GB2090068A - Horn antenna feeder - Google Patents

Abstract

A horn antenna (1) having a wide angle of flare in its H-plane is arranged to provide a substantially constant beamwidth over a broad frequency range. For operation over a very wide frequency range (e.g. 3:1), the horn (1) is combined with a waveguide feeder (2) having ridges (13, 14) the heights of which decrease with decreasing distance from the throat (8) of the horn (1), reaching zero adjacent the throat (8). Over the majority of the frequency range, the feeder (2) operates only in the fundamental TE10 mode, but at the upper end of the range, the ridges (13, 14) themselves generate in addition the higher-order TE30 mode. To alleviate the degradation of antenna performance above the TE30 cut-off frequency, the feeder (2) includes a short length (9) of plain waveguide between the throat (8) and the adjacent ends of the ridges (13, 14). <IMAGE>

Description

SPECIFICATION Horn antenna feeder The invention relates to an arrangement for feeding a horn antenna having a wide angle of flare in its H-plane.

Whereas the more-commonly used kind of horn antenna has a narrow angle of flare typically in the region of 20o300 (i.e. semi-angle 100--150), producing at the mouth of the horn a nominally planar wave front with a phase error thereacross of the less than 7r/2 radians, the flare angle of a wide-angle horn antenna is generally at least 600 and may typically be much greater; the wave front produced at the mouth of such an antenna is no longer nominally planar, and these properties can result in a beamwidth which is approximately constant over a wide frequency range, for example of an octave or more, whereas the beamwidth of a narrow-angle horn antenna varies substantially with frequency.

Antennas having approximately constant beamwidths over broad frequency ranges are useful in broad-band direction-finding systems.

For example, a system for determining the direction of arrival of a radio signal in azimuth may comprise a set of N nominally coincident antennas whose main beam axes are directed so as to be spaced at regular angular intervals of (360/N)O. An important characteristic of such a system is the power level (relative to peak) of the beam of an antenna at an angle (to its main beam axis) corresponding to the beam axis of an adjacent antenna; in the case where N = 8, the relevant angle is + 450, and it may be desired to keep the power level at that angle, over the whole operating frequency range of the set of antennas, within a fairly narrow band such as -9dB to -15dB. A wide-angle horn antenna may be more suitable for this application than, for example, a spiral antenna the performance of which may be inferior, for example as regards gain.

A problem which arises when it is desired to operate such a system over a very wide frequency range, for example greater than 2-:1, is the provision for each horn antenna of a feeder which when combined with the antenna does not degrade the performance to an unacceptable extent. One form of wide band feeder is a ridge waveguide; in particular, a symmetrical doubleridge waveguide has the advantage that the first higher-order mode which can propagate therein is the TE30 mode which has a cut-off frequency three times that of the fundamental TE,o mode, giving a nominal 3:1 bandwidth for operation in the TE10 mode alone.However, operation at frequencies near the cut-off frequency of the fundamental mode is not practicable in view of the large VSWR of the combination at such frequencies, and hence a very wide operating frequency range for the combination can include a band of frequencies above the cut-off frequency of the higher-order mode. It has been found that above this latter cutoff frequency, there is normally a marked deterioration in performance, particularly in the constancy of the beamwidth; this is believed to be due to the generation within the feeder itself of the higher-order mode.Thus in the case of a double-ridge waveguide the height of each ridge of which suitably decreases with decreasing distance from the throat, the change in height of the ridge will itself generate the TE30 mode to an extent which generally increases with the rate of change; where, for example, the rate of decrease of height increases with decreasing distance from the throat, the TE30 mode will primarily be generated at the ends of the ridges adjacent the throat of the horn.

According to a first aspect of the invention, a horn antenna having a wide angle of flare in its Hplane in combination with a feeder extending from the throat of the horn, wherein the feeder is adapted to supply microwave energy to the throat in substantially only a fundamental mode over a major part of an operating frequency range of the combination and in both the fundamental mode and a higher-order mode at the upper end of the operating frequency range and comprises means to generate said higher-order mode, is characterised in that said means are so spaced from the throat as substantially to minimise variations in the beamwidth of the antenna at frequencies immediately above the cut-off frequency of said higher-order mode.This aspect of the invention is based on the premise that unavoidable generation of the higher-order mode in the feeder can be so phased with respect to the antenna that the effect thereof on the beamwidth in the range of frequencies extending upwards from the cut-off frequency of the higher-order mode can be used to greatest advantage or to least disadvantage; considered in a slightly different way, variations with frequency in off-axis power level (for example at +450) may be contained within a desired band. The higher-order mode may be used so as at least partly to compensate for an underlying tendency for the beamwidth or power level to vary with frequency.

Said fundamental and higher-order modes may be TE,o and TE30 respectively, enabling a wide frequency band for operation in the fundamental mode alone.

Suitably, the feeder comprises a ridge waveguide which is to be understood to include finline. Such a waveguide is generally suitable for broad-band operation, and may comprise two opposed E-plane ridges the heights of which generally decrease with decreasing distance from the throat; this can provide a good broadband match between the throat and, for example, a coaxial transmission line connected to the ridges at a region remote from the throat. The height of each ridge may reach substantially zero at a point spaced along the feeder from the throat.

According to a second aspect, the invention provides a horn antenna having a wide angle of flare in its H-plane in combination with a feeder extending from the throat of the horn, wherein the feeder comprises a waveguide having two opposed E-plane ridges the height of each of which generally decreases with decreasing distance from the throat and reaches substantially zero with the edge of the ridge substantially inclined in the E-plane to the respective adjacent wall portion of the waveguide and with said inclined edge adjacent the throat but spaced along the feeder therefrom.

The feeder in a combination embodying the first or second aspect of the invention may comprise a rectangular waveguide; it may alternatively or additionally comprise a circular waveguide.

In an embodiment of the invention wherein the feeder comprises a waveguide having two opposed E-piane ridges the height of each of which generally decreases with decreasing distance from the throat, a transverse dimension of the waveguide suitably increases with decreasing distance from the throat. The cut-off frequency of the fundamental mode may thereby be maintained at or near a desired value along the feeder. Where the height of each ridge reaches zero at a point spaced along the feeder from the throat, the feeder may comprise a waveguide of substantially uniform internal cross-section between the throat and the points at which the heights of the ridges reach substantially zero.

A direction-finding system covering a bandwidth of, for example, 1-18GHz may comprise several sets of constant-beamwidth antennas, each set covering a respective portion of that bandwidth. By enabling the bandwidth of the antennas of a set to be increased, the number of sets required may be reduced; furthermore, the increased bandwidth of the antennas may make fuller use of the available bandwidth of other components for the system, such as amplifiers.

The H-plane angle of flare of an antenna for an embodiment of the invention may, for example, be in the range of 1000--1300.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view in the H-plane of a combination of a horn antenna and a feeder embodying the invention; Figure 2 is a schematic elevation in the E-plane of the combination of Figure 1, and Figure 3 is a graph showing variations with frequency of off-axis beam power.

The combination shown in Figures 1 and 2 comprises a horn antenna 1 and a feeder 2. The horn is bounded in the H-plane by planar walls 3 and 4 which are-disposed at an angle of 1150 to each other, this being the angle of flare in the Hplane, and is bounded in the E-plane by planar walls 5 and 6 which are disposed at an angle of 240 to each other. The edges of walls 5 and 6 bounding the mouth 7 of the horn are curved so that in the H-plane, the aperture is an arc of a circle centred at the intersection (by extrapolation) of walls 3 and 4.At the throat 8 of the horn, there is an abrupt change from the widely flared horn 1 to the feeder 2 which in this case comprises three portions 9, 10 and 11 of rectangular waveguide, the end portions 9 and 11 being of different respective uniform internal cross-sections and the cross-section of the intermediate portion 10 varying linearly along its length from one end portion to the other. The feeder includes ridge waveguide; in this case two symmetricallydisposed E-plane ridges 1 3 and 1 4 extend through portion 10 from the end thereof adjoining portion 9 into portion 11 where they end abruptly.The heights of the ridges decrease with decreasing distance from the throat with an exponential variation such that the rate of decrease of height increases with decreasing distance from the throat; the edge of each ridge is thus most steeply inclined in the E-plane to the adjacent wall portion of the waveguide at the end of the ridge (i.e.

where the height of the ridge reaches zero) adjacent the throat 8. At the junction of portions 10 and 11 in a miniature coaxial connector 12 of which the outer conductor is connected to the adjacent ridge 1 3 and the inner conductor is connected to the further ridge 14. The portion 11 is closed by a short-circuit 1 5 which for experimental purposes was made longitudinally adjustable (not shown).

With a horn antenna such as 1 in combination with a waveguide feeder having tapered ridges as in portions 10 and 1 , the performance of the antenna is affected by the presence of the TE30 mode at frequencies above the cut-off frequency (at the throat of the horn) of that mode, the mode apparently being generated by the ridges in the feeder This may be demonstrated by replacing the short portion 9 of plain waveguide by a much longer portion of waveguide of the same uniform internal cross-section, and measuring either the beam power at a fixed angle to the longitudinal axis of the combination, for example 450, or the beamwidth for a fixed power level relative to peak power (on the longitudinal axis), for example -3dB or OdB:: as the frequency increases large fluctuations in the measured parameter occur from the TE30 mode cut-off frequency upwards owing, it is thought, to the effect in the horn of the varying phase of the energy in the higher mode (which has a different phase velocity from the fundamental TE10 mode).For comparison, if the same horn antenna is combined with a plain (unridged) waveguide feeder of a very gradually decreasing internal cross-section from the throat and having aTE10 mode cut-off frequency at its narrower end somewhat below the cut-off frequency of the TE30 mode at the throat of the horn, it is found that these fluctuations do not occur, and the +450 beam power (for example) may remain within acceptable limits up to a frequency of some 33 times the TE10 mode cut-off frequency. (Such a plain waveguide feeder would of course be suitable for operation in a frequency range of less than 2:1). It may be mentioned that over the broadest frequency range used in these tests, the TE30 mode is thought to be the only significant higher-order mode and the coaxial line/ridge waveguide transducer at the end of the feeder remote from the horn operates only in the fundamental mode.

The disadvantageous effect of the TE30 mode can be mitigated by the use of a suitable short length of plain waveguide between the throat of the horn and the ends of the ridges 13, 14. The generation of the higher-order TE30 mode by the ridges is thought to occur predominantly where their edges are most steeply inclined in the Eplane to the respective adjacent wall portions of the waveguide, i.e. in this case at the ends of the ridges adjacent the throat. The length of the portion 9 can be selected to be such that the phasing of the TE30 mode in the horn has the optimum desired effect on the antenna performance in a range of frequencies immediately above the TE30 mode cut-off frequency; by keeping the length df the portion short, rapid and large fluctuations of performance with frequency may be mitigated.Figure 3 is a graph of beam power, relative to peak at + 450 to the axis for a range of frequencies extending upwards from below the TE30 mode cut-off frequency (indicated by an arrow on the frequency axis), showing the effect of varying the spacing between the throat and the adjacent ends of the ridges. For reference, with zero spacing, i.e.

omitting portion 9, the power charged sharply above the TE30 mode cut-off frequency, and at 13 GHz, for example, was about -20dB. The performance was tested with portions of various lengths (multiples of 1/1 6 inch, approximately 1.6 mm). In order to facilitate interchangeability, the portions were all of uniform internal crosssection along their length. Optimum performance was obtained with a length of 5/8 inch (15.9 mm), shown in curve A; curves B and C show performance with somewhat shorter and greater lengths of 1/2 inch (12.7 mm) and 11/16 inch (1 7.5 mm) respectively. The cut-off frequency for the TE30 mode is about 16.85 GHz. The performance below this cut-off frequency was similiar with all the lengths of portion 9, any differences becoming generally smaller with decreasing frequency owing, it is thought, to evanescent effects.The performance below about 1 6 GHz is indicated approximately in Figure 3 by curve D; it can be seen that the relative power level tends to become increasingly negative with increasing frequency, and the use of a portion 9 of suitable length can partly compensate for this.

With a length of 5/8 inch the + 450 beam power level remains within a range of -9dB to -15dB from 6.0 GHz up to at least 19.2 GHz. For each length of portion 9, the longitudinal position of short-circuit 1 5 was adjusted to obtain the best VSWR at the lower end of the operating frequency range. The beam performance was substantially independent of VSWR, but the lowest frequency for which an acceptable VSWR (better than 2:1 ) could be obtained was dependent on the length of the portion 9 and in the case of 5/8 inch was 6.4 GHz; nevertheless, this gave an operable frequency range of 3:1, whereas without a portion 9 of plain waveguide, the range was only 2.7:1.

The combination described above with reference to Figures 1 and 2 had the following dimensions. The H-plane aperture was 111.0 mm; the E-plane aperture was 36.9 mm at the centre of the mouth and 24.0 mm at the vertical edges of walls 3 and 4. The radius of curvature of the mouth in the H-plane was 65.8 mm. The internal dimensions of the throat of the horn (and thus also of the waveguide portion 9) were 26.7 mm x 12.6 mm, giving a TElo mode cut-off frequency of about 5.6 Ghz; the internal dimensions of the waveguide portion 11 were 11.3 mm x 8.2 mm, the decreasing width of the portion 10 with the increasing height of the ridges keeping the TE,o mode cut-off frequency roughly constant along the feeder. The length of portion 10 was 70.5 mm.The ridges in the feeder were 3.5 mm wide and extended 1.8 mm into portion 11; the profile of the edge of each ridge was given by the equation y = 0.235 exp (0.0467 x) where y is the distance of the edge of the ridge from the longitudinal axis of the feeder, and x is the distance along the feeder from the coaxial connector 1 2. The antenna has good polar diagrams in both the H-plane and the E-plane, the power typically varying smoothly from the peak level down to -25dB to -30dB; the gain increased with frequency over the operating range from about 5dBI to about 1 8dBI.

The ridges in the waveguide feeder may have a shape other than that described above. They may for example decrease in height linearly or according to a cosine function with decreasing distance from the throat. In the latter case, in a combination embodying the first aspect of the invention, the edge of each ridge may be generally S-shaped, with substantially no inclination in the E-plane to the respective adjacent wall portion of the waveguide where the height of the ridge reaches zero (and also where the coaxial connector is connected to it) and with the maximum rate of decrease in height (the predominant generator of the TE30 mode) mid-way along the ridge. The feeder may be other than a rectangular waveguide: it may for example be a circular waveguide (in which, for example, the modes corresponding to TE1o and TE30 in rectangular waveguide are TE" and TE3, respectively) and the horn antenna may similiarly be conical rather than generally rectangular.

Embodiments of the invention may be suitable for use with means adjacent the mouth of the antenna for rotating the plane of polarisation of radiation so that, for example, an antenna with its H-plane horizontal may be used for the reception of both horizontally and vertically plane-polarised radiation.

Claims (11)

1. A horn antenna having a wide angle of flare in its H-plane in combination with a feeder extending from the throat of the horn, wherein the feeder is adapted to supply microwave energy to the throat in substantially only a fundamental mode over a major part of an operating frequency range of the combination and in both the fundamental mode and a higher-order mode at the upper end of the operating frequency range and comprises means to generate said higher-order mode, characterised in that said means are so spaced from the throat as substantially to minimise variations in the beamwidth of the antenna at frequencies immediately above the cut-off frequency of said higher-order mode.

2. A combination as claimed in Claim 1 wherein said fundamental and higher order modes are TE,o and TE30 respectively.

3. A combination as claimed in Claim 1 or 2 wherein the feeder comprises a ridge waveguide.

4. A combination as claimed in Claim 3 wherein the waveguide comprises two opposed E-plane ridges the heights of which generally decrease with decreasing distance from the throat.

5. A combination as claimed in Claim 4 wherein the height of each ridge reaches substantially zero at a point spaced along the feeder from the throat.

6. A horn antenna having a wide angle of flare in its H-plane in combination with a feeder extending from the throat of the horn, wherein the feeder comprises a waveguide having two opposed E-plane ridges the height of each of which generally decreases with decreasing distance from the throat and reaches substantially zero with the edge of the ridge substantially inclined in the E-plane to the respective adjacent wall portion of the waveguide and with said inclined edge adjacent the throat but spaced along the feeder therefrom.

7. A combination as claimed in Claim 5 or 6 wherein the height of each ridge varies exponentially with distance along the feeder, the rate of decrease of height increasing with decreasing distance from the throat.

8. A combination as claimed in any preceding claim wherein the feeder comprises a rectangular waveguide.

9. A combination as claimed in Claim 4 or 6 or in any preceding Claim appendant to Claim 4 or 6 wherein the feeder comprises a waveguide a transverse dimension of which increases with decreasing distance from the throat.

10. A combination as claimed in Claim 9 as appendant to Claim 5 or 6 wherein the feeder comprises a waveguide of substantially uniform internal cross-section between the throat and the points at which the heights of the ridges reach substantially zero.

11. A horn antenna in combination with a feeder, substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.