Cassegrain-type feed for an antenna
View Patent ↗A cassegrain-type feed for a parabolic antenna is a dual-band feed and employs a waveguide feeding a dielectric cone feeding a subreflector. The waveguide has an end-portion adjacent a narrow end of the cone. The impedance of an inner wall of the cone is modified by the inclusion of a dielectric sleeve of thickness between ⅙ and ¼ of a wavelength relative to propagation in the sleeve at a mean value of the upper of the two frequency bands concerned. The sleeve helps to provide a rotationally substantially symmetric illumination of the subreflector in the upper frequency band and, when used with a parabolic main reflector, a similarly symmetric illumination of the main reflector also. The sleeve may be replaced by a series of grooves formed in the inner wall of the waveguide end-portion, these grooves being nominally ¼ of a mean wavelength deep.
1. A cassegrain-type feed for an antenna, comprising:
a) a waveguide section having an end-portion, the waveguide section having internal dimensions which support a propagation of a fundamental quasi-TE 11 mode in lower and upper frequency bands;
b) a dielectric cone having a small-diameter end and a large-diameter end, the small-diameter end adjoining the waveguide end-portion;
c) a subreflector adjoining the large-diameter end of the cone;
d) a multi-stage step transformer attached to the small-diameter end of the cone for matching an impedance of the cone to the waveguide section;
e) the feed being a dual-band feed covering the lower and upper frequency bands; and
f) the waveguide end-portion being provided at an inner wall thereof with a wall-impedance changing means comprising grooves formed in the inner wall of the waveguide section, and operative for changing an impedance of the inner wall to couple the quasi-TM 11 mode in the upper frequency band, to thereby achieve a rotationally substantially symmetric illumination of the subreflector in the upper frequency band.
2. The feed as claimed in claim 1 , wherein the grooves have a depth of approximately one-quarter of a mean wavelength of the upper frequency band, referred to propagation in the waveguide section.
3. The feed as claimed in claim 1 , wherein the waveguide section is of substantially uniform diameter throughout its length.
4. The feed as claimed in claim 1 , wherein a final stage of the dielectric transformer located at an aperture of the waveguide end-portion has a diameter which is approximately 75% of that of the waveguide end-portion.
5. The feed as claimed in claim 1 , wherein the subreflector has a central portion, and a disk at the central portion for reducing return loss in signals incident upon the subreflector.
6. A cassegrain-type feed for an antenna, comprising:
a) a waveguide section having an end-portion, the waveguide section having internal dimensions which support a propagation of a fundamental quasi-TE 11 mode in lower and upper frequency bands;
b) a dielectric cone having a small-diameter end and a large-diameter end, the small-diameter end adjoining the waveguide end-portion;
c) a subreflector adjoining the large-diameter end of the cone;
d) a multi-stage step transformer formed as an integral part of the cone at the small-diameter end of the cone for matching an impedance of the cone to the waveguide section;
e) the feed being a dual-band feed covering the lower and upper frequency bands; and
f) the waveguide end-portion being provided at an inner wall thereof with a wall-impedance changing means for changing an impedance of the inner wall to couple the quasi-TM 11 mode in the upper frequency band, to thereby achieve a rotationally substantially symmetric illumination of the subreflector in the upper frequency band.
7. A cassegrain-type feed for an antenna, comprising:
a) a waveguide section having an end-portion, the waveguide section having internal dimensions which support a propagation of a fundamental quasi-TE 11 mode in lower and upper frequency bands;
b) a dielectric cone having an outer flared surface, a small-diameter end and a large-diameter end, the small-diameter end adjoining the waveguide end-portion, the outer flared surface having a series of corrugations;
c) a subreflector adjoining the large-diameter end of the cone;
d) a multi-stage step transformer attached to the small-diameter end of the cone for matching an impedance of the cone to the waveguide section;
e) the feed being a dual-band feed covering the lower and upper frequency bands; and
f) the waveguide end-portion being provided at an inner wall thereof with a wall-impedance changing means for changing an impedance of the inner wall to couple the quasi-TM 11 mode in the upper frequency band, to thereby achieve a rotationally substantially symmetric illumination of the subreflector in the upper frequency band.
8. A cassegrain-type feed for an antenna, comprising:
a) a waveguide section having an end-portion, the waveguide section having internal dimensions which support a propagation of a fundamental quasi-TE 11 mode in lower and upper frequency bands;
b) a dielectric cone having a small-diameter end and a large-diameter end, the small-diameter end adjoining the waveguide end-portion;
c) a subreflector adjoining the large-diameter end of the cone;
d) a dielectric multi-stage step transformer attached to the small-diameter end of the cone for matching an impedance of the cone to the waveguide section;
e) the feed being a dual-band feed covering the lower and upper frequency bands; and
f) the waveguide end-portion being provided at an inner wall thereof with a wall-impedance changing means comprising a dielectric sleeve received in the waveguide end-portion, for changing an impedance of the inner wall to couple the quasi-TM 11 mode in the upper frequency band, to thereby achieve a rotationally substantially symmetric illumination of the subreflector in the upper frequency band.
9. The feed as claimed in claim 8 , wherein the dielectric sleeve is hollow and is further operative for stimulating excitation of a quasi-TE 12 mode in the upper frequency band.
10. The feed as claimed in claim 8 , wherein the dielectric sleeve has a thickness of between approximately one-quarter and approximately one-sixth of a mean wavelength of the upper frequency band, referred to propagation in the sleeve.
11. The feed as claimed in claim 8 , wherein the dielectric sleeve has a length which is greater than one wavelength in the partially filled waveguide at the highest frequency of the upper frequency band.
12. The feed as claimed in claim 8 , wherein the dielectric sleeve is formed as an integral, hollow cylindrical part of the dielectric cone.
13. The feed as claimed in claim 8 , wherein the waveguide section is of substantially uniform diameter throughout its length.
14. The feed as claimed in claim 8 , wherein the waveguide end-portion is of greater diameter than that of the rest of the waveguide section, such that a recess having a shoulder is formed, allowing a correct seating of the dielectric sleeve in the waveguide section to be established.
15. The feed as claimed in claim 8 , wherein a final stage of the dielectric transformer located at an aperture of the waveguide end-portion has a diameter which is approximately 75% of that of the waveguide end-portion.
16. The feed as claimed in claim 8 , wherein the subreflector has a central portion, and a disk at the central portion for reducing return loss in signals incident upon the subreflector.
17. The feed as claimed in claim 8 , wherein the wall-impedance changing means is operative for stimulating excitation of a quasi-TE 12 mode in the upper frequency band.
18. A parabolic antenna arrangement, comprising:
A) a parabolic reflector having a central portion, and
B) a cassegrain-type feed passing through the central portion, the feed including:
a) a waveguide section having an end-portion, the waveguide section having internal dimensions which support a propagation of a fundamental quasi-TE 11 mode in lower and upper frequency bands;
b) a dielectric cone having a small-diameter end and a large-diameter end, the small-diameter end adjoining the waveguide end-portion;
c) a subreflector adjoining the large-diameter end of the cone;
d) a dielectric multi-stage step transformer attached to the small-diameter end of the cone for matching an impedance of the cone to the waveguide section;
e) the feed being a dual-band feed covering the lower and upper frequency bands; and
f) the waveguide end-portion being provided at an inner wall thereof with a wall-impedance changing means comprising a dielectric sleeve received in the waveguide end-portion, for changing an impedance of the inner wall to couple the quasi-TM 11 mode in the upper frequency band, to thereby achieve a rotationally substantially symmetric illumination of the subreflector in the upper frequency band.