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Antenna Factor Calibration of Loop Antenna (9 kHz - 30 MHz)

  1. Abstract
  2. NMIJ provides cariblation services of specifed loop antennas at 8 frequencies (9 kHz, 10 kHz, 150 kHz, 500 kHz, 1 MHz, 5 MHz, 15 MHz, 30 MHz) in the range from 9 kHz to 30 MHz. In MF and HF bands (especially below 30 MHz), loop antennas are used for estimating the radiated magnetic field strength. It is important to accurately evaluate magnetic fields radiated from electric devices. Generally, this frequency bands have been used for the navigation systems of airplanes and ships, the amateurish radio communication and the radio controled toys, etc. Recently, loop antennas and magnetic antenna factors have been widely applied for the evaluation of power line communication (PLC) systems, induction heating (IH) systems, radio frequency identification (RF-ID) tag systems, RF shoplifting alarm systems, and signals for radio-controlled clocks. Hence, the antenna factor of the receiving loop antenna must be accurately determined. In NMIJ, the standard loop antenna whose diameter is 10 cm is calibrated by a three-antenna method and user's loop antennas are calibrated by a reference antenna method.

  3. Definition of the magnetic antenna factor
  4. Fig. 1. The definition of magnetic antenna factor.
    Fig. 1. Definition of the magnetic antenna factor.

    The magnetic antenna factor AFm is defined as

    AFm=H/V[S/m]     (1)

    where H is the magnetic field of the electromagnetic plane wave incident on the antenna element and V0 is the output voltage across the load (50 Ohm ) as shown in Fig.1. This means that we must obtain the characteristics under a far-field condition eventhough the wavelength is very long. The antenna factor, naturally, depends on the frequency f . Advantage to use the AFm that H can be easily estimated by multiplying the calibrated AFm to the measured V0.

    Here it is assumed that the antenna is located to obtain the maximum output response to the incident field. We should note that the terminal impedance of the ordinary receivers is adjusted to be 50 (refer to Fig.1). Advantage to use the AF is that the field strength can be easily estimated by multiplying the calibrated AF to the measured .

  5. Calibration of the magnetic antenna factor
  6. Fig. 2. Averaged magnetic field insideloop antenna.
    Fig. 2. Averaged magnetic field insideloop antenna.
    Generally, the magnetic antenna factor is defined in far-field condition and in free space. However, it is difficult to apply actual plane wave to a loop antenna and to realize far-field condition in the low frequency because the wave length is more than 10 m. Consequently there is non-uniformly distributed magnetic field inside the loop. Therefore the average magnetic field inside the receiving loop antenna [1] is ordinarily used instead of the ideal plane magnetic field. We adopted this average magnetic field for the three-antenna method [2] to calibrate standard loop antennas whose diameter is 10 cm. On the other hand, user's loop antennas are calibrated by the reference antenna method with the calibrated standard loop antenna. These calibration is carried out at the distance of a few tens centimeters in an anechoic chamber (The size is 15 m X 10 m X 8 m). Figures 3 and 4 show the schematic diagrams of the three-antenna method and the reference antenna method, respectively.


    Fig. 3. Schematic diagram of the three-antenna method Fig. 4. Schematic diagram of the reference antenna method.
    Fig. 3. Schematic diagram of the three-antenna method Fig. 4. Schematic diagram of the reference antenna method.

  7. Conditions of the calibration and uncertainty

REFERENCES


Contact information:

Electromagnetic fields section,
Electromagnetic waves division,
National Metrology Institute of Japan
National Institute of Advanced Industrial Sciense and Technology

Room 317, Tsukuba central 3
1-1-1, Umezono, Tsukuba 305-8563 Japan
Tel: +81-29-861-4177, Fax: +81-29-861-4957
E-mail:emf-cal@m.aist.go.jp
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Electromagnetic fields section
Electromagnetic waves division
NMIJ/AIST