1. Hanyang University 1/15 Antennas & RF Devices Lab. MODERN ANTENNA HANDBOOK by CONSTANTINE A.BALANIS ch. 3.1 - 3.2.4 Jeong Gu Ho

2. Hanyang University 2/14 2.2.2 Linear Dipole Antennas and Monopoles Above a Perfect Ground Plane –A graph of the variation of the feed-point resistance and reactance of a monopole antenna above ground is given in Figure 2.4 as a function of length h/λ –reactance X a depends on the conductor radius a, whereas the feed-point resistance is relatively independent of conductor radius a for thin antennas (a/λ≤1).

3. Hanyang University 3/14 3.1 INTRODUCTION -3.1.1 Aperture Antennas -3.1.2 Key Specifications 3.2 COMMON APPLICATIONS OF APERTURE ANTENNAS -3.2.1 Standard-Gain Horns -3.2.2 Prime-Focus Feeds -3.2.3 Secondary-Focus Feeds -3.2.4 Direct Radiators Contents

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5. Hanyang University 5/14 –Number of general properties to be considered when designing aperture antennas. (radiation pattern, return loss, mismatch) –Radiation pattern characteristics including beamwidth and the degree of side-lobe level and front-to-back ratio –Other important attributes of the radiation pattern include gain, the phase center and the main beam efficiency. For some horn-type antennas gain can be calculated accurately and these find use as gain standards. Knowing the Phase center of an aperture antenna is important if it is to be used as a feed on a reflector antenna or as an element in an array. Main beam efficiency is important parameter in designing feeds and has direct implications to the commonly used figure-of-merit parameter, G/T. G : Gain, T: antenna temperature Aside from the radiation pattern characteristics, the other major electromagnetic design consideration is the return loss, or mismatch.

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8. Hanyang University 8/14 3.2 Common applications of aperture antennas 3.2.1 Standard-Gain Horns –To evaluate the gain of an antenna, it is customary to compare its performance against a well calibrated "standard", so-called standard-gain horns are used for this purpose. –The most common type of horn used as a standard-gain horn is the pyramidal horn (figure 3.1), this enable easy manufacture and results in a low-cost antenna. –Conical horns (figure 3.2) can also be used as standard-gain horns where the antenna gain G dBi >15dBi –Conical corrugated horn (figure 3.3) as a standard-gain horn when G dBi >12dBi.

9. Hanyang University 9/14 3.2.2 Prime-focus feeds –Prime-focus feeds used mainly for circular-symmetrical or offset reflectors. The most common type of reflector used in the industry is the circular-symmetrical parabolic reflector defined by f/D ratio. F : the focal length of the parabola D : the aperture diameter. –The f/D ratio provides the antenna designer with an idea of the subtended angle of the antenna and also a rough description of the dish. –Offset-parabolic configuration is no feed blockage.

10. Hanyang University 10/14 –A circular-symmetrical parabolic reflector for a given f/D ratio, the half-subtended angel at the feed (θ e ) –In the case of an offset parabolic reflector,

11. Hanyang University 11/14 [8] G. L. James and B. MacA. Thomas, Comparison of G/T between dual-reflector and primary-focus antennas, Electron. Lett., Vol. 16, No.8, pp. 286–288, 1980.

12. Hanyang University 12/14 3.2.3 Secondary-focus feeds –Refers to feeds to feeds used to illuminate the subreflector of a dual- reflector antenna. Most common is the cassegrain antenna. –It is made of a parabolic main reflector and a hyperbolic subreflector. –From a feed design point of view, the important angle to consider is the half-subtended angle (θ e ), this is angle much smaller than that for the prime-focus case. Therefore the radiation pattern of the feed needs to be appropriately directive with the most common types of secondary feeds (conical horn, conical corrugated horn, dual-mode horn)

13. Hanyang University 13/14 3.2.4 Direct radiators –Direct radiators are horns or arrays that are used to radiate directily toward either a target or another antenna. (range illuminators, point to point communication, earth illumination from satellites) Range illuminators It is important to realize that the cross-polarizationn performance of the range illuminator will limit the accuracy. Point to point communication This is where highly directive horns or reflector antennas are pointed at each other to operate as areceive/transmit network for data transfer. Earth illumination from satellites Directive horn with low sidelobes is used to illuminate the full earth efficiently as seen from a satellite or a reflector antenna system is used to illuminate a "footprint " on the earth.

14. Hanyang University 14/15 Antennas & RF Devices Lab. Thank you for your attention Antennas & RF Devices Lab.