Tips for Making Fast, Accurate Antenna Measurements

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Agilent Technologies : Tips for Making Fast, Accurate Antenna Measurements : Article

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If the electromagnetic spectrum was traded as a commodity, its spot price would make gold prices look cheap in comparison. Like gold and other natural resources, some spectrum is of a “higher grade“ in terms of its usefulness but unlike other resources, it is found everywhere, we know where it is – and that when it’s fully utilized in a given area, there’s no more to be found. Read More... FROM WHERE WE SIT

January 2011

Tips for Making Fast, Accurate Antenna Measurements

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By Say Phommakesone, Agilent Technologies Wireless technology has and will continue to have a significant and transformative impact on every aspect of modern society, including the military. These days, wireless communications play a crucial role in everything from logistics (e.g., the wireless battlefield) and education/training to battlefield medical support. Keeping soldiers mobile and connected poses special challenges to the military and in turn, places a heavy burden on the components and systems that go into making that mobile connectivity possible, not the least of which is the antenna. In a wireless communication system, the antenna transmits and receives signals. Its characteristics therefore play a significant role in optimizing system performance. In the military, antennas are used in a range of applications including radar and satellite communications.

July 2013

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Today, advanced technology improvements and new architectures are enabling increasingly complex antennas. While this bodes well for the military applications in which the antennas will SEARCH be used, it also places increased pressure on the antenna test engineer tasked with accurately The MPD database has more than 250 measuring the antenna. Complex antennas like the phased array beam-steering antenna, for articles, 3,000 product descriptions, and example, can dramatically increase the amount of data needed to fully characterize complex ads f rom our advertisers f rom 2007 to the arrays and also increase overall test time. It is the antenna test engineer’s responsibility to current issue. ensure that the antenna is accurately characterized and meets specification, but doing so quickly and accurately given this complexity can be challenging. Let’s take a closer look at MILITARY MICROWAVE DIGEST antenna measurements and some key tips for making them fast and accurate. Antenna Measurements, RCS and Range To test an antenna, the engineer must measure its parameters (e.g., radiation pattern, gain, impedance, and polarization characteristics). Antenna patterns can be measured using one of the following techniques: far-field range or near-field range.

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To understand these techniques and how they work, first consider that a typical antenna-range Another Sad Moment For the FCC By Barry Manz

A significant number of rooftop antenna www.mpdigest.com/issue/Articles/2011/jan/Agilent/Default.asp

measurement system consists of a transmit site and a receive site (Figure 1). The transmit site is comprised of the microwave transmit source, optional amplifiers, transmit antenna, and communications link to the receive site. The receive site is comprised of the antenna-undertest (AUT), a reference antenna, receiver, LO source, RF downconverter, positioner, system software, and a computer. 1/7

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of rooftop antenna sites owned primarily by wireless carriers exceed FCC public and occupational exposure limits, make it impossible for workers to avoid standing in front of antennas, and are inadequately posted with warnings and barriers. Read More...


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Far-field antenna measurements require that the AUT is radiating in the far-field or Fraunhofer zone. In general, antennas produce a spherical wavefront; however, at great distances the spherical wavefront becomes almost planar across the aperture of the receive antenna. These planar waves are required for far-field testing. The generally accepted far-field criterion is as follows:

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A source antenna illuminates the AUT at a distance that is far enough away to create a nearplanar phase front over the AUT’s electrical aperture. Far-field measurements can be performed on both indoor and outdoor ranges. Indoor measurements are typically made in either a rectangular or tapered anechoic chamber specifically designed to reduce reflections off

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Agilent Technologies : Tips for Making Fast, Accurate Antenna Measurements : Article either a rectangular or tapered anechoic chamber specifically designed to reduce reflections off the walls, floor and ceiling.

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On a near-field range, the AUT is placed in the near-field and the fields on a surface close to the antenna are measured. Then, the data is mathematically transformed to obtain far-field behavior. Near-field measurements are performed on indoor ranges. Radar cross section (RCS) measurements are used to measure the angle-dependent echo characteristics of radar targets (e.g., how detectable an object is with radar). Measurement of an object’s RCS is performed at a radar reflectivity range or scattering range. The first type of range is an outdoor range where the object is positioned on a specially shaped low-RCS pylon that is some distance down range from the transmitters. An anechoic chamber is commonly used for indoor measurements. Here, the object is placed on a rotating pillar in the center of the chamber. The walls, floors and ceiling are covered by radar absorbing material to prevent corruption of the measurement due to reflections. While each of these techniques has its own set of benefits and limitations, which one the antenna test engineer selects to properly determine the amplitude and/or phase characteristics of an AUT depends on the antenna being measured and its application. Typically, far-field ranges are used for antennas under 10 wavelengths, while near-field ranges are used for antennas over 10 wavelengths. Due to the military’s ongoing effort to reduce the RCS of objects like missiles, ships, tanks and aircraft, the use of RCS measurements has also become increasingly popular. Selecting the Right Measurement Instrument When it comes to making fast and accurate antenna measurements, selecting the antenna range is only half of the equation; the other half is selecting the right measurement instrument. In this case, that means a measurement receiver—a radio receiver designed to measure the characteristics of radio signals. Today’s complex antennas require a measurement receiver that is fast, accurate, highly measurement sensitive, and able to handle large amounts of data. Exactly how fast, accurate, or sensitive the instrument must be will depend on the engineer’s specific measurement and application requirements. Nevertheless, the general criteria that any antenna test engineer should consider when determining which measurement receiver to purchase are: Speed Making antenna measurements can be a time consuming proposition, made all the more complicated when they need to be done in the field. Fast data acquisition and transfer rates are critical to reducing test times. Moreover, when coupled with LAN connectivity, the fast data transfer rates further decrease test time by enabling remote testing. Note that a measurement receiver with the ability to quickly obtain many data points per second (e.g., 400,000 points/sec) is ideal for far-field antenna range applications. Extremely fast data processing is particularly useful in applications where ranges include active array antennas and data acquisition is therefore quite intensive. Accuracy Measurement accuracy is essential to ensuring that the antenna is accurately characterized and meets specification. Features like support for synchronization with external signal generators is just one way to ensure the accuracy of the measurement receiver. Sensitivity Antenna test engineers often work on many different antennas and antenna applications, some


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of which may be highly complex. High sensitivity is therefore critical. A measurement receiver with fast throughput and low noise floor translates into increased sensitivity, while different intermediate frequency (IF) bandwidths enable the optimization of measurement receiver sensitivity versus measurement speed to accommodate the engineer’s requirements. If the instrument also boasts fast data acquisition speeds, then its IF bandwidth can be narrowed— significantly improving the measurement sensitivity without increasing total measurement times. Flexibility Since each engineer’s requirements will differ, instrument flexibility is essential. Compatibility with external signal sources, signal generators, remote mixers and frequency converters, for example, provides the engineer with the flexibility to create a setup optimized for any given antenna application. Multiple simultaneous test receivers (channels) provide even further measurement flexibility for the engineer. Security Many of the military’s wireless applications that employ antennas are of a sensitive or even secret nature, making security at all levels absolutely critical. A measurement receiver with a removable hard drive is ideal for secure environments as it completely ensures the security of the data it acquires.


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While selecting a measurement receiver that meets this criterion will not guarantee successful antenna measurements, it will ensure they can be made faster and with more accuracy (Figure 2). It also ensures antenna test engineers have the flexibility they need to meet their varying requirements. As an example, for far-field or large near-field antenna applications, the measurement receiverbased system may need to incorporate external instruments (e.g., broadband mixers and a distributed frequency converter) to make the required measurements. Compatibility of the measurement receiver with external instruments is therefore a critical factor, and one that should be verified prior to its purchase (Figure 3).


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For RCS measurements, high-power pulses are often used to overcome losses due to low device reflection and two-way transmission path loss. Receiver gating may therefore be required in a RCS configuration to avoid overloading the measurement receiver during the transmission of the pulsed-RF signal. As an example, this feature is available on the normal PNA-X vector network analyzer when using it as a receiver. Conclusion Given today’s increasingly complex antenna architectures and technologies, quickly and accurately characterizing an antenna and ensuring that it meets specification can be quite challenging. Selecting the right antenna range and measurement receiver is key to overcoming this challenge. Moreover, the measurement receiver must offer speed, accuracy, measurement sensitivity, flexibility, and security. It’s these capabilities that will ensure the measurement receiver is well suited for near-field, far-field antenna and RCS measurement applications. For more information about Agilent’s antenna test solutions go to our website. About the Author Say Phommakesone is a product marketing engineer and antenna test application specialist with Agilent Technologies’ Component Test Division in Santa Rosa, California. His work focuses on new business and product development for wireless, far-field and near-field antenna measurements. Say has amassed over 28 years of RF and microwave measurement expertise working for Hewlett-Packard and Agilent. Eight years were spent in manufacturing, supporting production test on network analyzers, sources and spectrum analyzers, followed by five years in the electrical calibration standard lab. For the past fifteen years, Say has worked in customer support, sales, technical support, and product marketing. He has developed and presented training materials for internal and external customers on a wide variety of network analyzer measurement topics.

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Tips for Making Fast, Accurate Antenna Measurements

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