`Antenna Measurements`

 Antennas (Home) Antenna Basics

Testing of real antennas is fundamental to antenna theory. All the antenna theory in the world doesn't add up to a hill of beans if the antennas under test don't perform as desired. Antenna Measurements is a science unto itself; as a very good antenna measurer once said to me "good antenna measurements don't just happen".

What exactly are we looking for when we test or measure antennas?

Basically, we want to measure many of the fundamental parameters listed on the Antenna Basics page. The most common and desired measurements are an antenna's radiation pattern including antenna gain and efficiency, the impedance or VSWR, the bandwidth, and the polarization.

The procedures and equipment used in antenna measurements are described in the following sections:

1. Required Equipment and Ranges

In this first section on Antenna Measurements, we look at the required equipment and types of "antenna ranges" used in modern antenna measurement systems.

The second antenna measurements section discusses how to perform the most fundamental antenna measurement - determining an antenna's radiation pattern and extracting the antenna gain.

The third antenna measurements section focuses on determining phase information from an antenna's radiation pattern. The phase is more important in terms of 'relative phase' (phase relative to other positions on the radiation pattern), not 'absolute phase'.

The fourth antenna measurements section discusses techniques for determining the polarization of the antenna under test. These techniques are used to classify an antenna as linearly, circularly or elliptically polarized.

The fifth antenna measurement section illustrates how to determine an antenna's impedance as a function of frequency. Here the focus is on the use of a Vector Network Analyzer (VNA).

The sixth antenna measurement section explains the useful concept of scale model measurements. This page illustrates how to obtain measurements when the physical size of the desired test is too large (or possibly, too small).

The final antenna measurement section illustrates the new field of SAR measurements and explains what SAR is. These measurements are critical in consumer electronics as antenna design consistently needs altered (or even degraded) in order to meet FCC SAR requirements.

Required Equipment in Antenna Measurements

For antenna test equipment, we will attempt to illuminate the test antenna (often called an Antenna-Under-Test) with a plane wave. This will be approximated by using a source (transmitting) antenna with known radiation pattern and characteristics, in such a way that the fields incident upon the test antenna are approximately plane waves. More will be discussed about this in the next section. The required equipment for antenna measurements include:

• A source antenna and transmitter - This antenna will have a known pattern that can be used to illuminate the test antenna

• A receiver system - This determines how much power is received by the test antenna

• A positioning system - This system is used to rotate the test antenna relative to the source antenna, to measure the radiation pattern as a function of angle.

A block diagram of the above equipment is shown in Figure 1.

Figure 1. Diagram of required antenna measurement equipment.

These components will be briefly discussed. The Source Antenna should of course radiate well at the desired test frequency. It must have the desired polarization and a suitable beamwidth for the given antenna test range. Source antennas are often horn antennas, or a dipole antenna with a parabolic reflector.

The Transmitting System should be capable of outputing a stable known power. The output frequency should also be tunable (selectable), and reasonably stable (stable means that the frequency you get from the transmitter is close to the frequency you want).

The Receiving System simply needs to determine how much power is received from the test antenna. This can be done via a simple bolometer, which is a device for measuring the energy of incident electromagnetic waves. The receiving system can be more complex, with high quality amplifiers for low power measurements and more accurate detection devices.

The Positioning System controls the orientation of the test antenna. Since we want to measure the radiation pattern of the test antenna as a function of angle (typically in spherical coordinates), we need to rotate the test antenna so that the source antenna illuminates the test antenna from different angles. The positioning system is used for this purpose.

Once we have all the equipment we need (and an antenna we want to test), we'll need to place the equipment and perform the test in an antenna range, the subject of the next section.

The first thing we need to do an antenna measurement is a place to perform the measurement. Maybe you would like to do this in your garage, but the reflections from the walls, ceilings and floor would make your measurements inaccurate. The ideal location to perform antenna measurements is somewhere in outer space, where no reflections can occur. However, because space travel is currently prohibitively expensive, we will focus on measurement places that are on the surface of the Earth. There are two main types of ranges, Free Space Ranges and Reflection Ranges. Reflection ranges are designed such that reflections add together in the test region to support a roughly planar wave. We will focus on the more common free space ranges.

Free Space Ranges

Free space ranges are antenna measurement locations designed to simulate measurements that would be performed in space. That is, all reflected waves from nearby objects and the ground (which are undesirable) are suppressed as much as possible. The most popular free space ranges are anechoic chambers, elevated ranges, and the compact range.

Anechoic Chambers

Anechoic chambers are indoor antenna ranges. The walls, ceilings and floor are lined with special electromagnetic wave absorbering material. Indoor ranges are desirable because the test conditions can be much more tightly controlled than that of outdoor ranges. The material is often jagged in shape as well, making these chambers quite interesting to see. The jagged triangle shapes are designed so that what is reflected from them tends to spread in random directions, and what is added together from all the random reflections tends to add incoherently and is thus suppressed further. A picture of an anechoic chamber is shown in the following picture, along with some test equipment:

The drawback to anechoic chambers is that they often need to be quite large. Often antennas need to be several wavelenghts away from each other at a minimum to simulate far-field conditions. Hence, it is desired to have anechoic chambers as large as possible, but cost and practical constraints often limit their size. Some defense contracting companies that measure the Radar Cross Section of large airplanes or other objects are known to have anechoic chambers the size of basketball courts, although this is not ordinary. universities with anechoic chambers typically have chambers that are 3-5 meters in length, width and height. Because of the size constraint, and because RF absorbing material typically works best at UHF and higher, anechoic chambers are most often used for frequencies above 300 MHz. Finally, the chamber should also be large enough that the source antenna's main lobe is not in view of the side walls, ceiling or floor.

Elevated Ranges

Elevated Ranges are outdoor ranges. In this setup, the source and antenna under test are mounted above the ground. These antennas can be on mountains, towers, buildings, or wherever one finds that is suitable. This is often done for very large antennas or at low frequencies (VHF and below, <100 MHz) where indoor measurements would be intractable. The basic diagram of an elevated range is shown in Figure 2.

Figure 2. Illustration of elevated range.

The source antenna is not necessarily at a higher elevation than the test antenna, I just showed it that way here. The line of sight (LOS) between the two antennas (illustrated by the black ray in Figure 2) must be unobstructed. All other reflections (such as the red ray reflected from the ground) are undesirable. For elevated ranges, once a source and test antenna location are determined, the test operators then determine where the significant reflections will occur, and attempt to minimize the reflections from these surfaces. Often rf absorbing material is used for this purpose, or other material that deflects the rays away from the test antenna.

Compact Ranges

The source antenna must be placed in the far field of the test antenna. The reason is that the wave received by the test antenna should be a plane wave for maximum accuracy. Since antennas radiate spherical waves, the antenna needs to be sufficiently far such that the wave radiated from the source antenna is approximately a plane wave - see Figure 3.

Figure 3. A source antenna radiates a wave with a spherical wavefront.

However, for indoor chambers there is often not enough separation to achieve this. One method to fix this problem is via a compact range. In this method, a source antenna is oriented towards a reflector, whose shape is designed to reflect the spherical wave in an approximately planar manner. This is very similar to the principle upon which a dish antenna operates. The basic operation is shown in Figure 4.

Figure 4. Compact Range - the spherical waves from the source antenna are reflected to be planar (collimated).

The length of the parabolic reflector is typically desired to be several times as large as the test antenna. The source antenna in Figure 4 is offset from the reflector so that it is not in the way of the reflected rays. Care must also be exercised in order to keep any direct radiation (mutual coupling) from the source antenna to the test antenna.