Frequency selective surface devices for separating multiple...

Details Frequency selective surface devices for separating multiple... Frequency selective surface devices for separating multiple...

1997-09-11

2001-03-27

Lee, Benny T. (Department: 2817)

Communications: radio wave antennas

Antennas

Antenna components

C333S134000, C333S202000

Reexamination Certificate

active

06208316

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to frequency selective surface devices for separating or combining two channels of electromagnetic radiation.
BRIEF DESCRIPTION OF THE PRIOR ART
Each channel so separated or combined may in turn be sub-divided or sub-combined using another frequency selective surface device of the kind to which the invention relates, or using another type of separator or combiner.
One example of a frequency selective surface is shown in FIG.
1
. Incoming energy having spot frequencies f
1
and f
2
is separated at the frequency selective surface
1
into a reflected beam f
2
and a transmitted beam f
1
. As shown, the frequency selective surface in
FIG. 1
separates the two frequencies f
1
and f
2
. However, the device is reciprocal and can be used for combining frequencies f
1
and f
2
if the directions of incidence are reversed. A possible frequency response for such a frequency selective surface
1
is shown in FIG.
2
. The transmission band is defined as the band of frequencies over which in excess of 90% of the incident energy is transmitted, and the reflection band is defined as the band over which in excess of 90% of the incident energy is reflected. While transmission and reflection bands are referred to in this text as for a 10% percentage loss in energy, it is possible to define the bands for other percentage transmission or reflection losses. In
FIG. 2
, the transmission band extends from a lower limit T
L
to an upper limit T
U
and the reflection band extends from a lower limit R
L
to an upper limit R
U
.
One use of such frequency selective surface devices is for increasing channel capacity of reflector antennas, particularly in satellite communications, but also in terrestrial use. A single transmit reflector may be fed by two or more feed horns, or a single receive reflector may direct radiation into two or more feed horns. The frequency selective surface device transmits a large percentage of the energy incident on it in one frequency band and reflects a large percentage of the energy incident on it in another frequency band, and the physical separation or combination of the beams permits the use of one reflector with two feed horns. Each feed horn can then be optimized to the reflector for its particular frequency band. The frequency selective surface device may be mounted in a waveguide assembly to filter energy as a waveguide beamsplitter. However, such frequency selective surface devices are also used as quasi-optical beamsplitters in multi-band radiometers (devices for detecting radiation, usually low-level and usually natural radiation). They are particularly applicable to high frequencies such as wavelengths in the region of centimetres, millimetres and in the sub-millimetre range and beyond into the infra-red region, but are of course generally applicable across the whole electromagnetic spectrum.
Frequency selective surfaces may be used singly or in cascade. Each such frequency selective surface has a conductive pattern on a substrate.
One such pattern is a lattice grid. In one proposal (U.S. Pat. No. 4,476,471), a three layer lattice grid has been proposed, the three layers
2
,
3
,
4
(
FIG. 3
) being used so that interactions between the layers generate a broad transmission band (FIG.
4
). Unlike the surface whose frequency response is illustrated in
FIG. 2
, which is a low pass arrangement, the lattice grid provides a high pass response. The response of a single layer is shown by the dotted line and the full line shows the effect of the three layers together. Even after the sharpening effect of the three layers, the ratio between the lower edge of the transmission band and the upper edge of the reflection band is still around 1:1.2.
Another proposed form of frequency selective surface consists of an array of conductive rings
5
(
FIG. 5
) which are printed onto a dielectric substrate
6
. (E. A. Parker and S. M. A. Hamdy, “Rings as elements for frequency selective surfaces”, Electron. Lett., Vol. 17, No. 17, 1981, pp 612-614). The individual rings are an integral multiple of the wavelength of the incident radiation in circumference and are therefore resonant, as well as being coupled to each other. The result of this is a sharper transition between transmission and reflection bands, as shown in full line in FIG.
6
. Nevertheless, the ratio between the lower edge of the reflection band and the upper edge of the transmission band is typically 2.5:1 to 3.01:1.
It has also been proposed to use “double resonant” elements on the substrate such as
7
or
8
. While these are shown in cutaway regions, in practice the entire array would be uniformly made of each of these elements in place of the rings. The rings
5
are single resonant in the sense that they can resonate at only one series of related frequencies (which will be harmonically related in the case of normal incidence and assuming that the electrical properties of the dielectric do not vary with frequency, but in which the higher order resonances in particular shift with frequency for inclined angles of incidence on the frequency selective device). The double resonant elements have smaller additional sections which are separately resonant. Thus, the double ring
7
is resonant at integral multiples of the circumference of the outer ring and integral multiples of the circumference of the inner ring (for normal incidence). The Maltese cross (also called a Jerusalem cross)
8
is resonant at integral multiples of the length of its dipoles as well as the integral multiples of the length of its endcaps (again, for normal incidence). The effect of these additional resonances is to produce an additional reflection band, as shown by the broken line in
FIG. 6
, so that the upper transmission band is pushed closer to the lower transmission band, and this reduces the ratio of the edge of the upper transmission band to the edge of the reflection band to around 1.3:1. The device is a high pass device. The printed resonant element array of
FIG. 5
is usually used singly, but proposals have been made to use an array of squares in cascade (R. Cahill, I. M. Sturland, J. W. Bowen, E. A. Parker, and A. C. de Lima, “Frequency selective surfaces for millimetre and sub-millimetre wave quasi optical demultiplexing”, Int. J. of Infrared and Millimetre Waves, Vol. 14, No. 9, 1993 pp 1769-1788), and also an array of Jerusalem crosses in cascade (J. A. Arnaud and F. A. Pelow, “Resonant Grid Quasi-Optical Diplexers”, Bell System Technical Journal, Feb. 1975 Vol. 54 No. 2 pp 263-283).
However, recently more stringent filtering requirements have been defined with the development of space-bome radiometers which are designed to survey emissions over the sub-millimetre band in the earth's upper atmosphere. Here certain species which are of interest to atmospheric chemists emit energy over frequency bands which are very closely spaced, with edge band ratios of 1.03:1 or less. Such radiometers are normally fed by a single reflector antenna
SUMMARY OF THE INVENTION
The invention provides a frequency selective surface device for separating or combining two channels, which comprises at least two frequency selective surfaces, each defining a transmission band and a reflection band of frequencies, each comprising an array of coupled resonant elements. These elements are resonant at only one series of related frequencies, so that the transmission and reflection bands defined are relatively broad, and wherein the spacing of the surfaces is such that multiple reflections between the surfaces results in the reinforcement of these reflections on emergence, whereby the transmission and reflection bands have a relatively sharp transition, permitting combination or separation of closely spaced channels.
The use of interference effects between the layers to provide reinforcement of the reflections on emergence, together with the use of an array of single resonant elements, permits frequency selective surface devices to be constructed which have channels spaced as closely as 1.03:1 ratios between

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