Antenna Factor Calibration of Dipole Antennas (30 - 1000 MHz)

1. Abstract

Dipole antenna factor calibration is avilable with the Japan Calibration Service System (JCSS; http://www.jcsslabo.or.jp/index_e.htm , http://www.nite.go.jp/asse/jcss/en/index.html ) at 24 frequencies in the range from 30 to 1000 MHz. Since dipole antennas [1] are mainly used for the evaluation of the test-site quality in this frequency range, the antenna shoul be calibrated correctly. In our institute the dipole antenna is holizontally located at 2.0 m above the ground plane. The electric field strength at a location can be easily estimated by multiplying the indicated voltage on the receiver display and the calibrated AF in the emission measurement. Thus, it is useful to measure the field strength. The primary standard of the diple antenna that containis 24 elements for the 24 calibration frequencies are calibrated by the three-antenna method on our open-area test site. The element length is approximitly half-wavelength at each frequency. The secondary standard dipole antennas of the acredited calibration laboratories are calibrated by the reference antenna method by replacing the primary standard antenna with the userfs antenna.

2. Definition of the antenna factor


Fig. 1. The definition of antenna factor.

The antenna factor (AF) is defined as the ratio of the incident electric field strength to the induced voltage across the terminal impedance and given by;

(1/m)     (1)

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 Ħ (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 .

3. Calibration of the antenna factor

Since many EMC measurements are carried out on an open-area test site, the AF is calibrated above a large ground plane with a sufficient obstruction free area [2]. However, the AF changes with respect to the antenna height and polarization due to the mutual coupling between the antenna element and its image. To avoid this problem the antenna under calibration is located horizontally at a height of 2.0 m above the ground plane in our calibration procedure [3]. By locating both of the transmitting and receiving antennas at a height above the ground plane the direct electromagnetic fields may be significantly reduced by the fields reflected by the ground plane. This will impact uncertainty of the calibration. Thus, the separation between antennas is appropriately determined in advance by the calculation to avoid the degraded calibration. Three antennas will be simultaneously calibrated by the three-antenna method (TAM) and the associated uncertainty for the three calibrated antennas must be the same. One of the antenna is specified as the standard dipole antenna and the secondary standard antenna of the accredited laboratories is calibrated by the reference antenna method (RAM). Figure 3 shows the schematic diagram of the calibration.

Fig. 2 AIST open-area test site	Fig. 3 Schematic diagram of the calibration.



• (1)The TAM [4][5]

The TAM is widely used to obtain AFs. Three antennas are necessary for this method. The site attenuations between three antenna pairs are measured. Then the AF of an antenna is given by;

     (2)

     (3)

where and are the direct and reflected path lengths between antennas, respectively. Note the subscript is introduced to specify the antenna pair.

• (2) The RAM

AFs of a user's antenna are calibrated by the RAM. The AF of the user's dipole is given by the ratio of the measured site attenuation the transmitting antenna and the standard antenna (whose AF is ) to the one between transmitting antenna and the user's antenna (whose AF is ). Let's assume the measurement system is ideal then we obtain;

.     (4)

4. Conditions of the calibration and uncertainty
• (1)Conditions of the calibration

The AF of a dipole antenna is calibrated at 2.0 m above a large flat conducting plane by locating antenna element horizontally. Calibration is carried out at following 24 frequencies.

Frequencies: 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000 MHz

• (2)Expanded uncertainty

0.7 dB (k=2)

Figure 4 shows an example of the calibrated AF of a dipole antenna and the associated uncertainties. Fig. 4 (2) contains uncertainties associated both with the TAM and the RAM [6].




Fig. 4 An example of the calibrated AFs and uncertainties associated with the calibration.

(1) Calibrated AF at 24 frequencies.	(2) Uncertainties associated with the AF calibration.

REFERENCES

• [1] M. J. Salter and M. J. Alexander, "EMC antenna calibration and design of an open-field site," in Proc. Inst. Elect. Eng., Sci. Meas. Technol., 1991, vol.2, pp. 510-519.
• [2] K. Komiyama and T. Morioka, "ETL open-area site for antenna measurement," CPEM2000, Sydney, May 2000.
• [3] A. Sugiura, T. Moriokawa, K. Koike, and K. Harima, "An improvement in the standard site method for accurate EMI antenna calibration," IEICE Trans. Commun., vol. EMCJ98-29.
• [4] ANSI C63.5, "American national standard for electromagnetic compatibility radiate emission measurements in electromagnetic interference (EMI) control-calibration of antennas (9 to 40 GHz)," 1998.
• [5] A. A. Smith, "Standard-site method for determining antenna factors," IEEE Trans. Electromagn. Compat., vol. EMC-24, no. 3, pp. 316-322, Aug. 1982.
• [6] T. Morioka and K. Komiyama, "Uncertainty analysis of dipole antenna calibration above a ground plane," IEEE Trans. Electromagn. Compat., vol. EMC-48, no. 4, pp.781-791, Nov. 2006.