Hanyang University
MODERN ANTENNA
HANDBOOK
by CONSTANTINE A.BALANIS
ch. 3.1 - 3.2.4
Jeong Gu Ho
1/15
Antennas & RF Devices Lab.
Hanyang University
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 Xa depends on the conductor
radius a, whereas the feed-point resistance
Ra is relatively independent
of conductor radius a for thin antennas
(a/λ≤1).
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Contents
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
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3.1 Introduction
3.1.1 Aperture Antennas
– The most common type of aperture antenna is horn-type antennas.
– Aperture antennas can be used radiation pattern measurements or
as an accurate gain standard.
– Alone or in an array, in point-to-point radio communication links
and are widely used as feeds in reflector antenna systems.
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–
–
–
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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3.1.1 Aperture Antennas
– Overall specifications are often given for a complete system, it is
necessary for the antenna designer to extract those key specifications
3.1.2.1 Bandwidth
– The frequency band over which the system is to operate is usually
specified as a range or ranges of frequencies where the antenna has to
satisfy a required return loss(or VSWR) and provide a radiation pattern
adequate for the application.
– To be specific, consider an antenna required to operate from fmin to fmax,
the bandwidth
– Using this definition, a bandwidth below 40% is generally considered in
this chapter to be narrowband whereas a bandwidth over 40% is
considered to be wideband.
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3.1.2.2 Gain/Antenna efficiency
– Of importance here is the gain of the whole antenna system
3.1.2.3 The working environment
– This is will dictate the choice of materials and also the mechanical aspects
of the system.
– Interference from neighboring antennas and other structures.
3.1.2.4 Cost
– An acceptable price to be reached.
– Cost can include choice of materials, finish, and the overall performance
design level acceptable within a given budget.
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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
GdBi>15dBi
Conical corrugated horn (figure 3.3) as a standard-gain horn when GdBi>12dBi.
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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.
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– A circular-symmetrical parabolic reflector for a given f/D ratio, the half-subtended
angel at the feed (θe)
y 2  4fx
p2
D
, y
16f
2
D
tan π  θ e   22
D
f
16f
D
16Df
2 
tan θ e  
2
d
2 16f 2  D 2
f
16f


f


8


D
 θ e  arctan
2

 16 f   1 


 D

x


– In the case of an offset parabolic reflector,
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<Spill over>
[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.
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3.2.3 Secondary-focus feeds
– Refers to feeds to feeds used to illuminate the subreflector of a dualreflector 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 (θe), 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)
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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.
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Thank you for your attention
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Antennas & RF Devices Lab.