Chapter 3 
ANTENNAS 
FUNDAMENTALS 
Function of Antennas 
TRANSMITTING ANTENNA converts the power 
delivered to it into electromagnetic radiation 
(neglecting losses); a receiving antenna abstracts 
power from an incident electromagnetic wave and 
delivers to the receiver that part which is not re- 
radiated or lost in the antenna. In the short and 
microwave region the power conversion is effected 
with a very small loss so that for most practical 
purposes the power loss inside the antenna may be 
disregarded. Apparent losses caused by reflection 
owing to mismatch between the antenna and its 
input circuit are of a different nature and are not 
included herein. 
For many purposes it is desirable to concentrate 
the power radiated into a beam of comparatively 
small angle as in this way the field strength in the 
preferred direction is enhanced. The gain of a 
directional antenna is defined by means of a com- 
parison of the given antenna radiation pattern with 
that of an electric doublet. 
The gain of an antenna is the ratio of power that 
must be supplied to a doublet to the power that must 
be supplied to the antenna considered in order that, 
at a given large distance, the electric field at the 
maximum of the antenna pattern is equal to the 
field at the same distance in the equatorial plane of 
the doublet. From the reciprocity principle it is 
found that the gain of a receiving antenna is equal 
to the gain of the same antenna used as a transmitter. 
A discussion of antenna gain and reciprocity is given 
in Chapter 2, p-339- 
Directive Antennas 
Polar plots of antenna radiation patterns are of 
two kinds: either the relative magnitude of the 
Poynting vector (power per unit area) is plotted 
along the radius vector, or the relative magnitude of 
the radiation electric field strength is plotted in the 
same way. Usually the value of the radius vector at 
the maximum ot the pattern is taken equal to unity. 
The Poynting vector plot is obtained from the field 
strength plot by squaring the radial distances 
(Figure 1). 
If an antenna system is designed so that most of its 
power is concentrated into a comparatively small 
cone, the corresponding part of the radiation pattern 
345 
is called the main lobe. Commonly there are a num- 
ber of secondary maxima (side lobes) much smaller 
than the main lobe. The width of the main lobe is 
measured by the angle between half-power points. 
Half-power points are those points in the polar 
diagram of the antenna pattern where the power 
per unit area is equal to one-half that at the maxi- 
mum, the field strength being 1/ V2 = 0.707 times 
that at the maximum. This angle is also referred 
to as the beam width. The beam width varies from a 
degree or less for some specialized radar antennas to 
very large angles such as 50 to 60 degrees, depending 
on the design and purpose of the antenna. The 
larger the beam width the smaller the gain. 
It should be noted that an antenna radiation 
pattern may have high directivity with respect to 
one plane going through the antenna and little or 
no directivity in another plane Thus a doublet 
antenna (for definition see texton p-336)is directive 
in a plane which contains the antenna itself but is 
nondirective in the equatorial plane perpendicular 
to the antenna (see Figure 11). 
Figure 1. 
Antenna radiation patterns. 
Antenna Pattern Factors 
in Ground Reflection 
With highly directive antennas the magnitude 
of the direct wave may differ appreciably from that 
of the ground-reflected wave owing to their differ- 
ence in angle of emergence from the antenna (Fig- 
ure 2). This must be taken into account by using the 
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Figure 2. Antenna pattern factors: 
