Subreflector Developed for Simultaneous S and X-Band from a Single Antenna

Communications News, June, 1985 by G. Seck, H. Briscoe

Satellites operating in two distinct frequency bands are becoming more common. In the case of earth resource satellites, previously developed space and ground segments continue to operate in S-band while the next generation satellites--(Landsat-D, SPOT, ERS)--will have both S and X-band down links to support the increased data rate of the improved high-resolution sensors.

In order to achieve high system G/T's in the dual bands, a dichoric subreflector has been developed as an integral part of its S and X-band antenna system. The S-band feed is located at the prime focus of the main reflector; the X-band feed operates in a Cassegrain configuration. The subreflector thus must be transparent at S-band and reflective at X-band. This frequency selective behavior is called "Dichronic," a term borrowed from the optics.

Dichroic literally means two colors. In this case the two colors are the two separate bands, S-band (2.2 to 2.29 GHz) and X-band (8.025 to 8.5 GHz). The dichroic subreflector design described here has been implemented in two S and X-band Landsat antenna systems. The antenna systems are currently installed and operational in Sweden and Kenya.

A main reflector, a dichroic subreflector, and S and X-band monopulse tracking feeds comprise the system. With a focal length of 150 inches, the main reflector is a 9.14-meter paraboloid of revolution. The reflecting surface is manufactured from 24 stressed-sheet aluminum panels attached to a rugged space frame. Measured RMS surface error is 0.019 inch.

The subreflector is a 57-inch diameter hyperboloid of revolution constructed of two frequency-selective surfaces supported by a hollow fiberglass mounting ring.

Printed on a dielectric substrate, the dipole lattice structure adheres to hyperbolic panels made of 0.060 thick polyester resin fiberglass. The front and back panels are resin coated. The front panel is both resin coated and painted to protect the dipoles from weathering.

Measured losses due to the frequency selective subreflector are less than 0.2 dB in both bands of operation. Monopulse autotrack feed patterns are unaffected by the subreflector.

The cassegrain configuration for the X-band portion of the antenna is an adaptation of a 17th century optical telescope design. Invented by William Cassegrain, a contemporary of Isaac newton, the folded-optical-bath approach uses a two-reflector system. By placing a small hyperboloid mirror (subreflector) in front of a larger paraboloid primary mirror, higher magnification is obtained with a physically shorter telescope.

The microwave application of this established optical principle allows the use of shorter transmission lines in the antenna system and significantly, in a dualband design, makes the volume at the prime focus of the main reflector available for installation of a second feed.

Benefits provided by a Cassegrain configuration include:

* Greater flexibility in design of the primary feed.

* Spillover past the subreflector is directed at cold sky.

* Low spillover past the paraboloid toward the ground at high elevation angles.

Design tradeoffs at X-band required that the subreflector be at least 57 inches in diameter. The manufacture of this size of dichroic subreflector is challenging, because it is extremely difficult to maintain acceptable tolerances. Fabrication with a lathe and mill was out of the question. Instead, special vacuum molding tooling and processes were developed to fabricate the large dichroic surfaces.

The length and spacing of the resonant elements are determined by the characterisitcs of the transmitted RF energy. The Landsat-D application uses circular polarization, requiring the symmetrical placement of resonant crossed dipoles.

Subreflector Changes Properties

The surface of the subreflector must change its transmission and reflection properties with the frequency. This property is called a frequency selective surface (FSS). An ideal dichroic surface is perfectly reflective at one microwave frequency while completely transparent at another frequency.

For this application, the dichroic, frequency selective surface is composed of a two dimensional array of printed circuit, crossed dipoles. The dipole resonance is designed for 8.2 GHz, making the surface highly reflective. At the lower frequencies of S-band, the dipoles are virtually invisible.

Initially, a design with only one FSS was considered. In order to meet mechanical requirements at dielectric thickness of 125 mils was needed. Analysis showed electrical performance at X-band would be acceptable.

At S-band there was a 1 to 2 dB transmission loss due to the dielectric material. To reduce S-band losses to 0.5 dB or less the dielectric thickness would have to be reduced to approximately 10 mils. The single FSS design waws therefore discarded based on mechanical considerations. A dual FSS design was then addressed, in this approach a second layer is placed behind the first layer. Separation between layers was held to a 1/4 wavelength at S-band.