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

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

With this geometry the two surfaces appear as a near perfect match in S-band because the reflections in the two layers cancel. The two-layer approach meets all requirements. Analysis indicated that the subreflector would exhibit less than a 0.25 dB loss at both bands.

Each of the two surfaces in the dichroic subreflector are made of crossed dipoles. The center of the reflection band depends on the dipole length and the dielectric on which the dipoles are placed. Bandwidth is controlled by the spacing of the dipoles.

Array spacing is expressed as the ratio of element gap to element length (G/L) for bandwith comparisons. The G/L ratio affects bandwidth (defined as the band of frequencies over which the reflection coefficient of the dichroic surface is higher than -0.5 dB).

Acting like a metallic surface, the subreflector becomes a mirror near the resonance frequency of the dipoles. The design length of the dipoles make the subreflector highly reflective in the X-band. On the other hand, the subreflector is a transparent to the s-band. At this much lower frequency, the dipoles appear short and almost invisible.

The radiation pattern of a central dipole in a planar array is calculated by a moment-method computer program. Dichroic surfaces are evaluated by analyzing a single diple in a cluster-by-moment program. Parameters are dipole length, width and spacing.

A purely analytical approach must be complemented by experimentation before actually constructing the subreflector. The theory may be accurate, but critical parameters such as the dielectric constant of a given complex material are only approximately known and may depend on construction techniques.

Making Hyperboloid Is Expensive

Manufacture of hyperboloid shapes is expensive. Testing that dichroic panels is a cost-effective beginning. Made from the same material as the subreflector, panels can be easily adjusted until the required performance is achieved. The panels are rotated from the perpendicular to 40 degrees to simulate the hyperboloid range of incident angles.

The development plan has five phases:

* Build a single layer FSS and test it at the X-band. Iterate the dipole length until the surface is resonant at the desired frequency.

* Build a second surface identical to the first.

* Space the two surfaces approximately 1/4 wavelength at the lowest frequency of operation. Measure the S-band performance as a function of the incident angle.

* Adjust the spacing until the acceptable performance (insertion loss) is achieved at S-band.

* Compute the appropriate subreflector spacing so that the mean path through the two layers approximates the spacing determined in Phase 3.

Reflection bandwitdth depends on both the G/L ratio of the dipoles and the incidence angle. Arrays with G/L ratios of 1.2, 1.4 and 1.6 were tested for reflectivity at several angles of incidence. Higher G/L ratios are preferred for the S-band. But the E-plane reflection (parallel polarization) coefficient is very frequently sensitive for larger incident angles.