Tips for Making Fast, Accurate Antenna Measurements

7/27/2019 Tips for Making Fast, Accurate Antenna Measurements

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7/28/13 Agilent Technologies : Tips for Making Fast, Accurate Antenna Measurements : Article

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Toward Mapping

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IN MY OPINION

FROM WHERE WE SIT

January 2011

Tips for Making Fast, Accurate Antenna MeasurementsBy Say Phommakesone, Agilent Technologies

Wireless technology has and will continue to have a s ignificant and transformative impact on

every aspect of modern s ociety, including the m ilitary. These days, wireless communications

play a crucial role in everything from logistics (e.g., the wireless battlefield) and

education/training to battlefield medical s upport. Keeping soldiers m obile 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 s ignals. Itscharacteristics 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.

Today, advanced technology improvements and ne w architectures are enabling increasingly

complex antennas. While this bodes well for the military applications in which the antennas will

be used, it also places increased pressure on the antenna test engineer tasked with accurately

measuring the antenna. Complex antennas like the phased array beam-steering antenna, for

example, can dramatically increase the amount of da ta needed to fully characterize complex

arrays and also in crease overall test time. It is the antenna test enginee rs respons ibility to

ensure that the antenna is accurately characterized and meets s pecification, but doing so

quickly and accurately given this complexity can be challenging. Lets take a closer look at

antenna measurem ents and som e key tips for making them fast and accurate.

Antenna Measurements, RCS and Range

To test an antenna, the engineer mus t measure its parameters (e.g., radiation pattern, gain,impedance, and polarization characteristics). Antenna patterns can be mea sured us ing one of

the following techniques: far-field range or near-field range.

To understand these techniques and how they work, first consider that a typical antenna-range

measurem ent system consists of a transmit site and a receive site (Figure 1). The transmit site

is compris ed of the microwave transmit source, optional amplifiers, transmit antenna, and

communications link to the receive site. The receive site is comprised o f the antenna-under-

test (AUT), a reference antenna, receiver, LO source, RF downconverter, positioner, system

software, and a computer.

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Far-field antenna measurem ents 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

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A source antenna ill uminates the AUT at a distance that is far enough away to create a near-

planar phas e front over the AUTs electrical aperture. Far-field measureme nts can be

performed on both indoor and outdoor ranges. Indoor measurem ents are typically made in

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either a rectangular or tapered anechoic chamber s pecifically designed to reduce reflections off

the walls, floor and ceiling.

On a near-field range, the AUT is placed in the near-field and the fields on a surface close to

the antenna are m easured. Then, the data is mathematically transformed to obtain far-field

behavior. Near-field measurements are p erformed on indoor ranges.

Radar cross section (RCS) measurements are used to meas ure the angle-dependent echo

characteristics of radar targets (e.g., how detectable an object is with radar). Measurement of

an objects 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 d istance down range from the transmitters. An anechoic chamber is commonlyused for indoor meas urements. 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 meas urement 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 a nd/or phase characteristics

of an AUT depends on the antenna being measured and its application. Typically, far-field

ranges are us ed for antennas under 10 wavelengths, while near-field ranges are used for

antennas over 10 wavelengths. Due to the militarys ongoing effort to reduce the RCS of objects

like miss iles, ships, tanks and aircraft, the use of RCS measuremen ts has also becom e

increasingly popular.

Selecting the Right Measurement Instrument

When it comes to making fast and accurate antenna measurements, selecting the antennarange is only half of the equation; the other half is selecting the right measu rement instrument.

In this case, that means a meas urement receivera radio receiver designed to m easure the

characteristics of radio s ignals. Todays complex antennas require a m easurement receiver

that is fast, accurate, highly measurement sens itive, and able to hand le large am ounts of data.

Exactly how fast, accurate, or sensitive the instrument mus t be will depend on the engineers

specific measureme nt and application requirements. Nevertheless , the general criteria that any

antenna test engineer should consider whe n determining which meas urement receiver to

purchase are:

Speed

Making antenna measurements can be a time cons uming proposi tion, 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 a ntenna range applications. Extremely fast data

processing is particularly useful in appl ications 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 m eets s pecification. Features like support for s ynchronization with external signal

generators is ju st one way to ensure the accuracy of the mea surement receiver.

Sensitivity

Antenna tes t engineers often work on many different antennas and antenna applications, som e
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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 en able the optimization of m easurement receiver

sensitivity versus measurem ent speed to accomm odate the engineers requirem ents. If the

instrument also boas ts fast data acquisition speeds, then its IF bandwidth can be narrowed

significantly improving the m easurement s ensitivity without increasing total measurement

times.

Flexibility

Since each engineers requirements will differ, instrument flexibility is es sential. Compatibility

with external s ignal s ources, signal generators, remote mixers and frequency converters, for

example, provides the engineer with the flexibility to create a setup optimized for any givenantenna application. Multiple s imultaneous test receivers (channels) provide even further

measurem ent flexibility for the engineer.

Security

Many of the militarys 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 s ecure environments as it completely ensures the security of

the data it acquires.


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While selecting a m easurement receiver that meets this criterion will not guarantee successful

antenna measurem ents, it will ensure they can be m ade faster and with m ore accuracy (Figure2). It also ensures antenna test engineers have the flexibility they need to meet their varying

requirements.

As an exampl e, for far-field or large near-field an tenna appl ications , the measurement receiver-

based s ystem m ay need to incorporate external instruments (e.g., broadband mixers and a

distributed frequency converter) to make the required measurements. Compatibility of the

measurem ent receiver with external ins truments is therefore a critical factor, and one that

should be verified prior to its purchase (Figure 3).


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7/28/13 Agilent Technologies : T ips for Making Fast, Accurate Antenna Measurements : Article


For RCS measurements, high-power pulses are often used to overcome loss es due to low

device reflection and two-way transmis sion path loss. Receiver gating m ay therefore be

required in a RCS configuration to avoid overloading the measurem ent receiver during thetransmissi on 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 todays increasingly complex antenna architectures and technologies, quickly and

accurately characterizing an antenna and ensuring that it m eets sp ecification can be quite

challenging. Selecting the right antenna range and measurem ent receiver is key to overcoming

this challenge. Moreover, the meas urement receiver must offer sp eed, accuracy, meas urement

sensitivity, flexibility, and security. Its these capabilities that will ensure the meas urement

receiver is well suited for near-field, far-field antenna and RCS measureme nt applications.

For more information about Agilents antenna test s olutions go to our website.

About the Author

Say Phommakesone i s a product marketing engineer and antenna test application specialist

with Agilent Technologies Component Test Division in Santa Rosa, California. His work

focuses on new bus iness and product development for wireless, far-field and near-field

antenna measurem ents. Say has am assed over 28 years of RF and microwave meas urement

expertise working for Hewlett-Packard and Agilent. Eight years were spent in manufacturing,

supporting production test on network analyzers, sources and s pectrum 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 ma terials for internal and external customers on a wide variety of network

analyzer m easurement topics.

AGILENT TECHNOLOGIES
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