Schwarzbeck HFRA 5170 Transmit Loop Antenna For Calibrations

The Transmit Loop Antenna HFRA 5170 was designed to generate well defined magnetic fields with moderate fieldstrength levels (< 0.1 A/m) in the frequency range DC to 30 MHz. With slightly reduced performance the loop can be used in the frequency range up to 50 MHz. The loop, consiting of one turn, is shielded against E-fields. The nickel-plated brass housing contains a 10 Ω resistor to monitor the loop current using a high impedant voltage measuring equipment (e.g. Oscilloscope or Voltmeter).

The voltage drop at the 10 Ω resistor can be used to calculate the loop current, being directly proportional to the generated magnetic fieldstrength. A monitor output voltage of 1 V corresponds to a loop current of 100 mA. A further built in resistor is used to obtain a flat frequency response over the entire frequency range. The TX-Loop can be connected to 50 Ω signal generators or power amplifiers directly. The loop can mounted at its female large camera thread.

The generated magnetic field is proportional to the loop current. A tabular indicates the fieldstrength values in dBµA/m to be expected with the maximum permissible current of 100 mA. Additionally the relative decrease of magnetic fieldstrength referred to the center of the loop antenna can be found.

Lower magnetic fieldstrengths can be achieved by scaling the current to the respective value. With a feed current of 10 mA instead of 100 mA the tabular values decrease by a factor of 10, which corresponds to a reduction by 20 dB in logarithmic measure. All specified magnetic fieldstrengths refer to the component which is perpendicular to the loop-plane.

Application:

The female BNC-connector "Input 250 Ω"is used to feed the loop, while the other can be used to monitor the voltage with a voltmeter (high impedance). The measurement distance to apply for the calibration of RXloops depends on the RX-loop dimensions. For large loop antennas a higher calibration distance is required in order to obtain a uniform field distribution over the complete loop area. The shorter the distance, the smaller the uniform area and the size of antennas to be calibrated. The following table indicates the recommended minimum calibration distances, depending on the loop diameter of the loop under calibration and the wanted field uniformity.

The error to be expected caused by the decay of fieldstrength within the RX-loop area is less than the specified field uniformity, because of the averaging characteristics of the loop area exposed to the magnetic field.