How to Figure EMC Antenna Factors

March 1, 1995

An EMC antenna is not a single entity but a set of specialized antennas with two primary functions:

o Receiving electromagnetic energy to produce data from an equipment under test (EUT) for comparison to specified limit values.

o Transmitting electromagnetic energy to determine if it will cause the EUT to malfunction, when the level striking the EUT is set as required by the test specification.

Both of these functions are critical, and ingenious design often allows a single antenna to be used for both purposes.

EMC Antenna vs Communications Antenna

Though often used for communications, EMC antennas are not designed for this purpose. Antennas designed for communications purposes will typically have higher gain and smaller bandwidth values than EMC antennas.

EMC antennas are designed to have the maximum values of bandwidth with a proportionate reduction in gain. In fact, EMC antennas may sacrifice other operating parameters, such as efficiency and good impedance matching, to assure usability over a wide frequency spectrum.

Common antenna design parameters include voltage standing wave ratio (VSWR) and radiation efficiency. For communications antennas, the parameters are much easier to achieve than for EMC antennas because the high efficiency and low VSWR are much easier to optimize over smaller portions of the frequency spectrum. The EMC antenna, with the emphasis placed on wide bandwidth, is a unique blend of parameters that allows EMC testing to be conducted with minimal time-consuming changes to the setup.

EMC Antenna Parameters

The main parameters of an EMC antenna are the antenna factors. There are two antenna factors of interest: the receive antenna factor (AF) and the transmit antenna factor (TAF).

For radiated emissions testing, the antenna factor is a measure of the relationship between the incident electromagnetic field (in uV/meter) and the voltage produced at the output port of the antenna (typically in dBuV). The actual relationship is given by the definition of the antenna factor:2

VV = EV/meter x AFmeter

or, in dB terms:

VdBuV = EdBuV/meter + AFdB meter

In normal usage, the voltage is the measured value and the AF is known from antenna calibration. Thus, the typical computation is:

EdBuV/meter = VdBuV + AFdB meter

producing a measured value for the incident electromagnetic field. The antenna factor is normally referred to as a calibration factor, since the units are changed, rather than being a correction factor where there is no change of units.

The TAF relates the value of the strength of the electromagnetic field produced, at a selected distance from the field-generating antenna, in terms of the RF voltage input to the antenna. Here the defining equation is:3

EV/meter = VRF input x TAFmeter-1

This relationship can be expressed in decibel terms:

EdBV/meter = VRF dBV + TAFdB meter-1

Here the TAF is furnished as a calibrated value, or can be calculated from the AF by:

TAFdB meter-1 = 20log(fMHz) – AFdB meter – 20log(rmeters) – 32.0

where: f = frequency in MHz

r = distance from the antenna where the field strength is desired, in meters

The AF and the TAF are not reciprocal; however, one can be calculated from the other. The TAF, when calculated from the AF, is for conditions that apply to the measurement of the free-space AF. These are typically measured on an open area test site (OATS) using the height search procedure as required in ANSI C63.5.4 Under these conditions, the calculation of the TAF from the AF produces values that can be used to estimate the electric field strength generated in a semianechoic chamber, an approximation of the OATS.

The EMC antenna, when used for a radiated emission measurement, is set up on a nonconducting tripod at a distance of 3 or 10 meters from the EUT, depending on the test standard requirements. The antenna is calibrated for these distances, and the antenna factor data of these distances is typically supplied with the antenna.

When the measurement is made for a FCC Class B requirement, the specified distance is 3 meters; therefore, the antenna is set 3 meters from the EUT. The frequency spectrum is scanned for signals radiating from the EUT and the voltage levels, after rotating the EUT 360o and searching for the maximum received value between 1 and 4 meters (in dBuV), are recorded.

Using this data, the E-field strength can be computed. You also must correct for the loss of the coaxial cable(s) used to connect the antenna and the receiving device and the gain of the preamplifier (if used). This calculation is:

EdBuV/meter = VdBuV + CLdB – GdB + AFdB meter-1

where: CL(dB) = measured cable loss at the frequency of the measurement, in dB

G(dB) = measured gain of the preamplifier at the frequency of the measurement, in dB.

As an example of this computation:

Let VdBuV = 47.1

CLdB = 2.6

GdB = -25.0

AFdB/meter = 12.2

Then EdBuV/meter = 36.9

The use of the TAF is similar, with the proviso that the computations are used to determine the value of the electric field strength at a specific distance from the antenna. In its pure form, the TAF requires these parameters: frequency, distance from the generating antenna and a level of the E-field required.

The frequency and distance from the generating antenna specify the value of the TAF to be applied. Given the TAF, the input to the antenna can be computed to determine the input RF voltage to the antenna.

TAF can be presented in terms of the input wattage required to develop an electric field strength at a specific distance. For IEC 801-3, the standard distance is 3 meters. Thus if the TAF for a specific antenna is -10 dBW at 3 meters, the equivalent statement is that the antenna will produce an electric field of 1 V/meter, at 3 meters, with an input of -10 dBm or 100 mW.

If an E-field strength of 10 V/meter is required, the input level should be increased by the appropriate number of dB in E-field strength, namely by

20log
= 20 dB

indicating that an input of +10 dBm (or 10 W) is required to generate the required E-field strength at that particular frequency.

As was the case with the AF, cable loss, amplifier gain and any other modifications to the level of the signal applied to the antenna input port must be made. Remember that the TAF, as well as the AF, can vary widely over the frequency range of the antenna being used.

Summary

The use of specialized antennas for EMC testing is not particularly complex, but the proper application of the information characterizing these antennas is necessary to achieve the desired results.

References

1. Antennas and Accessories for EMC Testing, The Electro-Mechanics Co., 1992, p. 13, 34.

2. Bronaugh, E.L., and Lambdin, W.S., Electromagnetic Interference Test Methodology, Interference Control Technologies, Inc., 1988, p. 2.57.

3. Osburn, J.D.M., Derivation of the Transmit Antenna Factor,

February 1995