Why Magnetic Field Measurements at Low Frequencies?

At frequencies below 30 MHz, wavelengths are very long (>10 meters), placing typical EMC measurements in the near-field region where electric (E) and magnetic (H) fields are not yet coupled into plane waves. In this region, inductive sources (transformers, motors, power supplies) create predominantly magnetic fields, while capacitive sources (high-impedance circuits) create electric fields. Loop antennas measure H-field, monopole antennas measure E-field. ANSI C63.4 and CISPR standards specify both measurements to capture all emission mechanisms.

⭕ AL-130R: 9 kHz - 30 MHz Active Loop (19" x 19")

Applications & Use Cases:

• ANSI C63.4 Commercial Testing (9 kHz - 30 MHz): Primary antenna for FCC Part 15 low-frequency radiated emissions. Measures magnetic field emissions from switching power supplies, DC-DC converters, motors, transformers, and inductive loads in commercial products. Required measurement per ANSI C63.4 Section 7 in addition to electric field measurements with monopole antenna.
• CISPR 11/22/32 Compliance: ISM equipment (CISPR 11), ITE (CISPR 22), and multimedia equipment (CISPR 32) magnetic field emissions testing for CE marking. European standards specify H-field measurements 9 kHz - 30 MHz to capture emissions from switched-mode power supplies increasingly used in modern electronics.
• Power Supply Emissions: Switching power supplies, AC-DC converters, and DC-DC converters create strong magnetic fields from inductor currents and high di/dt switching. Loop antenna positioned near power supply or PCB traces identifies problem frequencies and emission sources during pre-compliance debugging.
• Motor/Transformer Emissions: Electric motors, solenoids, relays, and transformers generate magnetic fields from current flow through windings. Loop measurements capture low-frequency emissions from these inductive devices that may not be detected by E-field antennas.
• Medical Device Testing (IEC 60601-1-2): Magnetic field emissions from medical equipment containing motors, transformers, and power conversion circuits. Critical for equipment used in magnetically-sensitive environments (MRI facilities, cardiac monitoring).
• Automotive Pre-Compliance: Early-stage emissions screening of automotive electronic modules (ECUs, motor controllers, inverters) during development. Loop antenna near harness or component identifies magnetic field coupling to vehicle wiring before formal CISPR 25 testing.
• Near-Field Scanning: Positioning loop antenna close to PCB traces, components, or cables during troubleshooting identifies specific emission sources. Small loop size (19") enables localized measurements for root cause analysis.
• Ambient RF Surveys: Measuring background magnetic field levels at test sites or facilities. Useful for site selection, interference troubleshooting, and baseline measurements before equipment installation.

H-Field to E-Field Conversion:

Battery Operation & Remote Monitoring:

• Battery Life: 6V NiMH rechargeable battery provides 10-12 hours continuous operation. Sufficient for full-day testing without recharge. Spare batteries recommended for multi-day site work.
• Fiber Optic Remote Interface (RAI-100): Optional fiber optic link enables monitoring of battery status and preamplifier saturation from control room outside chamber. Critical for unattended automated testing or when operator cannot physically access antenna during test.
• Saturation Indicator: Preamplifier saturation LED warns when input signal exceeds preamp linear range. Indicates need to move antenna farther from EUT or reduce EUT power. Saturated measurements are invalid.
• Battery Status: Low battery indicator provides advance warning before shutdown. Prevents test interruption due to unexpected battery depletion.

Setup & Usage:

1. Grounding: Connect counterpoise (ground plane) to facility ground per ANSI C63.4. Counterpoise provides reference potential for loop measurements and shields from ground currents.
2. Positioning: Place loop antenna at specified measurement distance from EUT (typically 1m or 3m per standard). Orient loop plane perpendicular to expected magnetic field direction (loop axis points toward field source for maximum response).
3. Battery Installation: Install fully-charged 6V NiMH battery. Verify battery indicator shows good status. Turn on preamplifier power switch.
4. Cable Connection: Connect BNC output to EMI receiver or spectrum analyzer using quality coaxial cable. Route cable to minimize coupling to ambient fields.
5. Calibration: Load antenna factor table (supplied with antenna) into receiver. Antenna factor converts received voltage (dBµV) to magnetic field strength (dBµA/m): H = V + AF.
6. Measurement: Perform frequency sweep 9 kHz - 30 MHz. Receiver applies antenna factor correction automatically if properly configured. Compare results to regulatory limits.
7. Saturation Check: Monitor saturation indicator during measurement. If saturation occurs, increase antenna-EUT distance or reduce EUT power until saturation clears.

Loop Antenna Physics

A loop antenna operates on Faraday's Law of electromagnetic induction: a time-varying magnetic field passing through the loop induces voltage proportional to the rate of change of magnetic flux. For a single-turn circular loop:

For sinusoidal fields at frequency f, the induced voltage is proportional to frequency and loop area. This frequency-dependent response means larger loops are more sensitive at low frequencies, while smaller loops work better at high frequencies. The AL-130R's 19" x 19" size optimizes sensitivity across 9 kHz - 30 MHz range.

Shielded Loop Design

The AL-130R uses a shielded loop construction where the conductive loop is enclosed in a grounded electrostatic shield with a small gap. This design provides several advantages:

• E-Field Rejection: Grounded shield blocks electric fields, ensuring loop responds only to magnetic fields. Critical for accurate H-field measurements in environments with strong E-fields.
• Common-Mode Rejection: Shield reduces common-mode currents on feed cable that could corrupt measurements.
• Improved Directionality: Shield gap defines loop's magnetic axis, improving directional null depth.
• Reduced Ambient Pickup: Shield reduces coupling to nearby conductors and cables.

The shield gap (typically 1-2 cm) prevents the shield from forming a short-circuited turn that would block magnetic flux. The gap must be small enough to provide good E-field shielding but large enough to allow H-field penetration.

Active Electronics Function

Preamplifier Role:

• Amplifies weak loop signal (typically µV levels) to overcome cable losses and receiver noise floor
• Typical preamp gain: 20-40 dB (frequency dependent)
• Low noise figure (<3 dB) preserves signal-to-noise ratio
• High input impedance (MΩ) matches loop's high impedance without loading

Impedance Matching:

• Loop antenna presents high impedance (thousands of ohms at low frequencies)
• EMI receivers expect 50Ω source impedance
• Active matching network transforms high impedance to 50Ω output
• Enables use of standard 50Ω coaxial cables and receivers without mismatch losses

Complete Low-Frequency Coverage:

For comprehensive EMC testing per ANSI C63.4 and CISPR, both loop (H-field) and monopole (E-field) measurements are required 9 kHz - 30 MHz. Many commercial products exhibit both magnetic and electric field emissions depending on circuit topology and layout. Performing both measurements ensures all emission mechanisms are captured and compared against limits.

Battery Care

• Charging: Use supplied charger or compatible 6V NiMH charger. Charge fully before first use and after each test session. Typical charge time: 4-6 hours.
• Storage: For long-term storage (>1 month), remove battery from antenna and store at 40-60% charge in cool, dry location. Fully discharge/recharge cycle every 3 months maintains battery health.
• Replacement: Battery capacity degrades over time. If runtime drops below 6 hours, replace battery. Use only specified 6V NiMH type to avoid damage to preamp circuitry.
• Field Spares: Carry spare charged battery for multi-day testing or when recharging not practical.

Common Issues & Solutions

• Issue: No signal / very low readings
  Solution: Check battery status - replace if low. Verify preamplifier power switch is ON. Check cable connections (BNC output). Verify receiver antenna factor calibration is loaded. Test with known signal source.
• Issue: Saturation indicator constantly lit
  Solution: Antenna too close to EUT or EUT emissions too strong. Increase antenna-EUT distance. Reduce EUT power if possible. Verify saturation is not false alarm from nearby RF source (cell tower, broadcast station).
• Issue: Intermittent readings / unstable measurements
  Solution: Check BNC connector for loose connection or corrosion. Inspect cable for damage. Verify counterpoise grounding is secure. Check for mechanical vibration affecting loop position.
• Issue: Readings don't match expected levels
  Solution: Verify antenna factor calibration file matches antenna serial number. Check receiver settings (resolution bandwidth, detector, units). Confirm loop orientation (plane perpendicular to field). Compare to monopole measurements to verify field polarization assumption.

Annual Calibration

Active loop antennas should be recalibrated annually or after any event that could affect performance:

• Physical damage to loop or housing
• Exposure to excessive fields (saturation for extended periods)
• Suspected preamplifier malfunction
• Quality system requirements for annual calibration
• Accreditation body requires traceable calibration

Calibration verifies antenna factor vs frequency, preamplifier gain, and saturation threshold. ISO 17025 accredited calibration available upon request.