The feed of the reflector antenna
The feed of the reflector antenna
The feed is the heart of reflector antenna.And its performance has a great influence on the overall antenna performance. Thereflector antenna requires that the feed possesses definite phase center,axisymmetric directionaldiagram, low cross polarization, fine VSWR, enough bandwidth and small blockedrange, etc. Feed forward antenna also requires that the top of the feeddirectional diagram is concave downward, as shown in Fig3-7. Thus, it cancompensate space attenuation and make aperture evenly distributed. Differenttypes of feed with high efficiency are developed.
Fig.3-7 Illustrated diagram of ideal direction of feed forward feed
1 Planar slot horn
(1) Radiation of open-ended circular waveguide
In microwaverange, waveguide is usually adopted to transmit electromagnetic energy. The waveguide ishollow metal tube. When electromagnetic wave propagating in the waveguide, on one hand,waveguide tube has the function of screen dispersion which enables electromagneticwave to limit the waveguide propagation in the waveguide tube, on the otherhand, electromagnetic field in waveguide should meet boundarycondition, which makes waveguide field different from electromagnetic fieldpropagating in free space. Different field structure in waveguide formsdifferent propagation mode. The dominate mode of circular waveguide is and its field structure is shown inFig.3-8.
Fig 3-8 Fielddistribution of modein circular waveguide
As for open-ended waveguide,its waveguide energy radiates from the opening to the space, and it is termedwaveguide radiator. The radiation pattern of circular waveguide is shown in Fig.3-9. Thefigure shows that, waveguide radiator pattern is quite wide. The maximumtheoretical efficiency can only achieve 74% when illuminating parabolic antennawith waveguide radiator. And when waveguide diameter varies from 0.8λ to 1.2λ, -10dBbeam width varies from 140° to 104°.
Fig.3-9 Illustrated diagram of circularwaveguide radiation pattern
(2) Planar slot horn
A flange with ring slot is put on thecircular waveguide, so it makes up planar slot horn which improves waveguideradiator illumination. According to different operational principle, planarhorn can be divided into two categories which are 90°corrugated horn and coaxial multimode horn. Thestructure of corrugated horn is shown in Fig. 3-10 and Fig. 3-11.
Fig. 3-10 Ku-band 90°corrugated horn Fig. 3-11 C-band 90°corrugated horn
There are 2~6 ring slots, the tooththickness δ is far less than operation wavelength, slot width W is not more thanλ/4 and slot depth h is about λ/4. The center waveguide generally works on dominate mode. If the maximum wavelength andthe minimum wavelength are λmax and λmin respectively, the waveguide radius Rshould meet the following condition:
Fromthe operation mode, 90°corrugated horn is a type of mixed modewhich is obtained by center waveguide stimulating 90°corrugated horn. The theoretical calculation of 90°corrugated horn is very complex. Now 90°corrugated horn is developed by the method combinedtheoretical analysis with experimental debugging.
Fig.3-10 is 90° corrugated horn operating at Ku-band andits actual directional diagram. It has 6 ring slots. Slot width W is equal to 2.3mm. Tooth thickness δ is equal to 1mm, L is equal to 2.3mm. Center waveguide radius R is equal to 8.15mm. And its normalizedradiation pattern is shown in Fig.3-12.
90°corrugatedhorn operating at C-band usually requires fewer ring slots. Fig.3-11 is thecorrugated horn with 3 ring slots. The horn which operates between 3.7GHz and4.2 GHz possesses nice equal direction diagram and nearly invariable beamwidth. The theoretical efficiency can achieve above 80％ if the aperture angle of feed forward parabolic antennais 120°.
Aperture angle θ (degree)
Fig.3-12 Illustrated diagram of radiationpattern of Ku-band 90°corrugated horn
Planar slot horn has the advantages of rotational symmetrical lobe, fixedphase center, sidelobe level less than 23dB, low cross polarization, simplestructure and low cost. Therefore, it is widely applied to common parabolicantenna.
2 Multi-mode horn
The open end of waveguide radiator isshaped like gradual opening, so horn antenna is formed. The field of commonconical horn is the extension and deformation of mode. Thelobe of E plane direction diagram and H plane direction diagram is not equal,phase center don’t coincide, cross polarization is high and sub-lobe level of Eplane is higher. This is because aperture field varies greatly in H plane whileit varies little in E plane. The high order mode is introduced so that aperturefield distribution in these two planes is nearly same and the direction diagramon these two main planes is approximately equal. And thus its performance isimproved.
Multi-mode horn is to motivate, control andapply high order mode. The motivation method of high order mode is to changestep and aperture, or add medium ring at the correct place of the horn. Thechange value can control mode ratio or mode conversion coefficient. The hornlength control can control the phase of high order mode. In this way aperturefield distribution can be obtained, as shown in Fig. 3-14. Due to differentmotivation and control method of high order mode, multi-mode horn has differenttypes that are stepped dual-mode horn, flare angle dual-mode horn and dielectric loaded dual-mode horn, as shown in Fig.3-15.
Dielectric loaded horn consists of commonconical horn added a section of dielectric ring. And the role of dielectricring is to motivate high order mode. Because of the limitation of horn size ofdielectric loaded section, mode is motivated and other high order modes are reduced soon. The changeof dielectric ring thickness and length and the distance to aperture can changehorn angle and also the ratio of high order mode and . If aperture phase condition of these two modes can meet 2nπ relation,aperture field distribution is obtained, as shown in Fig.3-12. The dielectrichorn of simple structure adopts PFTE ring added in common conical horn, whichrealizes multi-mode property. Dielectric loaded horn has the features ofsub-lobe (less than -20dB), rotational symmetrical lobe and low crosspolarization. Generally, its VSWR can achieve about 1.1.
Fig. 3-14 Illustrateddiagram of the directional pattern of multi-mode horn feed
(a)steppeddual-mode horn (b)flare angle dual-modehorn (c)dielectric loaded dual-mode horn
Fig.3-15 Different multi-mode horns
3. Conicalcorrugated horn
There are many ring slots cut in the insidewall of conical horn, as shown in Fig.3-16, thus conical corrugated horn isobtained. When there are 5 or more slots in the individual wavelength and theslot depth is between λ/4and λ/2, andthe corrugated wall for electric field and magnetic has the same boundary condition, hybrid mode is transmitted inside the horn. Whenthe slot depth is selected properly and balanced mixed condition Eρ=ZoHψ,Eψ=-ZoH ρ issatisfied, aperture field distribution of mode conical horn is as follows:
The horn field is zero, so the edgediffraction of sidelobe and back lobe is inhibited. The amplitude distributionof aperture field is axisymmetric, therefore, the radiation pattern ofcorrugated horn has good rotational symmetry and very low cross polarizationradiation. Conical corrugated horn with best illumination adopts corrugatedhorn as prime focus feed and its aperture efficiency can achieve 84%. It issuggested that balanced mixed horn is the best feed which is matched with focusfield on the basis of analyzing parabolic antenna focus field. The disadvantageof conical corrugated horn is complex processing and high cost. The corrugatedslot of corrugated horn with large angle is vertical to horn inwall. Thepattern of large angle horn is narrower, which is usually applied for the feedof feed back antenna.
Fig. 3-16Illustrated diagram of conical corrugated horn structure