Schwarzbeck HFRA 1356 Passive Magnetic TX Loop Antenna

The HFRA 1356 is a passive loop antenna which is matched to a resonance frequency of 13.56 MHz. The antenna is shielded against electric fields. The purpose of the antenna is to produce defined magnetic fields at 13.56 MHz efficiently, i.e. to test near field communication devices (NFC) or RFID equipment. NFC issues are described in the standards ISO 18092, ECMA 340, ETSI TS 102 190. NFC is used for data communication on distances of a few centimeter.

Signal generators are often rated with an output power level of approx. 16 dBm. This is sufficient to generate typ. 1 A/m with HFRA 1356. With 25 Watt generator power a magnetic fieldstrength of 25 A/m can be reached for short time operation.

Connect a signal generator to the “Input” port to generate a magnetic field and control the loop current at the "Monitor"-port. The current through the loop is measured as a voltage drop along the built in resistor with a 50 Ω voltmeter (i.e. a spectrum analyzer) If you use a device with an input with high impedance like some oscilloscopes or RF-voltmeters it is necessary to switch a 50 Ω load in parallel.

A measured voltage of 32 mV or 90 dBµV at the “Monitor” output equals a magnetic field strength of 1 A/m in the center of the loop.

The loop current is strictly proportional to the generated magnetic field strength. A magnetic field strength of H = 6 A/m is for instance generated if the monitored voltage is 192 mV or 105.6 dBµV.

The maximum allowed monitor voltage is 640 mV which corresponds to field strength of 20 A/m. Larger signal amplitudes will overheat the internal components of the antenna.

Do never apply the signal of the power generator to the “Monitor” output connector, otherwise the HFRA 1356 will be damaged.

The conversion between magnetic field strength and fictive electric field strength is independently of frequency 51.5 dB =20•log (377 Ω).

1 A/m ≙ 377 V/m ≙ 0 dBA/m
≙ 51.5 dBV/m ≙ 120 dBµA/m
≙ 171.5 dBµV/m