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.
The magnetic antenna factor 
is defined as
[S/m] (1)
where 
is the magnetic
field of the electromagnetic plane wave incident on the antenna element and 
is the output voltage across the load (50 
) 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 
. Advantage to use the that can be easily estimated by multiplying the calibrated to the measured .
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 .
). 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. |
The three-antenna method is one of the methods to obtain s. Three antennas are necessary for this method. The transmission S-parameters between three antenna pairs (
) are measured. Then the of an antenna 
is given by;
(2)
(3)
where 
is wave number 
is angular frequency, 
is permeability of free-space, 
is the load impedance matched to the transmission line (50 
), 
are radii of loop antennas and 
are distances between these two loop antennas. The average magnetic field inside the receiving loop antenna (shown in Fig.2) is used instead of the plane magnetic field assumption. Figure 3 shows the schematic diagram of the three-antenna method.
The magnetic antenna factor of the user's loop antenna 
is calibrated by the reference antenna method. The is given by;
. (4)
. (5)
where 
is the measured transmission S-parameter between the transmission loop antenna and the standard loop antenna (whose magnetic antenna factor is 
), 
is the one between the transmission loop antenna and the user's loop antenna, 
are radii of loop antennas and 
are distances between these two loop antennas. 
and 
mean correct coefficients for the average magnetic field inside the receiving loop antenna and they are depending on the radii of loop antennas and distances between loop antennas. Figure 4 shows the schematic diagram of the reference antenna method.
Frequencies : 9 kHz, 10 kHz, 150 kHz, 500 kHz, 1 MHz, 5 MHz, 15 MHz, 30 MHz
Antenna conditions : The diameter is from 50 cm to 60 cm. The diameter of antenna element is less than 23 mm. The feeding connector is PC-7 (Adaptor is available). The 
between transmission loop antenna (whose diameter is 10 cm) and user's loop antenna is more than -60 dB and less than -10 dB at 80 cm. You need to ask our person in charge, beforehand.
| Frequencies | Expandeduncertainty (k=2) |
| 9 kHz | 3.2 dB |
| 10 kHz | 3.2 dB |
| 150 kHz | 2.2 dB |
| 500 kHz | 2.0 dB |
| 1 MHz | 2.0 dB |
| 5 MHz | 2.0 dB |
| 15 MHz | 1.8 dB |
| 30 MHz | 1.6 dB |
The calibration is carried out by the reference antenna method with NMIJ's standard loop antenna. The standard loop antenna has been calibrated by the three-antenna method.
As the other calibration service, we can calibrate passive loop antennas (whose diameter is approximately 10 cm) by the three-antenna method.
Frequencies : 150 kHz, 500 kHz, 1 MHz, 5 MHz, 15 MHz, 30 MHz
Antenna conditions : The
diameter is from 10 cm (
6 mm). The diameter of antenna element is less than 6 mm. The
feeding connector is PC-7 (Adaptor is available). User's loop antenna must be a
passive type loop antenna without an amplifier. You need to ask our person in
charge, beforehand.
| Frequencies | Expanded uncertainty (k=2) |
| 150 kHz | 3.6 dB |
| 500 kHz | 3.6 dB |
| 1 MHz | 3.6 dB |
| 5 MHz | 3.6 dB |
| 15 MHz | 3.3 dB |
| 30 MHz | 2.7 dB |
Contact information:Copyright (C) 2004-2005 All Rights Reserved.
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
