EMC Test Antennas
Accurate EMC measurements depend critically on the characteristics of the antennas used for testing. Unlike communication antennas optimized for specific frequencies and patterns, EMC test antennas must operate over broad frequency ranges while maintaining well-characterized performance. Understanding the properties, limitations, and proper use of different antenna types enables engineers to select appropriate measurement equipment and correctly interpret test results.
EMC testing spans a wide frequency range, typically from below 30 MHz to several gigahertz for radiated emissions, and requires different antenna types to cover different portions of this spectrum. Each antenna type has distinct characteristics that affect measurement accuracy, sensitivity, and applicability to specific test scenarios. Proper antenna selection, use, and calibration are essential for obtaining meaningful, repeatable, and legally defensible test results.
Biconical Antennas
Biconical antennas are the workhorse of EMC measurements in the 30 MHz to 300 MHz frequency range. Their broad bandwidth and relatively simple construction make them ideal for radiated emissions testing in this frequency band.
Physical Construction
A biconical antenna consists of two conical elements arranged tip-to-tip, creating a structure resembling a bow tie when viewed from the side. The cones are typically constructed from solid sheet metal, mesh, or a framework of rods. The size of the cones determines the low-frequency limit; larger cones extend the response to lower frequencies. Standard EMC biconical antennas have element lengths of approximately one meter, providing useful response down to about 20-30 MHz.
Electrical Characteristics
The biconical geometry provides a relatively constant input impedance over a wide frequency range, typically targeting 50 ohms for compatibility with standard test equipment. The broad cone angle maintains impedance stability, though practical antennas deviate from the ideal infinite biconical structure and exhibit some impedance variation. The antenna pattern is toroidal, with maximum response perpendicular to the antenna axis and nulls along the axis.
Antenna Factor
The antenna factor (AF) of a biconical antenna relates the received voltage at the antenna terminals to the incident electric field. Biconical antennas typically have antenna factors ranging from about 10 dB at 30 MHz to 25 dB or more at 300 MHz. The increasing antenna factor with frequency reflects the decreasing effective aperture of the antenna at higher frequencies. Accurate antenna factor calibration is essential for converting measured voltages to field strength values.
Practical Considerations
Biconical antennas are physically large and require adequate spacing from the ground plane and test equipment. Height scanning during emissions measurements accounts for ground reflection effects. The antenna's polarization is linear along the axis of the cones, requiring rotation to measure both horizontal and vertical polarization components. Biconical antennas are typically used outdoors, in semi-anechoic chambers, or in open-area test sites.
Log-Periodic Antennas
Log-periodic dipole arrays (LPDAs) extend EMC measurement capability into the VHF and UHF ranges, typically covering 200 MHz to several gigahertz. Their frequency-independent characteristics make them essential for broadband emissions testing.
Operating Principles
Log-periodic antennas achieve broadband operation through a series of dipole elements of varying length, arranged with a constant ratio between adjacent element lengths and spacings. At any given frequency, only the elements that are resonant (approximately half-wavelength) actively contribute to reception. As frequency changes, the active region shifts along the antenna structure. This design principle yields nearly constant characteristics across the operating band.
Physical Characteristics
LPDAs have a distinctive appearance with elements graduated in size from front to back. The longest elements at the rear determine the low-frequency limit, while the shortest elements at the front set the high-frequency limit. Typical EMC log-periodic antennas span 200 MHz to 1 GHz or higher. The antenna has a unidirectional pattern, with maximum response in the forward direction (toward the smaller elements) and significant front-to-back ratio.
Gain and Directivity
Log-periodic antennas provide moderate gain, typically 5-7 dBi, which improves measurement sensitivity compared to lower-gain alternatives. The directional pattern requires proper alignment toward the equipment under test. The beamwidth narrows with increasing frequency, which must be considered when scanning for emissions from extended sources. Unlike biconical antennas, LPDAs have significant gain variation with angle, making source localization possible.
Usage Guidelines
Log-periodic antennas require careful alignment during measurements to ensure maximum response toward the equipment under test. They are typically mounted on a tripod or mast with provisions for rotation between horizontal and vertical polarization. The antenna factor generally decreases with frequency (opposite to biconical behavior) due to increasing effective aperture. LPDAs are standard equipment for FCC and CISPR emissions testing above 200 MHz.
Horn Antennas
Horn antennas provide high gain and excellent directivity for EMC measurements at microwave frequencies, typically above 1 GHz. Their predictable characteristics and ease of calibration make them valuable for both emissions testing and immunity applications.
Horn Types
Several horn configurations serve EMC applications. Pyramidal horns flare in both planes and are most common for general EMC use. Sectoral horns flare in only one plane, providing different beamwidths in the E and H planes. Conical horns have circular apertures and are used with circular waveguide feeds. Double-ridged horns incorporate ridges to extend bandwidth, trading some pattern purity for broader frequency coverage.
Bandwidth Considerations
Standard waveguide horns have limited bandwidth, typically a 2:1 frequency range determined by the waveguide dimensions. Double-ridged horns overcome this limitation, achieving decade bandwidths (10:1 or greater) suitable for broadband EMC measurements. A single double-ridged horn might cover 1-18 GHz, eliminating the need to change antennas during testing. The tradeoff is typically lower gain and less ideal patterns compared to conventional horns.
Gain and Directivity
Horn antennas offer substantial gain, typically 10-20 dBi depending on size and frequency. Higher gain improves measurement sensitivity and immunity field generation efficiency but requires more precise alignment. The highly directional pattern concentrates energy in a well-defined beam, which is advantageous for immunity testing where uniform field illumination of a specific area is needed. Pattern measurements show the expected gradual rolloff from boresight with minimal sidelobes.
Immunity Testing Applications
Horn antennas are frequently used to generate fields for radiated immunity testing at microwave frequencies. The high gain allows achieving required field strengths with moderate amplifier power. The well-defined beam enables predictable field coverage of the equipment under test. Standard immunity test methods specify the relationship between horn aperture, test distance, and field uniformity requirements.
Loop Antennas
Loop antennas respond to the magnetic field component of electromagnetic waves, providing capabilities complementary to electric field antennas. They are particularly useful for near-field measurements and low-frequency EMC testing.
Magnetic Field Response
Unlike dipole-type antennas that respond primarily to electric fields, loop antennas couple to the magnetic field component. This characteristic is valuable for near-field measurements where electric and magnetic fields may not have the fixed relationship of far-field radiation. Loop probes can determine whether a source is predominantly electric or magnetic in nature, guiding appropriate mitigation strategies.
Shielded Loops
EMC measurement loops are typically shielded to reject electric field pickup, ensuring pure magnetic field response. The shield surrounds the loop conductor with a gap at one point to prevent circulating currents in the shield. This construction provides better measurement accuracy in the presence of strong electric fields. Active shielded loops incorporate a preamplifier within the loop assembly to improve sensitivity.
Near-Field Probing
Small loop antennas serve as near-field probes for diagnostic measurements. When held close to a circuit, a loop probe responds to current-carrying conductors, allowing identification of noise sources and coupling paths. The directional response of the loop (null when the loop plane is parallel to the conductor) aids in localizing specific current paths. Multiple loop orientations provide three-dimensional field mapping capability.
Low-Frequency Measurements
For EMC measurements below 30 MHz, loop antennas are standard equipment. Their response extends to very low frequencies limited primarily by amplifier noise rather than antenna physics. Military and aircraft EMC standards often require magnetic field measurements using calibrated loop antennas. The antenna factor for loops typically increases with frequency at 20 dB per decade, reflecting the frequency-dependent coupling mechanism.
Rod Antennas
Rod antennas, also called monopole or whip antennas, are simple vertically polarized antennas used for EMC measurements primarily below 30 MHz. Their straightforward construction and well-understood behavior make them reliable measurement tools.
Physical Configuration
A rod antenna consists of a vertical conductor mounted on a ground plane, forming a quarter-wave monopole structure. For EMC applications, the rod is typically one meter in length, providing a convenient reference dimension. The ground plane may be the actual earth for outdoor measurements, a metallic ground plane for controlled test setups, or part of the measurement system structure. Active rod antennas incorporate a preamplifier at the base.
Electric Field Response
Rod antennas respond to the vertical electric field component, complementing loop antenna magnetic field measurements. The combination of rod and loop antennas allows complete characterization of the electromagnetic field at low frequencies. For far-field measurements, either antenna type theoretically suffices, but both types are used for verification and in situations where near-field effects may be present.
Active Rod Antennas
Modern EMC rod antennas are typically active designs incorporating a high-impedance preamplifier. The preamplifier presents high impedance to the short rod element, maintaining sensitivity even though the rod is electrically short compared to wavelength. Active designs provide flat response over the measurement band and adequate sensitivity for emissions testing. The preamplifier requires power, typically supplied through the coaxial cable or from batteries.
Ground Plane Effects
Rod antenna performance depends significantly on the ground plane characteristics. An ideal infinite ground plane produces the textbook monopole pattern, but real ground planes are finite and imperfect. Ground conductivity, size, and proximity to other structures all affect the antenna pattern and impedance. Standardized measurement setups specify ground plane requirements to ensure repeatable results.
Hybrid Antennas
Hybrid antennas combine features of different antenna types to cover extended frequency ranges or provide enhanced capabilities. These designs reduce the number of antennas required for broadband EMC testing.
Bilog Antennas
Bilog antennas combine biconical and log-periodic elements in a single structure, covering the full 30 MHz to 1 GHz range common in commercial EMC testing. The biconical section handles lower frequencies while the log-periodic section addresses higher frequencies. A switching network or broadband design seamlessly transitions between sections. This combination eliminates antenna changes during the measurement sweep, improving efficiency and repeatability.
Combined Loop-Dipole
Some antenna designs incorporate both loop and dipole elements, providing simultaneous magnetic and electric field measurement capability. These antennas are useful for near-field characterization where both field components are of interest. The combined response helps identify whether sources are predominantly electric or magnetic in character.
Multiband Designs
Various multiband antenna designs address specific EMC applications requiring coverage of multiple frequency bands. These may include trap dipoles, multiple-element arrays, or frequency-selective structures. While not as common as single-purpose antennas, multiband designs serve niche applications where test setup constraints favor reduced antenna count.
Antenna Factors
The antenna factor is a critical parameter that relates measured voltage to incident field strength. Accurate knowledge of antenna factors across the measurement frequency range is essential for meaningful EMC measurements.
Definition and Units
Antenna factor (AF) is defined as the ratio of incident electric field strength (E) to the voltage (V) at the antenna terminals: AF = E/V. Expressed in decibels: AF(dB) = 20 log(AF). Units are typically dB/m (decibels referenced to one meter). A higher antenna factor means less voltage is produced for a given field strength. Field strength is calculated from measured voltage by adding the antenna factor: E(dBuV/m) = V(dBuV) + AF(dB/m).
Frequency Dependence
Antenna factor varies with frequency according to the antenna's electrical characteristics. For electrically small antennas, AF typically decreases with frequency at 20 dB/decade as the effective aperture increases. For resonant antennas, AF is relatively constant near resonance. Broadband antennas like biconicals and LPDAs have AF values that vary smoothly across their operating range, typically specified in calibration data at discrete frequency points.
Measurement System Integration
When making EMC measurements, the complete signal path from antenna to receiver must be considered. Cable losses between the antenna and receiver add to the effective antenna factor. System antenna factor includes these losses: AF(system) = AF(antenna) + Cable Loss. Modern EMC software automatically applies antenna factors and cable corrections, but engineers must verify that correct calibration data is used.
Calibration Traceability
Antenna factor calibration must be traceable to national or international standards for measurements to have legal standing. Calibration laboratories provide antenna factors at specified frequencies, measurement uncertainties, and traceability statements. The calibration environment (free space, above a ground plane, etc.) should match the intended measurement application. Recalibration intervals, typically annual, ensure continued accuracy.
Balun Effects
Baluns (balanced-to-unbalanced transformers) connect balanced antenna elements to unbalanced coaxial transmission lines. Balun performance significantly affects measurement accuracy, particularly at lower frequencies.
Purpose and Function
Most EMC antennas have balanced element structures (both elements at equal but opposite voltages relative to ground), while coaxial cables are unbalanced (center conductor at signal potential, shield at ground). A balun provides the interface between these systems, preventing common-mode currents on the cable shield that would distort the antenna pattern and affect measurements. The balun must maintain balance across the antenna's operating frequency range.
Balun Types
Several balun designs serve EMC antenna applications. Ferrite sleeve baluns use high-permeability cores to choke common-mode current on the cable. Transformer baluns use wound structures for impedance matching and balance. Tapered baluns provide gradual transition from unbalanced to balanced impedance. Each type has tradeoffs in bandwidth, balance quality, insertion loss, and power handling appropriate for different applications.
Impact on Measurements
Poor balun performance allows common-mode current on the feed cable, which then acts as an unintended antenna element. This modifies the antenna pattern, creates unwanted sensitivity to cable position, and introduces measurement uncertainty. Effects are most pronounced at lower frequencies where the ferrite cores may not provide adequate choking impedance. Well-designed EMC antennas include baluns appropriate for their intended frequency range.
Ferrite Loading
Additional ferrite cores on the antenna feed cable further suppress common-mode currents. These supplementary chokes are particularly important for measurements below 100 MHz where integral baluns may be inadequate. Multiple ferrites spaced along the cable provide broadband suppression. The ferrite material must be appropriate for the frequency range; different materials optimize performance at different frequencies.
Calibration Requirements
Proper calibration is the foundation of accurate EMC measurements. Calibrated antennas with traceable antenna factors are required for compliance testing and meaningful comparisons between measurements.
Calibration Methods
Standard reference antenna method compares the unknown antenna to a reference antenna of known characteristics. Three-antenna method uses three antennas in three measurement configurations to solve for each antenna's parameters without requiring a reference. Site attenuation comparison measures insertion loss between transmit and receive antennas at a calibrated site. Each method has specific requirements and achievable uncertainties documented in relevant standards.
Calibration Facilities
Antenna calibration requires controlled electromagnetic environments. Open-area test sites (OATS) meeting CISPR or ANSI specifications provide validated free-space-like conditions. Anechoic chambers eliminate reflections through absorptive treatment of walls, ceiling, and floor (or floor reflections for semi-anechoic chambers). The calibration environment must match or be correctable to the environment where the antenna will be used.
Calibration Intervals
Most accreditation bodies require annual antenna calibration for compliance testing. More frequent calibration may be necessary for antennas subject to harsh conditions, frequent transport, or critical applications. Between calibrations, antennas should be inspected for physical damage that could affect performance. Consistent measurement results using check sources or comparison measurements provide confidence in continuing calibration validity.
Calibration Documentation
Calibration certificates document the antenna factor at specified frequencies, measurement uncertainties, calibration method, equipment used, and traceability. This documentation is essential for audit purposes and legal defensibility of test results. Engineers should verify that calibration data is correctly entered into measurement software and that the calibration date is current.
Antenna Selection Guidelines
Selecting the appropriate antenna for an EMC measurement requires considering frequency range, sensitivity requirements, physical constraints, and the nature of the equipment under test.
Frequency Coverage
The antenna must cover the required frequency range with acceptable performance. For commercial EMC testing, this typically means rod and loop antennas below 30 MHz, biconicals from 30-200 MHz, log-periodic antennas from 200 MHz to 1 GHz, and horn antennas above 1 GHz. Hybrid antennas can reduce the number of required antenna changes. Gaps or overlaps in frequency coverage should be addressed in the test plan.
Sensitivity and Dynamic Range
The antenna factor directly affects measurement sensitivity. Lower antenna factors yield higher receive voltages for a given field strength, improving signal-to-noise ratio. However, very sensitive systems may experience receiver overload from strong ambient signals or high-level emissions. The measurement system must have adequate dynamic range to handle both minimum detectible signals and maximum expected levels without overload or excessive noise.
Physical Considerations
Antenna size affects minimum measurement distance, chamber requirements, and practical handling. Large antennas like biconicals require adequate space and stable mounting. Weight considerations affect tripod and mast requirements. For immunity testing, antenna power handling capability must exceed the maximum amplifier output. Environmental ratings may be necessary for outdoor use or harsh conditions.
Standards Compliance
Specific EMC standards may mandate particular antenna types or characteristics. Understanding applicable standards ensures that selected antennas meet requirements. Standards may specify antenna factor ranges, calibration methods, or construction details. Using non-compliant antennas for regulated testing can invalidate results regardless of technical adequacy.
Summary
EMC test antennas are precision instruments requiring careful selection, use, and maintenance for accurate measurements. Each antenna type has characteristics suited to particular frequency ranges and applications. Understanding antenna factors, balun effects, and calibration requirements enables engineers to obtain reliable, repeatable, and legally defensible test results. Proper antenna selection considers not just frequency coverage but also sensitivity, physical constraints, and standards requirements. The investment in quality calibrated antennas and their proper use is fundamental to meaningful EMC testing.