ANTENNA RADIATION PATTERN
1.1 To plot the radiation pattern of Dipole Antenna in E & H planes on log & linear scales on polar and Cartesian plots.
1.2. To measure the beamwidth (-3 dB), front to back ratio, side lobe level and its angular position, plane of polarization and directivity and gain of Dipole Antenna.
Antennas are a fundamental component of modern communication systems. By definition, an antenna acts as a transducer between a guided wave in a transmission line and an electromagnetic wave in free space. Antennas demonstrate a property known as reciprocity, which is an antenna will maintain the same characteristics regardless if it is transmitting or receiving. When a signal is fed into an antenna, the antenna will emit radiation distributed in space a certain way. A graphical representation of the relative distribution of the radiated power in space is called a radiation pattern. The radiation pattern of the antenna is of principle concern when engineering a communications system. Let’s assume that a signal needs to be sent from an antenna on the ground to a satellite in orbit. This would require a radiation pattern with the majority of its radiated power focused into orbit. If the antenna is not engineered to do so, contact cannot be established between the signal source and its target. There are many different ways to manipulate a radiation pattern to meet the demands of a specific task. These concepts are the principle focus of this lab assignment. Implementing this lab assignment, students will examine the radiation patterns of several antennas by hands on field testing. Only the most fundamental antennas were chosen for this lab assignment. This allows us to see visually how the most common types of real-world antenna designs function.
The following is a glossary of basic antenna concepts.
An antenna is a device that transmits and/or receives electromagnetic waves. 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.
We often refer to antenna size relative to wavelength. For example: a half-wave dipole, this is approximately a half-wavelength long. Wavelength is the distance a radio wave will travel during one cycle. The formula for wavelength is shown on the next page. Note: The length of a half-wave dipole is slightly less than a half-wavelength due to end effect. The speed of propagation in coaxial cable is slower than in air, so the wavelength in the cable is shorter. The velocity of propagation of electromagnetic waves in coax is usually given as a percentage of free space velocity, and is different for different types of coax.
Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power. The definition used is based on VSWR. Bandwidth is often expressed in terms of percent bandwidth, because the percent bandwidth is constant relative to frequency. If bandwidth is expressed in absolute units of frequency, for example MHz, the bandwidth is then different depending upon whether the frequencies in question are near 150, 450, or 825 MHz A mathematical analysis of bandwidth is provided below.
PERCENT BANDWIDTH IS DEFINED AS:
FH is the HIGHEST FREQUENCY IN THE BAND
FL is the LOWEST FREQUENCY IN THE BAND
FC = (FH + FL)/2
2.4 Directivity and Gain
Directivity: Given a set of spherical polar coordinates we can determine the power density in watts/ (square meter) for both the antenna being investigated, and the isotropic reference antenna, which is radiating the sane total power. The ratio of these power densities gives the directivity of the unknown antenna in the direction at a distance R from the antenna. If the direction is not specified, the “directivity” is taken to be the maximum directivity of any of the directions of radiation. The quoted definition is: “The directivity of an antenna is defined as the ratio of the radiation intensity in a given direction from the antenna, to the radiation intensity averaged over all directions; The average radiation intensity is equal to the total power of the antenna divided by 4. If the direction is not specified the directivity refers to the direction of maximum radiation intensity”.
Gain: The gain in any direction is power density radiated in direction divided by power density this would have been radiated at by a loss less (perfect) isotropic radiator having the same total accepted input power. If the direction is not specified, the value for gain is taken to mean the maximum value in 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.
2.5 Gain Measurement
One method of measuring gain is by comparing the antenna under test against a known standard antenna. This is technically known as a gain transfer technique. At lower frequencies, it is convenient to use a 1/2-wave dipole as the standard. At higher frequencies, it is common to use a calibrated gain horn as a gain standard, with gain typically expressed in dBi. Another method for measuring gain is the 3 antenna method. Transmitted and received power at the antenna terminals is measured between three arbitrary antennas at a known fixed distance. The Friis transmission formula is used to develop three equations and
three unknowns. The equations are solved to find the gain expressed in dBi of all three antennas.
2.6 Radiation Patterns
Radiation Pattern: The antenna radiation pattern is a measure of its power or radiation distribution with respect to a particular type of coordinates. We generally consider spherical coordinates as the ideal antenna is supposed to radiate in a spherically symmetrical pattern. However antennae in practice are not omni directional but have a radiation maximum along one particular direction. For e.g. Dipole antenna is a broadside antenna wherein the maximum radiation occurs along the axis of the antenna. The 3-D radiation pattern of a typical dipole antennais shown in figure 1
FIGURE 1:- RADIATION PATTERN OF A DIPOLE ANTENNA
2.7 Absolute and Relative Patterns
Absolute radiation patterns are presented in absolute units of field strength or power. Relative radiation patterns are referenced in relative units of field strength or power. Most radiation pattern measurements are relative pattern measurements, and the gain transfer method is then used to establish the absolute gain of the antenna.
2.8 Near-Field and Far-Field Patterns
The radiation pattern in the region close to the antenna is not exactly the same as the pattern at large distances. The term near-field refers to the field pattern that exists close to the antenna; the term far-field refers to the field pattern at large distances. The far-field is also called the radiation field, and is what is most commonly of interest. The near-field is called the induction field (although it also has a radiation component). Ordinarily, it is the radiated power that is of interest, and so antenna patterns are usually measured in the far-field region.
Distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength. The accepted formula for this distance is:
rmin=minimum distance from antenna
D=largest dimension of antenna
When extremely high power is being radiated (as from some modern radar antennas), the near-field pattern is needed to determine what regions near the antenna, if any, are hazardous to human beings.
Depending on the radio system in which an antenna is being employed there can be many definitions of beamwidth. A common definition is the half power beamwidth(HPBW). The peak radiation intensity is found and then the points on either side of the peak represent half the power of the peak intensity are located. The angular distance between the half power points traveling through the peak is the beamwidth. Half the power is —3dB, so the half power beamwidth is sometimes referred to as the 3dB beamwidth.
2.10 Antenna Pattern Types
2.10.1 Omnidirectional Antennas
For mobile, portable, and some base station applications the type of antenna needed has an omnidirectional radiation pattern. The omnidirectional antenna radiates and receives equally well in all horizontal directions. Many of the antennas measured in the laboratory experiment are of this type, because of they are common-place in the real world. The gain of an omnidirectional antenna can be increased by narrowing the beamwidth in the vertical or elevation plane. The net effect is to focus the antenna’s energy toward the Horizon. Selecting the right antenna gain for the application is the subject of much analysis and investigation. Gain is achieved at the expense of beamwidth: higher-gain antennas feature narrow beamwidths . Omnidirectional antennas with different gains are used to improve reception and Transmission in certain types of terrain. A 0 dBd gain antenna radiates more energy higher in the vertical plane to reach radio communication sites that are located in higher places. Therefore they are more useful in mountainous and metropolitan areas with tall buildings. A
3 dBd gain antenna is the compromise in suburban and general settings. A 5 dBd gain antenna radiates more energy toward the horizon compared to the 0 and 3 dBd antennas to reach radio communication sites that are further apart and less obstructed. Therefore they are best used in deserts, plains, flatlands, and open farm areas.
2.10.2 Directional Antennas
Directional antennas focus energy in a particular direction. Directional antennas are used in some base station applications where coverage over a sector by separate antennas is desired. Point to point links also benefit from directional antennas. Yagi,horn,hellical and panel antennas are directional antennas.
2.10.3 Antenna Polarization
Polarization is defined as the orientation of the electric field of an electromagnetic wave. Polarization is in general described by an ellipse. Two often used special cases of elliptical polarization are linear polarization and circular polarization. The initial polarization of a radio wave is determined by the antenna that launches the waves into space. The environment through which the radio wave passes on its way from the transmit antenna to the receive antenna may cause a change in polarization.
With linear polarization the electric field vector stays in the same plane. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turns for each RF cycle. The rotation may be right-hand or left-hand. Choice of polarization is one of the design choices available to the RF system designer. For example, low frequency (< 1 MHz) vertically polarized radio waves propagate much more successfully near the earth than horizontally polarized radio waves, because horizontally polarized waves will be canceled out by reflections from the earth. Mobile radio systems waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios.
At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization, although in specific applications, there may be some possible advantage in one or the other. Circular polarization has also been found to be of advantage in some microwave radar applications to minimize the "clutter" echoes received from raindrops, in relation to the echoes from larger targets such as aircraft. Circular polarization can also be used to reduce multipath. The majority of the antennas utilized in this experiment are vertically polarized because of their predominance in antenna applications.