26-07-2014, 11:31 AM
BASIC ANTENNA THEORY
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ANTENNA
An antenna is a conductor or group of conductors used either for radiating electromagnetic
energy into space or for collecting it from space. Electromagnetic waves are often referred to as
radio waves. Most antennas are resonant devices, which operate efficiently over a relatively
narrow frequency band. An antenna must be tuned to the same frequency band that the radio
system to which it is connected operates in, otherwise reception and/or transmission will be
impaired.
ANTENNA CHARACTERISTICS
As it has been defined, an antenna is a conductor or group of conductors used either for radiating
electromagnetic energy into space or for collecting it from space. Electrical energy from the
transmitter is converted into electromagnetic energy by the antenna and radiated into space. On
the receiving end, electromagnetic energy is converted into electrical energy by the antenna and
is fed into the receiver.
Fortunately, separate antennas seldom are required for both transmitting and receiving rf energy.
Any antenna can transfer energy from space to its input receiver with the same efficiency that it
transfers energy from the transmitter into space. Of course, this is assuming that the same
frequency is used in both cases. This property of interchangeability of the same antenna for
transmitting and receiving is known as antenna RECIPROCITY. Antenna reciprocity is possible
because antenna characteristics are essentially the same for sending and receiving
electromagnetic energy.
RECIPROCITY OF ANTENNAS
In general, the various properties of an antenna apply equally, regardless of whether you use the
antenna for transmitting or receiving. The more efficient a certain antenna is for transmitting, the
more efficient it will be for receiving on the same frequency. Likewise, the directive properties
of a given antenna also will be the same whether it is used for transmitting or receiving.
Assume, for example, that a certain antenna used with a transmitter radiates a maximum amount
of energy at right angles to the axis of the antenna, as shown in figure 1, view A. Note the
minimum amount of radiation along the axis of the antenna. Now, if this same antenna were used
as a receiving antenna, as shown in view B, it would receive best in the same directions in which
it produced maximum radiation; that is, at right angles to the axis of the antenna.
BANDWIDTH:
The bandwidth of an antenna is the range of frequencies over which it is effective, usually
centered around the resonant frequency.
The bandwidth of an antenna may be increased by several techniques, including using thicker
wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like
in a feed horn), and combining multiple antennas into a single assembly and allowing the natural
impedance to select the correct antenna.
RADIATION RESISTANCE:
Radiation Resistance RA is the equivalent resistance which would dissipate the same amount of
power as the antenna radiates.
Radiated energy is the useful part of the transmitter's signal. However, it represents as much of a
loss to the antenna as the energy lost in heating the antenna wire. In either case, the dissipated
power is equal to I2R. In the case of heat losses, the R is real resistance. In the case of radiation,
R is an assumed resistance; if this resistance were actually present, it would dissipate the same
amount of power that the antenna takes to radiate the energy. This assumed resistance is referred
ANTENNA INPUT IMPEDANCE:
Radiation from an antenna is a direct result of the flow of RF current. The current flows to the
antenna through a transmission line, which is connected to a small gap between the conductors
that make up the antenna. The point on the antenna where the transmission line is connected is
called the antenna input terminal or the feed point. The feed point presents an ac load to the
transmission line called antenna input impedance and is simply the ratio of the antenna’s input
voltage to input current. The input impedance measured at the antenna input terminals. In general
it is complex and has two real parts and one imaginary part:
ANTENNA GAIN G:
Gain is given in reference to a standard antenna. The two most common reference antennas are
the isotropic antenna and the resonant half-wave dipole antenna. The isotropic antenna radiates
equally well in "all" directions. Real isotropic antennas do not exist, but they provide useful and
simple theoretical antenna patterns with which to compare real antennas. An antenna gain of 2 (3
dB) compared to an isotropic antenna would be written as 3 dBi. The resonant half-wave dipole
can be a useful standard for comparing to other antennas at one frequency or over a very narrow
band of frequencies. To compare the dipole to an antenna over a range of frequencies requires an
adjustable dipole or a number of dipoles of different lengths. An antenna gain of 1 (0 dB)
compared to a dipole antenna would be written as 0 dBd.
Therefore, gain is the logarithm of the ratio of the intensity of an antenna's radiation pattern in
the direction of strongest radiation to that of a reference antenna.
Simply Antenna Gain G: The ratio of the radiated power in the maximum direction to the
radiated power of an isotropic antenna. The gain of an antenna represents the ability to focus its
beam in a particular direction – an isotropic antenna has a gain of 0 dB.
EFFECTIVE ISOTROPICALLY RADIATED POWER (EIRP):
A frequently used concept in communication system is that of effective (or equivalent)
isotropically radiated power, EIRP. It is formally defined as the power gain of a transmitting
antenna in a given direction multiplied by the net power accepted by the antenna from the
connected transmitter. EIRP, effective radiated power, is similar to EIRP but with antenna gain
relative to that of a half-wave dipole instead of relative to an isotropic antenna. As an example of
EIRP, suppose an observer is located in the direction of maximum radiation from a transmitting
antenna with input power Pt; then
LOG PERIODIC ANTENNA
A log-periodic antenna is another form of frequency-independent antenna and has a structural
geometry such that its impedance and radiation characteristics repeat periodically as the
logarithm of frequency. In practice, the variations over the frequency band of operation are
minor.
Diameter-to-wavelength ratio of supporting boom
0.002 0.006 0.01 0.02 0.03 0.04
0.005
0.010
0.015
0.020
0.025
0.030Antennas lecture notes. By M. D. Kabadi 42
The final phase in this metamorphosis of log-periodic antennas is the use of only parallel wire
segments. This is the log-periodic dipole array of Figure 11. The log-periodic dipole array
(LPDA) is a series-fed array of parallel wire dipoles of successively increasing lengths outward
from the feed point at the apex. Note that the interconnecting feed lines cross over between
adjacent elements.
A particularly successful method of constructing an LPDA is shown in Figure 2. A coaxial
transmission line is run through the inside of one of the feed conductors. The outer conductor of
the coax is attached to that conductor and the inner conductor of the coax is connected to the
other conductor of the LPDA transmission line.