Example Experiment #6: Antenna Basics
Developed by: Robert Stigliano
Acknowledgement: Margery Hines
Lab Exercise: Antenna Basics
Experiment Objectives:
In this lab you will analyze received radar signals from the P400 to gain an understanding of the basics of
electromagnetic antennas. This lab will demonstrate the importance of electromagnetic wave polarization, antenna
radiation patterns, and the law of reciprocity.
Part 1: Overview
1.1
Introduction
Understanding antennas is a key component to understanding electromagnetics and their numerous applications.
There are many types of antennas, but all antenna performance can be described by two aspects: its impedance and
radiation properties. These properties are dictated by the shape, size, and material of the antenna. The impedance of
the antennas determines the amount of energy that is coupled to the antenna from the generator when the antenna is
transmitting or, vice-versa, when the antenna is receiving. The simplest case is when the antenna is properly
matched, thereby resulting in full transmission. The radiation properties, which are of greater interest for this lab,
include the radiation pattern and the polarization of the radiated field.
Polarization is a critical aspect of antenna design and usage. In general, electromagnetic waves are time varying
electric and magnetic fields that are a function of spatial coordinates and are coupled by Maxwell’s equations. A
uniform plane wave is characterized by electric and magnetic fields which do not have any components along the
direction of propagation and have uniform properties across a plane perpendicular to the direction of propagation.
The polarization of a uniform plane wave describes the shape and locus of the tip of the E vector in the plane
orthogonal to the direction of propagation, and depends on the relative phase between the field components, Figure
1. In the most general case, a wave is elliptically polarized, meaning that the vector of the E field traces an ellipse as
it propagates. The rotation can be either clockwise or counterclockwise depending on the properties of the wave, and
dictated by the direction of propagation. In certain cases, the field can simplify to circular or linear polarization.
Figure 1: Example of circular polarization
In Figure 1, the blue and green lines depict the Ex and Ey components which are propagating in the z direction. The
red line is the combined effect of the E field components, since E x and Ey are 90 out-of-phase, the result is circular
Lab Instructions: Experiment #6
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Example Experiment #6: Antenna Basics
Developed by: Robert Stigliano
Acknowledgement: Margery Hines
polarization. In this lab we will focus on linear polarization. If a wave is traveling in the z direction, the wave is said
to be linearly polarized if it contains only one field component (either E x or Ey) , or if the Ex and Ey components are
either in-phase or out-of-phase by 180 . In these particular cases, at any specified value of z, the tip of the E field
will trace a straight line in the x y plane. For instance, considering Figure 1, if the Ey field component (green) was
zero, then for any z value, the Ex field component would appear to be a line along the x-axis changing from positive
to negative.
Depending on an antenna’s physical properties, the radiated field can be elliptical, circular, or linear in nature. Aside
from radiating a particular polarization type, the antenna will also only be able to receive that particular polarization
type. For example, if the transmitting antenna only radiates an E x component, then given a receiving antenna which
cannot receive Ex components no power will be received. If the transmitting antenna is circularly polarized, and the
receiving antenna is linearly polarized (e.g. in E x), then since the transmitting antenna radiates both an E x and Ey
component, only part of the transmitted power will be received.
The antenna pattern, also referred to as the radiation pattern, describes the strength of the radiated wave in the threedimensional space around the antenna. It is the far-field directional properties of the antenna measured at a fixed
distance from the antenna. An isotropic antenna is one that radiates equal power in all directions; it is not physically
realizable but it is used as a reference to calculate antenna properties such as antenna gain. Measuring an antenna’s
radiation pattern can be difficult, as is graphically displaying the three-dimensional results. Most often, radiation
patterns are discussed in terms of the spherical coordinates, and typically they are shown using either a threedimensional plot, or two separate plots for orthogonal planes. The two planes most commonly used are the elevation
and azimuth planes. The elevation plane, also called the plane, corresponds to a constant; whereas the azimuth plane,
also called the plane, corresponds to the x y plane where = 90.
The antenna radiation patterns can vary greatly depending on the application; however two general classes are
omnidirectional antennas or directional antennas, Figure 2. Omnidirectional antennas radiate power uniformly in all
directions in a single plane, usually defined as the azimuth plane, and then have decreasing power in the elevation
plane. Conversely, directional antennas, also called beam antennas, radiate greater power in one or more directions.
Directional antennas are often described in terms of their lobes, where the maximum power is confined to the main
lobe, and additional power results in the undesirable side and back lobes. Radiation patterns not only define the
directions along which an antenna will transmit power, they also define the directions in which an antenna can
receive power. If an antenna is directional, and does not transmit energy in a certain direction, it also will not be able
to receive energy coming from that direction. This is the definition of reciprocity, and is a fundamental property of
antennas. Reciprocity means that the transmitting and receiving antennas can be switched, and the resulting signal
will be exactly the same.
Figure 2: 3D representation of an omnidirectional radiation pattern (left) and a directional radiation pattern
(right).
Lab Instructions: Experiment #6
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Example Experiment #6: Antenna Basics
Developed by: Robert Stigliano
Acknowledgement: Margery Hines
1.2
Background
The P400 emits an electromagnetic pulse that has a center frequency of 4.3GHz and a bandwidth of 2GHz, Figure
3. The antennas are planar elliptical dipole antennas, which produce a linearly polarized electromagnetic field. The
radiation patterns can be found in the Broadspec UWB Antenna data sheet which was included in your Time
Domain information packet. This specification sheet will be used for comparison to the experimental data collected.
Note that the antennas are omnidirectional in the azimuthal plane, and have greater variability in the elevation plane.
Also, note that the radiation patterns are shown for individual frequencies. In the following experiments, the peak
received signal will be used to analyze the radiation pattern. Since we are considering the results in the time domain,
these measurements will include the entire bandwidth of the P400 excitation.
Figure 3: P400 excitation in the time domain (left) and frequency domain (right). Images from Time Domain.
Part II: Antenna Experiment
2.1
Required Equipment
Hardware: Hardware: Laptop, P400 radar, coax cables, assortment of fixed attenuators, tape measure,
string (1 meter), protractor
Software: MATLAB, MATLAB scripts (DataAcquistion.m, Initialize.m, getScan.m)
Note: In this lab you will be rotating and moving the antennas often, and you should be sure to check that the
coaxial connections are still tight after each move.
2.2
Setup:
Step 1: Mount the antennas some distance apart, with the flat sides facing each other. The antennas should be level,
at the same angle, and orthogonal to the ground. This setup (see Figure 4a) will be referred to as Position 1
and will be the starting point for various data sets. Recall our lab about multipath reflections, and assure
that the first received signal will be the direct signal between the two antennas and will not constructively
interfere with any reflections. (This can be accomplished by mounting the antennas, for example on tripods
or cardboard boxes, such that there are no RF reflectors close to either antenna. In this case a reflector is
“too close” if it closer to either antenna by a distance less than the distance between the two antennas.)
Step 2: In Matlab open DataAcquisition.m
Lab Instructions: Experiment #6
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Example Experiment #6: Antenna Basics
Developed by: Robert Stigliano
Acknowledgement: Margery Hines
Step 3: Using the provided coaxial cables, attenuators, and transmitGain setting in Initialize.m, determine a
combination where the received signal is strong but not clipped.
2.3
Test Procedure:
Step 4: Use DataAcquisition.m to record a scan with the current setup at Position 1.
Step 5: Next, rotate one of the antennas 90o, and record a scan. Repeat for 180o, and 270o. Save the current data
set. How does the signal change? Does this agree with the antenna pattern given on the Broadspec UWB
Antenna specifications?
Step 6: Start recording a new data set. With the antennas back at Position 1, record a new scan.
Step 7: Tie a string around the center of the transmitter, and tie the other end around the receiver. Next, move one
of the antennas around the other antenna, and keep the string taunt so that the antenna separation is the
same, Figure 4. Both antennas should still be orthogonal to the ground at all positions. Record scans at
different positions. How do the results change? Is this expected?
(a)
(b)
(c)
Figure 4: Diagrams for Procedure Part 7; (a) Tie a string connecting the antennas at Position 1; (b) Keeping the
string taunt, raise the receiver some angle above the transmitter; (c) position the receiver directly above the
transmitter
Lab Instructions: Experiment #6
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Example Experiment #6: Antenna Basics
Developed by: Robert Stigliano
Acknowledgement: Margery Hines
Step 8: Again, start recording a new data set with the antennas back at Position 1. Using a protractor, change the
angle of the receiver so that is no longer orthogonal with the ground. Move the antenna in increments of 10
degrees from 0 degrees (orthogonal to the ground) to 90 degrees (parallel with the ground), Figure 5. What
changes? Why? Do you need to repeat this experiment for the transmitter, why or why not?
2.4
Lab Report
Prepare a Lab report which summarizes your experiment and specifically addresses the questions asked in the
experimental procedure.
2.5
References
1. Balanis, Constantine A. Advanced Engineering Electromagnetics. New York: Wiley, 1989.
2. Ulaby, Fawwaz. Fundamentals of Applied Electromagnetics. Prentice Hall, 2006.
Lab Instructions: Experiment #6
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