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# What exactly is the "Antenna Factor"?

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Antenna Factor Calculations and Deviations
In recent years, the use of a term call "Antenna Factor" in EMC and spectrum pollution work has become very important. There has been a great need for measuring field intensity and/or power density as accurately and conveniently as possible.

Classically in RFI/EMI measurements, the use of antenna factors is very common. These antenna factors were, by definition, the conversion of two-terminal receiver voltages to field intensity. This is a very convenient term, particularly when expressed in decibels, and the receiver measures in dB above one millivolt. Historically, antenna factors were supplied by equipment manufactures, or in some cases, designated in military specifications. In many cases, these factors were used incorrectly because the user did not understand fully the mathematical significance of the antenna factor. It is the purpose of this paper to establish the relationship of antenna factor and antenna gain.

Antenna gain is a common term utilized by antenna design engineers as well as communication electronic system designers. The gain is the ratio of the power density produced by antenna, at a certain range in a certain direction, to the average power density at that range.

The recent deployment of computer controlled receiving systems with analysis capable capabilities has added a new need for examining the antenna factor. The desired data output from such systems is usually a field intensity expressed in volts per meter as a function of frequency. From the standpoint of strictly application of antenna factors it would seem an easy test to store the antenna factor as a “look up table” in the computers core memory; however as we shall see, it may not necessaryily be the most cost effective or the best utilization of computer memory.

ANTENNA FACTOR CALCULATIONS
Antenna Factor AF is used in EMC/EMI testing to convert received voltage VR to the radiated field intensity ED. Depending on the type of test that antenna may include factors such as:

• Loss due to mismatch of impedance between the antenna output connector and transmission line.
• Loss due to attenuation of the transmission line.
• Loss due to VSWR at the antenna and/or the receiver.
• Gain due to a preamplifiers located at the antenna.
• Loss due to the mismatch of impedance at the input of the receiver.

Although all of the above factors are very real losses of received signal, they may be treated separately and independently. They may be calculated, measured, or obtained from published charts. If any of the above factors are included in an antenna factor, it should be stated so as to avoid misinterpretation.

Let us assume a condition where the antenna is so designed that the above five factors are negligible, that is, well-matched antenna output/cable/receiver, short low loss cable, no preamplifier gain, low VSWR.

APPLICATIONS TO COMPUTER PROCESSING
If you want to calculate and display field intensity from a real-time data collection system, it is important to have the value for antenna factor available in the computer for field intensity calculations. Since the receivers in the system are controlled by the computer, the frequency is always known.

Antenna Factor and Gain Calculations

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A.H. Systems has been established since 1974 and manufactures a complete line of affordable, reliable, EMI test equipment. Our individually calibrated EMI Test Antennas, Preamplifiers, Current Probes and Low-Loss Cables satisfy many test standards including CISPR, MIL-STD, FCC, EN, VDE, IEC and SAE. With a wide variety of mounting configurations, we can also offer tripods and accessories that compliment other EMI testing equipment used to complete your testing requirements. We are also committed to providing all of our clients with no cost prompt and professional technical support. Manufacturing high quality products at competitive prices with immediate shipment plus prompt technical support are our goals to improve the quality of your testing requirements.