frequency antenna feeding having numerous electrical and mechanical advantages discussed above. It is apparent that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel features and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.