Com-Power Corporation

Whitepapers from Com-Power Corporation

Applications of HF Current Probes

This technical article explores advanced uses of high-frequency RF current probes beyond basic current measurement, using the Com-Power CLCE-400 as a reference tool throughout. It opens by explaining transfer impedance — the ratio of probe output voltage to current in the conductor under test — and how to use it to calculate common mode (CM) current levels. Three main applications are then presented. The first demonstrates how to predict radiated emissions pass/fail outcomes by measuring CM currents on cables and calculating the expected electric field using a standard formula. The second shows how to isolate common mode from differential mode conducted emissions using a current probe, enabling more targeted filter design decisions. The third describes a three-step bench-level troubleshooting process for radiated emissions: near-field probing to identify noise sources, current probe measurements on cables to quantify and locate dominant harmonics, and close-spaced antenna measurements to confirm actual radiation. The article concludes with five practical troubleshooting tips covering probe positioning techniques, dual-probe comparisons, and coupling type identification.

Near Field ProbesCurrent Probes

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Changes to Measurement Arrangement for Tests Performed With Rod (Monopole) Antenna

This technical note from Com-Power Corporation describes the significant change made to the monopole antenna test setup between MIL-STD-461E and MIL-STD-461F, which was released in December 2007. Under previous versions of the standard, the 1.04-meter rod antenna counterpoise was positioned at the same height as the EUT ground plane, with a bonding strap of sheet metal or aluminum foil bridging the gap between the two. In MIL-STD-461F, this arrangement was revised based on experimental findings that the bonding strap was distorting results — enhancing readings at some frequencies while suppressing them at others. In the new arrangement, the counterpoise height is decoupled from the ground plane height, with the vertical center of the rod element now fixed at 120 cm above the shielded room floor. The bonding strap is eliminated entirely, and the only ground connection is made through the coaxial antenna output cable shield, which is bonded to the floor directly beneath the antenna. A ferrite with 20 to 30 ohms of impedance is also required on the output cable between the antenna port and the floor grounding point. The standard additionally requires that antennas with isolated coaxial bulkhead connectors be modified to allow electrical contact between the connector shell and the matching network enclosure.

MIL-STD-461Antennas

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Comb Generator as a Reference Source

This application note from Com-Power Corporation explains what a comb generator is and how it is used as a reference source in EMC testing environments. A comb generator produces a repetitive pulse with very fast rise and fall times, generating a series of harmonically related frequency components that, when viewed on a spectrum analyzer, resemble the teeth of a comb. Its compact size, battery power, instant on/off operation, and simultaneous multi-frequency output make it far more practical for lab use than a conventional RF signal generator. The primary application discussed is EMC test site validation, where the comb generator is used before each test session to verify that all components of the measurement chain — antenna, cables, connectors, preamplifier, and receiver — are working correctly. Discrepancies from previously recorded reference readings indicate a problem that should be investigated before data is collected. The document outlines six desirable characteristics for a radiated reference source, covering radiation pattern (ideally omnidirectional), battery-only operation during use, temperature stability, appropriate output power level relative to specification limits, harmonic step size to control energy density, and usable frequency range. For conducted reference sources, ease of connection to the LISN and stable output are highlighted. The note also covers how a comb generator can be used to evaluate the quality of anechoic chamber absorber material by comparing readings taken at two slightly different positions within the chamber.

Comb Generator

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Considerations when Selecting an Antenna for an RF Immunity Test System

This brief technical paper from Com-Power Corporation outlines the key factors beyond frequency range that engineers must evaluate when choosing an antenna for RF immunity testing. Test distance is addressed first, showing mathematically that required amplifier power scales with the square of the test distance, meaning a change from one to three meters demands nine times the power. VSWR is then explained as a measure of impedance mismatch between the antenna and feed cable, with high VSWR causing significant power reflection and the need for increased drive power to achieve the required field strength. Physical antenna size is flagged as a concern in chamber environments, where long antenna elements can interact with walls or the ceiling and degrade field uniformity. Antenna gain over isotropic (expressed in dBi) is presented as a key figure of merit that directly reduces the power needed to generate a target field strength, though excessively narrow beam widths can complicate uniform illumination of the calibration plane. Return loss, the dB expression of the reflection coefficient, is explained as a complementary metric to VSWR where higher values indicate better impedance matching.

AntennasImmunity TestingRadiated Immunity

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Does my Comb Generator Require Calibration?

This brief technical note from Com-Power Corporation addresses the common question of whether comb generators used in EMC labs need to be formally calibrated. The answer is grounded in how the device is actually used. Since the comb generator's primary role is as a reference source for periodic site verification checks rather than as a precision measurement instrument, there is no external standard against which it can be formally calibrated and no defined pass/fail criteria for its output. Any third-party calibration of a comb generator would amount to nothing more than a characterization of its output levels at a point in time and at a different test site, which would hold limited relevance because the test environment itself influences the measured values. The document explains the correct process: initial reference measurements are taken when the measurement system is already confirmed to be in good order, ideally immediately after all individual components have been calibrated and site attenuation verified. These become the baseline. Before each subsequent test, the same measurements are repeated and compared to the baseline within a tolerance of typically ±3 dB. This reference measurement process is itself considered the effective calibration of the comb generator. ISO 17025 accredited laboratories are also noted to be permitted to calibrate their own equipment, making external calibration of a comb generator unnecessary in that context as well.

Comb Generator

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Pre Compliance Emissions Testing: Accounting for Ambient Noise

This technical note from Com-Power Corporation addresses one of the central challenges of pre-compliance radiated emissions testing outside an anechoic chamber: the presence of ambient RF signals that can obscure or mask emissions from the equipment under test. Three practical techniques for improving signal-to-noise ratio are presented and illustrated with spectrum analyzer screenshots. The first involves narrowing the resolution bandwidth (RBW) of the spectrum analyzer, which separates closely spaced EUT emissions and ambient signals that appear merged at wider bandwidths, though at the cost of longer sweep times. The second technique is rotating the test antenna polarization; since ambient noise sources typically have a fixed polarization, rotating the antenna 90 degrees relative to the ambient source can substantially suppress it while the EUT's own emissions remain visible. The third approach is reducing the measurement distance: because EUT emissions diminish with distance while ambient signals from far-off sources remain roughly constant, moving the antenna closer to the EUT significantly improves the EUT signal above the noise floor, with the document demonstrating improvements from 3.5 dB at 10 meters to over 20 dB at 1 meter.

EMC Pre ComplianceSpectrum AnalyzerAntennas

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Reducing Scepticism in Emissions Measurements

This article from Com-Power argues that deeper familiarity with RF emissions test system design is essential for building confidence in lab pass/fail decisions. It walks through the core field strength equation (E = V + AF + C – G) and explains why none of the system components have flat frequency responses, requiring correction data to be loaded at calibration and verified with comb generators before each test run. The article then applies the Friis noise equation to demonstrate why a pre-amplifier must be placed before the long interconnecting cable rather than after it — a counterintuitive but mathematically clear conclusion that yields a 10 dB improvement in signal-to-noise ratio. Practical solutions such as integrated active horn antennas are presented. The conclusion emphasizes that an engineer who understands the system's design limitations will inspire far greater confidence in test results than one who operates the equipment blindly.

EMI / EMC Theory

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Selecting the Right EMI Antenna

This application note from Com-Power provides a practical guide to choosing the correct antenna for radiated EMI emissions and immunity testing. It covers the major antenna types used in EMC work — monopole, dipole, biconical, log-periodic, biconical-log (CombiLog), horn, and loop — describing the frequency range, physical characteristics, and typical application context for each. The document explains antenna factor, how it varies with frequency, and how it fits into the E-field calculation alongside coax loss, preamplifier gain, and attenuator loss. It includes worked examples for calculating compliance limits at non-standard test distances and covers typical test setups for consumer, commercial, automotive, and military/aerospace testing under standards such as CISPR 11/32, CISPR 25, and MIL-STD-461. Brief guidance on antenna calibration methods is provided at the close.

Antennas

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Use of Antenna Factors in EMI Measurements

This Com-Power application note (AN-106) explains the concept of antenna factor as it is used in EMI radiated electric field measurements. Antenna factor is defined as the ratio of the incident electromagnetic field to the voltage at the antenna connector, and in logarithmic terms it can simply be added to the receiver voltage reading to obtain the E-field in dBµV/m. The note walks through a full worked example incorporating preamplifier gain and cable loss into the calculation and presents a sample data table formatted as it would appear on an FCC Part 15 radiated emissions report. It also discusses the key limitation of the antenna factor concept — the assumption of a uniform incident field — and describes both the two-antenna and three-antenna calibration methods, noting that antenna factors can vary with calibration distance and that this must be accounted for to minimize measurement error.

AntennasAntenna Factor

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Using the RF Current Probe

This application note introduces the RF current probe as one of the most essential tools in an EMC engineer's troubleshooting kit, explaining both the theory behind it and how to use it effectively. The document begins by explaining how common mode currents arise from finite impedance in circuit board ground return planes — a phenomenon sometimes called ground bounce — and why these currents, flowing in the same direction on signal and return conductors, are the primary cause of radiated emissions failures. The operating principle of the current probe is described as a clamp-on RF current transformer where the conductor under test forms the primary winding, allowing non-intrusive high-frequency measurement. Full specifications for the Com-Power CLCE-400 model are provided, including its 10 kHz to 400 MHz frequency range and 7 ohm transfer impedance. The transfer impedance concept is explained with equations for converting probe voltage readings into actual current values. Practical measurement technique is covered, including the value of sliding the probe along cables to account for standing wave resonance. The note also covers probe calibration procedures using a dedicated calibration fixture and network analyzer. The document emphasizes that all of this troubleshooting can be performed at the designer's workbench, avoiding costly time at a third-party test facility.

Current Probes

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What is Antenna Factor?

This Com-Power application note (AN-107) offers a conceptual and technical exploration of antenna factor for EMC engineers. It defines antenna factor as the ratio of the electric field strength surrounding an antenna to the voltage induced at its output terminals, expressed in dB microvolts per meter. The document explains why the concept is so widely used in EMI testing — its linearity at any given frequency makes it a simple additive correction — while also laying out the practical limitations engineers must keep in mind. These include the assumption of a uniform field (rarely met in practice), the dependence of antenna factor on calibration distance, the importance of careful handling and regular calibration verification, the need to ensure active antennas remain in their linear range, and the impact of impedance mismatch (VSWR) on voltage readings. The note closes with the mathematical derivation linking antenna gain to antenna factor, yielding the expression AF = 9.7 / (λ√G).

AntennasAntenna Factor

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Why So Many Different Types of LISNs?

This application note from Com-Power Corporation explains the purpose of the Line Impedance Stabilization Network (LISN) and why so many variants exist. The LISN performs four core functions during conducted emissions testing: it provides a stable, normalized impedance on the power line so that measurements are consistent and repeatable regardless of the facility's wiring; it prevents external RF noise from the mains from coupling into the measurement; it allows clean power to pass through to the equipment under test; and it safely couples the low-level RF noise from the EUT to the 50-ohm input of the spectrum analyzer or EMI receiver while blocking the higher mains voltage. The document then explains why a single LISN design cannot serve all applications. Differences in test frequency range require different inductor values and filter topologies, with the standard 50 µH design suited to FCC and CISPR testing from 150 kHz to 30 MHz, while a 250 µH two-stage design is needed for testing down to 10 kHz. The 50 µH value was originally chosen to represent the inductance of approximately 50 meters of power distribution wiring on ships or cargo aircraft, while smaller platforms like fighter aircraft call for 5 µH designs. Variations in current handling requirements, operating voltage, AC versus DC operation, and the number of power phases further expand the range of LISN types needed.

LISN

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