High performance multimode horn

Details High performance multimode horn High performance multimode horn

2001-04-13

2002-05-28

Phan, Tho G. (Department: 2821)

Communications: radio wave antennas

Antennas

Wave guide type

Reexamination Certificate

active

06396453

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a horn for use in RF signal transmitters or receivers, and more particularly to a multimode horn having higher order modes generated through discontinuities such as corrugations, smooth profiles, chokes and/or steps.
BACKGROUND OF THE INVENTION
Modern broadband high capacity satellite communication systems give rise to a host of challenging antenna design problems. High-gain Multi-Beam Antennas (MBAs) are probably the best example of such challenging antenna designs. The MBAs typically provide service to an area made up of multiple contiguous coverage cells. The current context assumes that the antenna configuration is of the focal-fed type, as opposed to an imaging reflector configuration or a direct radiating array. It is also assumed that each beam is generated by a single feed element and that the aperture size is constrained by the presence of adjacent feed elements generating other beams in the contiguous lattice.
Impact of feed performance on MBA Performance
It is well known that in order to achieve an optimal reflector or lens antenna performance, the reflector illumination, including edge-taper, needs to be controlled.
FIG. 1
illustrates the EOC (Edge Of Coverage) gain of a typical MBA as a function of reflector illumination taper, assuming a cos
q
-type illumination. The first-sidelobe level is also shown, on the secondary axis. Depending on sidelobe requirements,
FIG. 1
shows that a reflector edge-taper of 12 to 13 dB (decibels) is close to optimal. A slightly higher illumination edge-taper will yield a better sidelobe performance with a minor degradation in gain.
In multiple beam coverages, ensuring an adequate overlap between adjacent beams, typically 3 or 4 dB below peak, requires close beam spacing. In such applications where reflector or lens antennas are used and where each beam is generated with a single feed element, this close beam spacing leads to a feed array composed of tightly clustered small horns. The performance of such antennas is limited by the ability to efficiently illuminate the antenna aperture with small, closely-packed feed elements producing a relatively broad primary pattern. The main factors limiting antenna performance include:
1—High antenna spill-over losses, degrading gain performance; and
2—Limited edge illumination taper, leading to relatively high sidelobe levels.
Multiple reflectors generating sets of interleaved alternate beams have been proposed as a mean of alleviating the performance limitations described above. By using multiple apertures, the feed elements are distributed, hence the spacing and size of elements on a given feed array can be increased, resulting in a narrower, more directive, primary pattern for each feed element. The element size approximately increases as the square root of the number of apertures used. For example, interleaving the beams produced by four reflectors, as shown in
FIG. 2
, yields an element whose size is increased by a factor of about two (2). This greatly reduces spill-over losses and consequently improves the co-polarized sidelobe levels. The four different beam labels, identified by letters A, B, C & D in
FIG. 2
, refer to beams generated by the four apertures having corresponding designations.
Although multiple apertures significantly improve antenna performance by increasing the physical element size, it can be easily demonstrated that even with four apertures, the performance of MBAs employing a single feed element per beam is still limited by the aperture efficiency &eegr; of the feed element defined as:
&eegr;=
g
*(&lgr;/&pgr;
d
)
2
where g is the peak gain, or directivity, &lgr; is the lowest wavelength of the signal operating frequency band and d is the physical diameter of the feed element, or feed spacing.
Assuming a cos
q
-type feed pattern, it can be derived that the illumination edge-taper (ET) of a four-reflector system is:
ET
(
dB
)≈13*&eegr;
where &eegr; is the feed aperture efficiency. This means that for a four-reflector system, feed elements with at least 92% aperture efficiency are needed in order to achieve the 12 dB illumination taper, identified as optimal in FIG.
1
. Achieving a higher edge-taper, for better sidelobe control, necessitates even higher feed aperture efficiency.
Similarly, we find that if three reflectors are used instead of four, the reflector illumination edge taper can be approximated as:
ET
(
dB
)≈9.75*&eegr;
In reality, the relationship between ET and &eegr; is not exactly linear. A more rigorous analysis shows that as the edge-taper increases, the reflector size also needs to be increased in order to maintain the same beamwidth. This increase in reflector size results in a second-order increase in reflector edge-taper.
As illustrated in
FIG. 3
, a parametric analysis shows that the MBA gain is optimal for a feed aperture efficiency of about 95%. Selection of another beam crossover level would affect the location of the optimal point, but in general the optimal feed efficiency will always be between 85% and 100%.
Conventional solutions
It has been established that high aperture efficiency elements are required to maximize the performance of MBAs. Although conical horns offer reasonable aperture efficiency (typically between 80% and 83%), they suffer from bad pattern symmetry and poor cross-polar performance. Dual-mode or hybrid mode horns have been developed to ensure excellent pattern symmetry and cross-polar performance. Conventional dual-mode horns include the well-known Potter horn and hybrid multimode horns are usually of the corrugated type, as illustrated in
FIGS. 4 and 5
respectively.
Potter horns typically offer 65-72% efficiency, depending on the size and operating bandwidth. Corrugated horns can operate over a wider band but yield an even lower efficiency, due to the presence of the aperture corrugations that limit their electrical diameter to about &lgr;/2 less than their physical dimension.
Consequently, as shown in
FIG. 3
, conventional dual-mode or hybrid mode feedhorns do not allow to achieve an optimal MBA performance, since insufficient reflector edge-taper results in high sidelobe levels and a gain degraded by high spill-over losses.
OBJECTS OF THE INVENTION
It is therefore a general object of the invention to provide an improved horn that obviates the above noted disadvantages.
Another object of the present invention is to provide a multimode horn having a series of discontinuities for altering the mode content of the signal transmitted and/or received there through.
A further object of the present invention is to provide a multimode horn that alters the mode content of the signal transmitted and/or received there through via regular and/or irregular corrugation, smooth profile, choke and/or step discontinuities.
An advantage of the present invention is that the multimode horn uses the full size electrical aperture even though corrugation type discontinuities are present.
Another advantage of the present invention is that the multimode horn feeding an antenna is tailored relative to a plurality of performance parameters including at least one of the following: horn on-axis directivity, horn pattern beamwidth, antenna illumination edge-taper, antenna illumination profile and antenna spill-over losses.
Still a further advantage of the present invention is that the multibeam antenna is fed with multimode horns, each having a series of discontinuities for altering the mode content of the signal transmitted and/or received there through, to maximize the overall performance of the antenna relative to its application.
Another advantage of the present invention is that it is possible to design a multimode horn feeding an antenna that is optimized with discontinuities altering the mode content to achieve a balance between a plurality of performance parameters of said antenna over a pre-determined frequency range of said signal, thus maximizing the secondary radiation pattern and overall performance of the antenna.
Other objects and advantages of the present in

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