The Care and Feeding of EMC Antennas

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A version of this article ran in the December-January 1999 edition of Test & Measurement Europe. Download the article as a PDF.

I have two cats. Every day before breakfast, I set out fresh water for them and refill their food bowl. Throughout the day, when they need some attention, I’ll scratch them behind the ears. Every week, I trim their claws, and every year or so, I take them to the vet for a checkup and whatever booster shots they need.

What do I get in return for all the attention I pay to them? I get their unconditional love and a couple of companions that like to plop down on my lap when I watch TV.

EMC antennas are like cats in that they also need proper care. You don’t have to feed them, but you should send them to a cal lab once every year or so. They may not appreciate a scratch behind the ears, and they won’t jump onto your lap after a hard day of work, but if you treat them right and handle them with care, your EMC antennas will reward you with many years of faithful service.

Handle Horns Gently
The most important step in caring for EMC antennas is to handle them as gently as possible. This is especially true for antennas used in the higher frequency ranges, such as horn antennas. Most horn antennas look rugged, but when operating at gigahertz frequencies, dimensions are critical. Mechanical shocks can change an antenna’s dimensions by a millimeter or two, which will change the antenna’s characteristics.

You also can damage horn antennas if you connect cables improperly during setup. According to Larry Caney, president of Raven Engineering (San Diego, CA), most horn antennas use an N type connector. Caney notes that if you’re not careful when connecting a cable to the antenna, you will push the female pin in the antenna’s connector into the antenna cavity, changing the antenna’s characteristics. Usually, this damage is permanent. When the cable is disconnected, the female pin probably will not return to its original position.

To prevent damage to connectors, Caney suggests using “connector savers.” You carefully install such a device on an antenna and then connect the cable to it. If damage should occur, you wreck the adapter and not the antenna. These adapters also save the antenna’s connector from the wear and tear of multiple connects and disconnects.

Connector savers range in price from about $100 to $500 and are available from Maury Microwave (909-987-4715; www.maurymw.com), Midwest Microwave (800-521-4410; www.midwest-microwave.com), and Narda Microwave (516-231-1700; www.nardamicrowave.com). They may seem expensive, but since repairs can cost $1,000 or more, these adapters provide inexpensive insurance.

Weathering and electrostatic discharge (ESD) also affect an antenna’s characteristics. If you leave an antenna outside in the rain, antenna elements can become corroded and water can seep into cable assemblies. Ron Bethel, senior calibration technician for EMC Test Systems (Austin, TX), suggests bringing antennas inside if the weather looks bad. ESD mainly affects active antennas that have amplifiers with FET (field-effect transistor) front ends. FETs are quite sensitive to ESD, and what’s worse, an ESD event may not render an amplifier inoperative, but only decrease its gain.

Keep It Calibrated
In addition to handling your EMC antennas carefully, you should also calibrate them regularly. This usually means sending them off to a cal lab once a year. While some standards, such as MIL-STD-461D,1 specify a two-year calibration interval, most call for one-year calibrations, and most antenna manufacturers recommend annual calibrations.

The first thing a cal lab will do is inspect the antenna for physical damage. If the antenna looks to be in good shape, the engineers will make a series of measurements to determine the antenna factor, or the ratio of the field strength to the measured voltage.

ANSI/IEEE C63.5-19982 specifies several different methods for determining the antenna factor, including the standard site (i.e. three-antenna), reference antenna, equivalent capacitance substitution, standard transmitting loop, standard antenna, and standard field methods. The most commonly used method is the reference antenna method. Using this method, the lab uses an antenna with known characteristics to generate a field of known strength and then measures the field with the antenna under test.

Since most antennas cover a wide frequency range, the lab will measure the antenna factor at many different frequencies. Normally, they’ll measure the antenna factor at 30 to 40 points across the frequency range of the antenna. If desired, you can specify how many frequencies and at which frequencies they measure.

Of course, you may need to have your antennas calibrated more frequently than every year. How can you tell if your antenna is out of calibration? Bethel suggests that before each use you check the antenna for obvious physical damage. He also says that sudden changes in readings from an antenna or changes in the power required to reach a given field level may indicate that you may need to recalibrate your antenna.

Another way to tell if an antenna’s characteristics have changed, according to Bethel, is to make a series of VSWR (voltage standing-wave-ratio) measurements over the antenna’s frequency range when you first put the antenna into service. You then can periodically make the same measurements and compare them to the original ones. If they deviate significantly, schedule an early calibration.

Choose the Right Lab
To keep your antennas in tip-top shape, it helps to choose the right lab. Caney suggests you ask the following questions:

• How will the lab supply the calibration data? If the lab can supply the data in an electronic format, you won’t have to retype the data once you receive it.
• Does the lab also measure VSWR; and will it supply that data as well?
• Is the lab accredited by any standards body?
• Can the lab supply a loaner antenna if necessary?
• What kind of technical support does the lab provide? This may be especially important if you are just getting into EMC testing.

The good news is that most antennas now on the market are well built and offer good performance, and taking care of them is not a big chore. With the proper care and feeding, you’ll be able to make accurate measurements with your EMC antennas for a long time. (For a list of companies that manufacture antennas, see the product survey chart.) T&MW

FOOTNOTES
1. MIL-STD-461D, Requirements for the Control of Electromagnetic Interference Emissions and Susceptibility, January 1993. www.jsc.mil/jsce3/emcslsa/library.htm
2. ANSI/IEEE C63.5-1998, American National Standard for Electromagnetic Compatibility—Radiated Emission Measurements in Electromagnetic Interference (EMI) Control - Calibration of Antennas (9 kHz to 40 GHz), November 1998. American National Standards Institute, New York, NY. 212-642-4900; www.ansi.org