Gain Calibration Services for Standard Horn Antennas (1 GHz to 40 GHz)

1. Abstract

NMIJ provides a service for calibrating gains of specified standard horn antennas from 1 GHz to 40 GHz, whose frequency is divided into 10 bands (L, R, S, G, C, X, P, K, and U). The calibrations are using a gain-transfer method [1] and the reference antennas that were calibrated by NPL (National Physical Laboratory, The UK�fs National Measurement Laboratory). The calibrations are possible to do at specified frequencies (principally the lowest, center, and highest frequencies at each band). The L band is divided into two band at 1.15 GHz. The uncertainties depend on the frequency bands(0.25 dB to 0.41 dB).

2. Definition and Measurement of Antenna Gain

The gain calibrations are provided at the one direction that is normal to the aperture plane of the horn antennas. The gain is defined as the ratio of the power radiated per unit solid angle to the assumed power isotropically radiated per unit solid angle from the input power Pin as [2]

     (1)

where is a solid angle. As a method to measure the gains, the Friis transmission formula is used to represent the transmission between two antennas at sufficiently large distance. It is [2]

     (2)

where Prec, p, ƒ¡, ƒÉ, R are the receiving power, the polarization mismatch factor, the wavelength, and the distance respectively. The subscript r and t represent the quantities with respect to receiving and transmitting antennas. The polarization mismatch factor can be 1 if both antennas are linearly polarized. The factor must be measured by another method.

3. Calibration using the gain-transfer method

Fig. 1 and Pic. 1 show a schematic and a scene for measurement. In the measurements, a reference antenna or an antenna under calibration (AUC) is used as a transmitting antenna and one antenna is used as a receiving antenna. From (2), the ratio between each pair of antennas, the gain of the AUT is given as

     (3)

Where the letters AUT and S represent the AUT and the reference antenna, and S21 is the one of S-parameters between a pair of two antennas. The reference antenna used is the antenna calibrated by NPL, or the one calibrated by the antenna. In strict sense, (3) requires the two assumptions; 1. the multiple reflections between antennas can be neglected, 2. the distance between antennas is sufficiently large. The assumption 1 can be realized by using averaging the quantities where the receiving antenna is moved. The assumption 2 can be realized by using the near-field correction [3] as

     (4)

where

     (5)

Ar is an aperture of the receiving antenna. , , are the magnetic and electric fields, the unit normal at the aperture respectively. , are the magnetic and electric fields at the aperture of the transmitting antenna specified by the letter i at the distance R respectively. The S-parameters between antennas is measured by a vector network analyzer (VNA) that is calibrated by the SOLT method below 1.7 GHz and the TRL method above 1.7 GHz. The measurement uncertainty is evaluated from the axis errors, the distance error, the polarization error, and etc.

Fig. 1 Schematic of the gain-transfer method	Pic. 1 Measurement scene

4. Plan

The calibration is limited to specified types of standard horn antennas and the specified frequencies at each band. To remove the limitations, we have a plan to calibrate any standard horn antennas at any frequencies using the planar near-field scanning method and provide the antenna gain and pattern, and the polarization.

References

• [1] G. Mayhew-Ridgers, J.W. Odendaal, J. Joubert, �hHorn Antenna Analysis as Applied to the Evaluation of the Gain-Transfer Method,�h IEEE Trans. Instrum. Meas., Vol. IM-34, No. 6, pp. 490-503, Dec. 1985.
• [2]R.E.Collin, Antennas and Radiowave Propagation, McGRAW-HILL, 1985.
• [3]T.S.Chu and R.A.Semplak, �gGain of Electromagnetic Horns,�h Bell Syst. Tech. J., Vol. 44, pp. 527-537, March 1965.