Recent Progress in Development of Multiband Feed Horns (Review)

International Conference on Antenna Theory and Techniques, 17-21 September, 2007, Sevastopol, Ukraine pp. 44-50

978-1-4244-1584-7/07/$25.00©2007 IEEE

RECENT PROGRESS IN DEVELOPMENT OF MULTIBAND FEED HORNS (REVIEW)

1 Dubrovka F. F., 2 Dubrovka R. F., 1 Ovsianyk Yu. A. and 1 Rospopa Ya. O. 1 National Technical University of Ukraine “Kyiv Polytechnic Institute”,

E-mail: [email protected], [email protected] 2 Queen Mary, University of London, E-mail: [email protected]

Abstract This paper proposes overview of the multiband feed system for reflector-type an-

tennas. Coaxial, multimode Potter’s type structures, multiband corrugated and dielec-tric loaded feed horns is discussed below.

Keywords: Multiband feed system, multimode horn, corrugated horn, dielectric loaded horn.

1. INTRODUCTION Reflector antennas have a widespread application in satellite communication, defence and radio astronomy. Simultaneous multifrequency functionality at widely separated frequency bands, low level cross-polar and side lobes within each operating range are the basic requirements to such antennas nowadays.

Reflector-type antennas performances strongly de-pend on the radiation characteristics of the feed system. This paper reviews earlier works and recent progress in development of multiband feed systems. It focuses on different types of multimode horn, multiband corrugated horns, multiband dielectric loaded horns and multiband coaxial dielectric loaded horns.

2. MULTIMODE HORN DESIGN In 1963 novel horn design technique was proposed by Potter [1]. This technique, "dual-mode conical horn," util-izes a conical horn excited at the throat region the domi-nant TE11 mode and the higher-order TM11 mode. These two modes are then excited in the horn aperture with the appropriate relative amplitude and phase to effect sidelobe suppression and beamwidth equalization (Fig. 1). Phase center coincidence follows as a result. The predicted radia-tion pattern characteristics are easily derived, as is the technique for mode generation and control.

In general, it was only the first step in the technique of utilizing orthogonal waveguide modes to produce practical horn antennas with new radiation characteristics.

Nowadays, a lot of different multiband horns using this technique are known.

A simple dualmode feed horn is described in [2]. Conical waveguide section is exited by TE11. Aperture diameter (D1) is approximately one wavelength in the lower frequency band so as to produce substantially equal power patterns in the E and H planes in the lower

frequency band (Fig. 2). The slope (β ) generate TM11 mode for the higher frequency band.

Other simple dual mode horn [3] comprises two waveguide sections to lock in through the double axi-ally symmetrical ring discontinuities. It generates TM11 mode in both working frequency ranges.

Potters horn with scalar rings is shown on Fig. 3. At the higher frequency band, the TM11 mode launching diame-ter step ratio, the mode phasing section, and the aperture diameter have primary control of the horn radiation [4–6]. The scalar horn rings placed outside the waveguide do not have any appreciable effect on the higher frequency band radiation, particularly if they are placed behind the aper-ture. This is because of two main reasons. First, the aper-ture size in wavelengths is larger at the higher frequency, and therefore the aperture itself has higher gain. Second, the proper phase addition of the TE11 and TM11 energy in the aperture plane reduces the E-plane field intensity near the edges of the aperture, thus reducing the E-plane cou-pling into the scalar rings. At the lower frequency band,

Fig. 1. Vector diagram of the dominant mode, higher

order modes and the resulting mode in circu-lar waveguide (horn).

Fig. 2. Dualmode feed horn.

Recent Progress in Development of Multiband Feed Horns (Review)

International Conference on Antenna Theory and Techniques, 17-21 September, 2007, Sevastopol, Ukraine 45

this horn step does not launch the TM11 mode. (Dimension of the second waveguide section is set so that the TM11 mode is below cutoff).

Multimode feed systems with different discontinuities are proposed in [7–11]. The feed horns include slopes, profile section, axially symmetrical ring steps and etc. Fig. 4 illustrates different dual-band horns shapes.

The horn described in [12] uses a corrugated input sec-tion to excite the desired ratio of the TE11 and TM11 modes in a smooth-wall horn to achieve low cross-polarization performance. This corrugated-to-smooth-wall transducer has substantially more bandwidth and is less dispersive than the conventional Potter step. It may be designed to operate in two discrete bands which are widely separated in frequency. Furthermore, the corrugated junction can occur at a cross-sectional diameter that is substantially larger than the cutoff diameter of the TM11 mode. This reduces the mode phasing errors at the horn aperture. Fig. 5 illustrates this type of dual-band horn.

The input to the horn is a circular waveguide which is excited in the TE11 mode. A dual-depth mode transducer section of conical waveguide is used to transition from the input TE11 mode to a hybrid HE11 mode. This is fol-lowed by the inverse ring-loaded section described above. The low frequency hybrid mode continues to propagate through this section without substantial

change in its modal characteristics. At the output of this section is an abrupt junction with a smooth-wall circular conical waveguide. At this junction, the hybrid mode converts to the desired ratio of TE11 and TM11 modes.

The input section also converts the TE11 mode to a hybrid HE11 mode at the high frequency. At the junction with the inverse ring-loaded section, this hybrid-mode wave converts to a pair of modes with characteristics similar to those of smooth-wall circular waveguide TE11 and TM11 modes. These modes continue to propagate virtually unchanged into the smooth-wall horn. The key feature of this inverse ring-loaded section is that it per-mits the desired phasing of the two modes in the smooth-wall horn to be achieved in two widely separated fre-quency bands.

A dual-band, dual-polarization, coaxial feed system for the Parkes radio telescope is proposed in [13, 14].

The feed horn is a coaxial structure where the HF-band is received by the inner circular waveguide, which forms the core of the structure, and the LF-band by the coaxial waveguide itself. To improve the radiation pat-tern symmetry of the inner waveguide for the HF-band, a choke is located between the two radiating apertures (see Fig. 6). The coaxial waveguide receiving the LF-band has inherently poor return loss and therefore irises are used to improve the match.

The LF-band is extracted by using a six-port coaxial orthogonal mode junction (OMJ). The incoming signals are coupled from the OMJ to four rectangular waveguides through two matching steps. The signals are then re-combined using two coaxial T-junctions. The coaxial OMJ was designed using a full-wave method. The HF-band is extracted through a conventional or-thomode transducer (OMT) whose design is based on previous work.

Compact circularly-polarised coaxial dual-band feed where the frequency ratio between the lower and upper bands is approximately 2:3 is proposed in [15].

The horn is designed for minimum crosspolarisation over the full usable bandwidth of circular waveguide between the cut-off frequencies of the TE11 and TM11 modes (Fig. 7).

The chokes are of equal depth and approximately one half wavelength deep at the top of the band, where they behave as a conducting ground plane around the aper-ture, which is –1.1 wavelengths in diameter and thus radiates with low crosspolarisation. In the lower band the chokes behave more as conventional current suppressors around a circular waveguide with diameter near 0.7

Fig. 3. Potters horn with scalar rings.

Fig. 4. Dual-band feed horns.

Fig. 5. Dual-band feed systems.

Fig. 6. Coaxial feed system for the Parkes radio

telescope.

Dubrovka F. F., Dubrovka R. F., Ovsianyk Yu. A. and Rospopa Ya. O.

46 International Conference on Antenna Theory and Techniques, 17-21 September, 2007, Sevastopol, Ukraine

wavelengths, again giving low crosspolarisation. The use of a multiple-step Potter-type horn design is

described in [16], in meeting a requirement to provide simultaneous S/X-band operation and tracking of an Earth-resources satellite. Fig. 8 shows a cross-sectional view of the complete feed design where, within the inner section of a basic coaxial-waveguide structure, the X-band system (including provision of tracking) is housed, and the S-band system is contained within the coaxial-waveguide geometry.

In both the inner smooth-walled X-band conical-waveguide section, and the smooth-walled S-band coaxial section, multiple steps were used in the throat region of the horn. This provided a symmetrical low-cross-polarization radiation pattern over the ~ 5% bandwidth of each band.

3. MULTIBAND CORRUGATED HORNS The corrugated surface may be considered as an arche-typical artificial surface, which has been used since the sixties, mainly to design horn antennas with low cross polarization and rotationally symmetric beams. The first

articles on this subject were presented by Kay [17, 18], Rumsey [19], and Minnett and Thomas [20] in 1966 following earlier work by Cutler [21].

The corrugations were used to create a zero boundary condition of the vertical field component at the surface, and, consequently, to stop vertically polarized waves from propagating along the surface. The horizontally polarized field at the surface is also zero, being enforced by the me-tallic ridges between the grooves. Therefore, the same zero boundary condition could be created in both E and H planes of the horn antenna aperture, resulting in a rotation-ally symmetric radiation pattern with low cross polariza-tion. The corrugations had also been used before this, as chokes to reduce coupling (stop waves) in both electro-magnetic compatibility and antenna applications. Thus, the stopband characteristics of the transversely corrugated surface were well understood, as well as the surface waves appearing at the boundaries of the stopband.

Dual-band double corrugated horn is shown on Fig. 10. The feed system comprises of three main sec-tions: feed waveguide, mode transducer and corrugated horn [22]. The feed waveguide consists of two concen-tric, circular waveguides that are excited in the TE11 co-axial and circular waveguide modes for the low and high bands, respectively.

The mode transducer, which is critical to the perform-ance of the feed, provides a single mode, low return loss transition, for both bands, between the feed waveguide and the corrugated horn. This is achieved by converting the TE11 circular waveguide modes into the fundamental hybrid, HE11 mode (Fig. 9) of the corrugated horn. The corrugated horn, which is a stepped-slot configuration, is designed to achieve a smooth transition from the mode transducer and to produce the desired radiation character-istics at both frequency bands.

Also, double stepped-slot corrugated horn is discussed in [23]. A corrugated horn excited in the HE11 and HE12 hybrid modes and using alternating corrugation depths to optimize performance over two discrete bands. The horn uses a moderate flare angle to minimize phase distortion across the horn aperture. The beamwidth is equated in the two bands by introducing the HE12 mode to broaden the beam in the up-link band. The beam shape of the dual-

Fig. 7. Circularly-polarised coaxial dual-band feed

horn.

Fig. 8. An S/X-band tracking feed.

a) b) c)

Fig. 9. Field patterns in focal plane: a) TM1N component; b) TE1N component; c) HE1N hybrid mode.



mode horn can be controlled to virtually match the beam shapes at the two centerband frequencies by providing the proper amount of higher order mode content with the cor-rect phase at the higher frequency.

Coaxial corrugated horn operates simultaneously and independently in two widely separated bands [24, 25]. The LF-band feed horn is a corrugated horn that has a plurality of corrugations formed on an interior surface defining a profile (Fig. 11).

The profile extends substantially from a throat of the first feed horn and along a tapered portion of the first feed horn. The profile substantially minimizes an interac-tion of the corrugations with the second feed horn. A HF-band feed horn is positioned coaxially within the first one. For the HF-band symmetrical radiation pattern has been achieved using quarter wavelength choke on the outer conductor surface.

Dual-band corrugated horns with different surface-wave-type antennas are shown on Fig. 12. Outer corru-gated horn provides symmetrical radiation pattern and low cross-polar for the LF-band. In the HF-band are used di-electric rod [26, 27], disk-on-rod [28, 29], disk-in-tube and ring antennas [30]. All this surface-wave-type antennas perform radiation pattern symmetry and high efficiency for the HF-band. Dielectric rod and disk-on-rod are well known surfacewave-type antennas. DITA is dielectric-metal-disk “sandwich” type structure. DITA inside the circular horn excludes excitation of the TEM mode in such structure. Metal rings on dielectric rod takes shape RINGA-type antenna. Such structure has lowest radial dimensions among surface-wave-type antennas.

The same horn operable at two widely spaced fre-quency bands with nearly identical radiation patterns is achieved by using a corrugated horn and a combiner to excite the horn in its two frequency bands [31], and operating in a beamwidth saturation mode (Fig. 13).

The combiner, which must have a low loss at the X-band (excited through the apex of the horn) of less than 0.02 dB for useful application, is comprised of a circum-ferential slot for S-band injection and designed with a choke for the X-band rejection. The circumferential slot position in the horn is chosen to obtain good S-band im-pedance matching. Excitation of the slot is through four equally spaced waveguide ports. Two opposite ports are fed with 180° phase difference to yield a horizontal polari-zation, and the other two ports are fed with 180° phase difference to yield a vertical polarization. Feeding the ports around the circumferential slot with a 90° phase dif-ference from port to port clockwise or counter-clockwise yields a circular polarization of one sense or the other.

Dual-band horn with longitudinal corrugation is pro-posed in [32–34]. Longitudinally corrugations on the inner surface of the horn provide equal radiations pat-terns for both planes of the LF-band (Fig. 14). Dielectric rod antenna produces symmetry radiation characteristics for the HF-band.

A multifrequency [35], broadband, dual-polarized cor-rugated conical horn antenna is simultaneously fed a mul-tiplicity of signals, two for each of five frequencies, with each of a pair of signals fed in each of two orthogonal

Fig. 10. a) Dual-band double corrugated horn b) Stepped-slot configuration.

Fig. 11. Dual-band coaxial corrugated horn.

Fig. 12. Corrugated horns with disk-on-rod antenna.

Fig. 13. a) Dual-band corrugated horn; b) Combiner (cross-section).

Fig. 14. Dual-band horn with longitudinal corruga-

tions.



planes for excitation of a desired spherical hybrid mode (HE11). The lowest frequency is fed into the horn through orthogonal pairs of colinear slots, each pair being fed by coaxial tee power dividers (Fig. 15). Other signals are fed through a circular waveguide connected to the vertex. Band reject cavities block the next higher frequency from passing through the low frequency feed slots. The highest frequency signals are fed through orthogonal ports near the far end of the circular waveguide. The intermediate frequency signals are fed through orthogonal ports spaced along the waveguide. Filtering is incorporated for each to maintain isolation and low insertion loss, a quarterwave step transformer is used between the highest frequency (37 GHz) ports and the two next lower frequencies (21 GHz and 18 GHz) to provide a short circuit for these two lower frequencies, and a TM11 mode generator for the highest frequency is used as a short circuit for the next lower frequency (10.69 GHz).

Also, multifrequency corrugated horns with consecu-tive feeds are discussed in [36, 37].

Preliminary results for a horn antenna with good cross-polar performance in two discrete frequency bands in addition to desirable VSWR characteristics are pre-sented in [38]. The horn combines two different design concepts. It uses corrugations to control the crosspolari-sation in the lower frequency band when the corrugation depth is approximately /4λ . At the upper frequency limit the corrugation depth tends towards /2λ and the horn begins to behave more like a smooth walled horn in term of its cross-polar performance. Under these condi-tions the addition of a mode transducer in the horn throat can be used to excite the TM11 mode at the centre fre-quency of the transmit band. The transducer can take various forms although provisional designs have focused on the use of a step change, the size and position of which are chosen to ensure the TM11 mode, the funda-mental TE11 mode and other higher order modes, com-bine in the aperture with the correct amplitude and phase to produce minimum crosspolarisation.

4. MULTIBAND DIELECTRIC LOADED HORNS

Conical horns loaded with dielectric cones became of in-terest when it was realised that they could radiate a low level of cross-polar power over a wide band of frequencies using relatively inexpensive materials. They have compa-rable performance to the well known corrugated horn but are much simpler to construct. They can be used as high performance feeds for reflector antennas, particularly at millimetre waves where the corrugated horn is difficult to construct. The dielectric cone loaded horn creates a bal-anced hybrid mode by the inhomogeneously filled conical horn as well as corrugated horn.

The potential advantages of the dielectric-loaded horn are that it is cheaper than the corrugated horn to construct and it has inherently very wide band capabilities 5:1 and more [39]. The corrugated horn has reached an upper de-sign limit, where acceptable match and phase-centre sta-bility, together with good pattern symmetry (and hence

relatively low cross-polar response) can be maintained over, at most, a 2.4:1 continuous bandwidth [40].

In [41, 42] is demonstrated profiled dielectrically-loaded, hybrid-mode dual-band horn. Lower band has a bandwidth of 24 % while the upper band has a bandwidth of 19 %, with the bands separated ratio of ~ 3.7:1.This feed system is shown in Fig. 16.

The hybrid mode is supported by the dielectric mate-rial which almost fills the interior of the (smooth-walled) horn except for the ‘gap’ τ , which is chosen to optimize the performance of the horn. This parameter is not criti-cal and the gap need not be air, as a dielectric material can be used provided it has a lower permittivity, ε than that of the material in the main body of the horn. The value of τ is dependent on the relative permittivities be-tween these two materials.

In this case was found that a value for dielectric per-mittivity of about 1.13 is required. However, since suit-able low-loss materials with this permittivity were unavailable, it has been simulated the dielectric in the horn by means of a sandwich structure where thin layers of low-loss Teflon separated by layers of low-loss, low-density foam of appropriate thickness to provide an aver-age permittivity of 1.13.

Compact dual-band dielectric loaded feed systems with different surface-wave-type antennas for the HF-band are proposed in [43].

Low loss, low permittivity dielectric material is partly filed primary horn provides high efficiency and low cross-polar for the LF-band. Dielectric rod, disk-on-rod, disk-in-tube (DITA) and ring antennas (RINGA) per-

Fig. 15. Multifrequency corrugated conical horn.

Fig. 16. Profiled dielectrically-loaded, hybrid-mode dual-band horn with OMT.



form radiation pattern symmetry and low cross-polar for the second HF-band (Fig. 17).

A novel multiband hybrid-mode dielectric-loaded feed horn for satellite communications has been proposed in [44]. A dual-band coaxial feed horn is schematically shown in the Fig. 18.

It consists of two smooth-wall profiled horns with partially filled by a dielectric material. Hybrid-modes in the low frequency coaxial horn are excited and supported by a coaxial dielectric core that partially fills the coaxial horn forming two gaps between metal surfaces of the coaxial horn and dielectric core. Radiation characteristics depend on the gaps-width, core dielectric permittivity and outer-to-inner horns diameter ratio. Numerical inves-tigations have shown that better electrical performance can be achieved in case of using high dielectric permit-tivity materials, such as low loss polystyrene ( 2.5ε = ) or Teflon ( 2.05ε = ). Cross-polar and side lobe levels in coaxial horn strongly depend on outer-to-inner coaxial conductor diameter ratio of the horn. For the Ku-band inside circular horn is mounting dielectric cone, sepa-rated from the metal wall by a dielectric layer (air) with lower permittivity then for the core material. Symmetri-cal radiation patterns and low cross-polar (less –30 dB) radiation without any side lobes over wide frequency range have been obtained.

5. CONCLUSION Different multiband feed systems for reflector antennas have been discussed.

At present due to the excellent radiation characteristics the most widespread application for reflector antennas have got corrugated horns. Those include satellite and earth stations antennas, spacecraft antennas, radio astron-omy antennas and warfare systems antennas. On the other

hand corrugated horns have some limitations, namely: operating bandwidth is limited to 2.4:1 and separation between operating bands in multiband corrugated horns is less than 2:1.

Dielectric loaded horns are alternative to the corru-gated horns. They not so expensive as corrugated horns and have very wide frequency range capabilities (5:1 and more). Novel coaxial dielectric loaded horns have no upper separation limitations between operating bands. The larger separation between operating bands the better radiation characteristics.

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Fig. 17. Dual-band dielectric loaded feed horn with

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Fig. 18. Dual-band coaxial dielectric-loaded feed horn.



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Recent Progress in Development of Multiband Feed Horns (Review)

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