What is antenna factor and how is it used in EMC measurements?

The antenna factor (AF) is the calibration parameter that converts the voltage measured at the antenna's output terminals to the electric field strength at the antenna location. The relationship is: E(dBuV/m) = V(dBuV) + AF(dB/m), where E is the electric field strength in dBuV/m, V is the received voltage in dBuV, and AF is the antenna factor in dB/m. For a lossless antenna, AF can be derived from the antenna's effective area: AF = sqrt(4*pi*Z0/(lambda^2*Ae)) = (9.73/lambda) * sqrt(Z0/Rr), where Z0 is the system impedance (50 ohms), lambda is the wavelength, and Rr is the radiation resistance. For a biconical antenna: AF is relatively flat across 20-300 MHz, typically ranging from 10 dB/m at 20 MHz to 20-25 dB/m at 300 MHz. The gradually increasing AF compensates for the antenna's decreasing effective area at higher frequencies. In practice, AF is measured (not calculated) using standardized calibration procedures: (1) Three-antenna method (ANSI C63.5): three antennas are measured pairwise on a ground plane site, and the individual AF values are derived from the three insertion loss measurements using three simultaneous equations. (2) Standard site method (SSM): uses a reference antenna with known AF and a ground plane, measuring the unknown antenna by comparison. Calibration uncertainty: +/-1-2 dB for laboratory calibrations, +/-2-3 dB for field calibrations. Cable loss must be measured and added to the antenna factor or subtracted from the measurement to get the correct field strength at the antenna reference point.

How is a biconical antenna used in CISPR 16 radiated emissions testing?

CISPR 16 (and its derivatives EN 55032/55035 for IT/multimedia, EN 55011 for industrial) specifies the biconical antenna as the standard receive antenna for radiated emissions measurements from 30 MHz to 300 MHz (below 30 MHz uses loop antennas or rod antennas). The test setup per CISPR 16-2-3: (1) The equipment under test (EUT) is placed on a non-conductive table (80 cm height) at a distance of 3, 5, or 10 meters from the receive antenna (depending on the standard and emission class). (2) The biconical antenna is mounted on a mast that scans from 1.0 to 4.0 meters height (for 3-10 m distance) to find the maximum field strength at each frequency. The height scan accounts for ground plane reflections that create constructive and destructive interference patterns. (3) The receiver (EMI receiver per CISPR 16-1-1) scans the frequency range with the appropriate detector (quasi-peak, average, or peak depending on the standard) and resolution bandwidth (120 kHz for 30-1000 MHz). (4) The EUT is rotated 360 degrees (or the antenna is moved around the EUT in a fully anechoic room, FAR) to find the maximum emission direction. (5) The measured field strength is: E(dBuV/m) = V(dBuV) + AF(dB/m) + Cable_Loss(dB). This is compared to the emission limit (e.g., Class B residential: 30 dBuV/m at 10m from 30-230 MHz, 37 dBuV/m from 230-1000 MHz per EN 55032). Measurement uncertainty budget: antenna factor (+/-2 dB), cable loss (+/-0.5 dB), receiver accuracy (+/-1 dB), site imperfections (+/-1-3 dB). Total expanded uncertainty: typically 4-6 dB at 95% confidence.

What are the alternatives and companion antennas for EMC testing?

The biconical antenna covers 20-300 MHz, but full EMC compliance testing requires coverage from 9 kHz to 40 GHz depending on the product standard. Companion and alternative antennas include: Log-Periodic Dipole Array (LPDA): covers 200 MHz to 1 GHz (or higher). Combined with the biconical, forms the 'bilog' antenna that covers 20 MHz to 3 GHz with a single feed point (electronic or mechanical switching between biconical and LPDA elements). The bilog is the most common single antenna for pre-compliance testing. Horn antennas: cover 1-40 GHz for standards requiring measurements above 1 GHz (e.g., EN 55032 for computing equipment above 1 GHz, automotive EMC per CISPR 25 to 6 GHz). Standard gain horns provide known gain with low uncertainty, enabling accurate field strength measurements. Ridged waveguide horn: covers 1-18 GHz with a single antenna (dual-ridge horn), providing broader bandwidth than standard horns. Used in fully anechoic rooms (FARs) for automotive and military EMC testing. Rod antenna (monopole): covers 9 kHz to 30 MHz for conducted emissions and low-frequency radiated measurements per CISPR 16. Typically a 1-meter active monopole antenna. Loop antenna: covers 9 kHz to 30 MHz for magnetic field measurements per MIL-STD-461 RE101. Pre-compliance vs. full compliance: pre-compliance testing can use uncalibrated or simplified antennas for screening, but final compliance reports submitted to regulatory bodies (FCC, CE) must use calibrated antennas with documented uncertainty budgets per CISPR 16-1-4 and ISO/IEC 17025 accredited calibration laboratories.

EMC / Test Antenna

Biconical Antenna EMC

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Calibrated broadband receive antenna for radiated emissions measurements covering 20–300 MHz per CISPR 16-1-4, ANSI C63.5, and MIL-STD-461. Antenna factor AF = 20 log(E/V) converts received voltage to field strength (typical 10–25 dB/m). Height scan 1–4 m, 3/5/10 m distance. Combined with LPDA as "bilog" for 20 MHz–6 GHz full EMC coverage.

Band: 20–300 MHz

AF: 10–25 dB/m

Cal. uncert.: ±1–2 dB

Understanding Biconical EMC Antennas

In radiated emissions testing, the biconical antenna serves as the calibrated field sensor that converts received voltage to electric field strength. The antenna factor (AF) is the key calibration parameter: E(dBμV/m) = V(dBμV) + AF(dB/m). Accurate AF calibration (per ANSI C63.5 three-antenna method) is essential, as it directly determines whether a product passes or fails its emission limits.

CISPR 16 specifies the biconical for the critical 30–300 MHz band where most digital emissions concentrate (clock harmonics, switching regulator emissions). The test procedure includes a 1–4 m height scan to account for ground reflection interference patterns, and 360° EUT rotation to find the maximum emission direction.

Antenna Factor Calculation

Field Strength from Measurement:
E(dBμV/m) = V(dBμV) + AF(dB/m) + L_cable(dB)

AF from Antenna Parameters:
AF = 9.73/λ × √(Z₀/R_r)
Z₀ = 50 Ω, R_r = radiation resistance

Typical AF Range:
20 MHz: AF ≈ 10 dB/m
100 MHz: AF ≈ 15 dB/m
300 MHz: AF ≈ 22 dB/m

EMC Test Setup (CISPR 16)

Parameter	Specification	Purpose
Frequency	30–300 MHz (biconical)	Digital emissions band
Distance	3, 5, or 10 m	Per product standard
Height scan	1.0–4.0 m	Find ground reflection max
EUT rotation	360°	Find max emission angle
Detector	QP, average, peak	Per standard requirement
RBW	120 kHz	CISPR Band C/D

EMC Antenna Coverage Matrix

Antenna	Frequency	Standard	Combined As
Rod (monopole)	9 kHz–30 MHz	CISPR 16	Standalone
Biconical	20–300 MHz	CISPR 16	Bilog (low)
LPDA	200 MHz–3 GHz	CISPR 16	Bilog (high)
Horn	1–40 GHz	CISPR 25, 461	Standalone

Common Questions

Frequently Asked Questions

What is antenna factor?

AF converts received voltage to field strength: E(dBμV/m) = V(dBμV) + AF(dB/m) + cable loss. Calibrated per ANSI C63.5 three-antenna method. Typical biconical: 10–25 dB/m across 20–300 MHz. Uncertainty: ±1–2 dB (lab), ±2–3 dB (field).

CISPR 16 test procedure?

EUT on 80 cm table, biconical at 3/5/10 m, height scan 1–4 m, 360° rotation, EMI receiver with QP/average detector, 120 kHz RBW. Compare E(dBμV/m) to emission limit (Class B: 30 dBμV/m at 10 m, 30–230 MHz). Total measurement uncertainty: 4–6 dB.

Full EMC coverage?

Biconical (20–300 MHz) + LPDA (200 MHz–3 GHz) = bilog for pre-compliance. Horn for 1–40 GHz. Rod for 9 kHz–30 MHz. Full compliance requires ISO 17025 calibrated antennas with documented uncertainty budgets.

Related Terms

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