Antenna Signal Gain
The effective signal gain, or power gain, of an antenna
is the ratio between the power required in the antenna
and the power required in an isotropic radiator to
achieve the same field strength in the favored direction
of the antenna under measurement
(Figure 15). Direc-
tive gain may be expressed as the power ratio, in units
called decibels (dB). Referring to the illustration, the
power gain of the antenna under test, placed at the
center of the sphere, illuminates only a portion of the
sphere and the power gain is the ratio of the surface
area illuminated by the isotropic antenna to that area
illuminated by the test antenna. Since the field pattern
of radiation of any antenna is not clear, but blends into
nothingness at the extremities, the practical pattern is
defined as that illuminated portion of the sphere which
lies between the "half-power" angles of the radiator
field. On the usual polar plot of an antenna pattern,
these points are the "-3 dB" power points.
DIRECTIONAL ANTENNA
® · RAD IATION FROM ANTENNA 1
® · RAD IAT ION FROM ANTENNA 2
Figure 13. Radiation Pattern from Two
Antennas
Wave interference patterns created by two adjacent an-
tennas. Radio waves from two adjacent sources of the
same frequency reinforce or cancel each other to pro-
vide wave pattern in space adjoining the antennas. In
this representation the waves reinforce each other along
radial lines OA, OB, 00', OC, and OD. Midway between
these lines the waves cancel each other. This pattern
represents an antenna array having five lobes.
Figure 14. Antenna Pattern of Directional
Array
cqually well in all directions) is known as an isotropic
rodiator, and only exists as a mathematical concept.
Such an antenna, if placed at the center of a sphere,
would "illuminate" the inner surface of the sphere uni-
formly.
Figure 15. Antenna Power Gain Over
Isotropic Radiator
The effedive power gain of an antenna is the ratio of
power required in the antenna and the power required
in an isotropic radiator to achieve the same field
strength in the favored diredion of the antenna under
measurement. The power gain of a half-wave dipole
over an isotropic radiator is 1.64. The gain of a direc-
tional antenna over an isotropic radiator is expressed by
the formula in the illustration.
The power gain over an isotropic radiator, or over a
simple dipole, is the measuring stick for antenna per-
formance. The power gain over a dipole may be com-
puted from the formula shown in the illustration,
which provides a quick method of determining the
power gain of an antenna by measuring the radiation
pattern at the -3 dB power points.
Closely allied to the concept of power gain is the
problem of suppressing unwanted radiation from the
sides and rear of a directive antenna system. Unwanted
energy radiated to the rear of the directional antenna
may be compared to the energy radiated from the front
of the array and is expressed as a power ratio in deci-
bels termed the front-to-back ratio.
Simple antennas often have a symmetrical radiation
pattern and may even possess modest gain without hav-
ing appreciable front-to-back ratio. More complex an-
tenna arrays exhibit higher gain and front-to-back
ratio, but seldom will maximum power gain and maxi-
20-9
RADIATION AND PROPAGATION
mum front-to-back ratio occur at the same condition of
antenna adjustment.
Power gain implies horizontal or vertical directivity
in the antenna pattern which can be best expressed as a
directive pattern which is a graph showing the relative
radiated field intensity expressed in terms of the azi-
muth angle for horizontal directivity and in terms of
the elevation angle for vertical directivity (Figure 16).
B
50o 60o 70o 80o 90o 80o 70 6( 5rW
Figure 16. Directivity Patterns for Dipole
Antenna One-Half Wavelength Above
Perfect Ground Plane
Plotted field intensity for dipole antenna. Azimuth angle
for horizontal directivity is shown in (A). Vertical angle
(elevation angle) is shown at (B).
Antenna Bandwidth
The bandwidth of an antenna is a measure of its ability
to operate over a specified range of frequencies. Unlike
other antenna properties, bandwidth does not have a
unique definition, as it depends on the operational re-
quirement of the antenna. Bandwidth may be limited
by loss in gain, change of antenna pattern, excessive
SWR on the feed system, or change in input impe-
dance. One of these factors, such as gain or impedance,
usually limits the low-frequency limit of operation,
whereas change of pattern shape might determine the
high-frequency limit. In amateur practice, bandwidth is
usually specified in terms of a maximum SWR limit on
the transmission line feeding the antenna system.
Mutual Impedance
A conductor placed in the field of an antenna will have
a current induced in it by virtue of the voltage applied
to the antenna. In the case of two adjacent antennas, if
a voltage is applied to the terminals of the first antenna
and the induced current measured at the terminals of
the second antenna, then an equal current will be found
at the terminals of the first antenna if the original volt-
age is applied to the terminals of the second antenna.
This classic theory can be expanded into the concept
of mutual impedance between two coupled antennas
and accounts for the fact that the feed impedance of an
individual element in an array of antennas may differ
considerably from its free-space impedance because of
the effect of mutual coupling with the other elements of
the array. In an antenna array where the current distri-
bution in the elements is critical because of pattern
requirements, it is necessary to adjust the coupling sys-
tem between the elements to provide correct current
distribution and to match the input impedance of the
array, rather than the self-impedance of the input
element.
The input impedance is the sum of the self-impe-
dance of the fed element and the mutual impedance
with all other elements in the array. The magnitude and
phase of the mutual impedance depend on the ampli-
tude of the current induced in the fed antenna by the
other elements and this, in turn, is a function of the
spacing and tuning of the additional elements. Induced
currents in the fed element are greatest when the ele-
ments of the array are close together, resonant, and
parallel.
The induced current may be in phase, or out of
phase, with the fed-element current and the impedance
of the array may be higher, or lower, than that of the
fed element. In addition, the elements may introduce
reactance into the fed element, detuning it from a reso-
nant condition. All of these effects are interlocking,
and changes in spacing or tuning can create vast differ-
encesin the performance of an antenna array.
The mutual impedance between antennas of an array
is important as this factor determines the current that
flows in the system for a given amount of power. The
current determines the power in a given array and if the
mutual impedance between the elements of an array is
such that the resulting currents are greater (for the same
amount of power) than if the antenna elements were
not coupled, then the power gain of the system is
greater.
The Antenna Above a Ground Plane
The properties of an antenna placed near a large con-
ducting ground plane will be modified by the effect of
ground reflection. In the hf region, the ground is a
basic part of the antenna system and affects both the
radiation pattern of the antenna as well as its radiation
raistance. To estimate the effects of the ground plane,
an image antenna is introduced below the ground plane
as shown in Figure 17. The electric charges of the
master antenna above the ground are reversed in the
imaginary ground image antenna. In addition, the ver-
tical components of the image are in the same direction
as those in the master antenna, while the horizontal
components are reversed in direction. The radiated
feld of the master antenna above the ground plane can
be determined by replacing the ground plane with the
image antenna and computing the resulting field of the
two antennas. In a similar manner, the effect of the
ground on the radiation resistance of the antenna can
be determined by image theory.
Polar plot shows antenna radiation as compared to an
omnidiredional antenna. Signal gain varies with the
number and adjustment of antenna elements in the ar-
ray. The diredive pattern is termed the "main lobe" of
the antenna, with the unwanted lobe termed the "rear
lobe." The ratio between the two lobes is called the
"front-to-back ratio" of the array.
