Calibration Loop Antenna for Multiple Probe Antenna Measurement System

In order to achieve high accuracy in measurements of the multiple probe antenna measurement system (MPAMS), an omnidirectional calibration antenna with a small gain variation in the horizontal polarization (named calibration loop antenna, in convention) is required. This article presents such an antenna design. The proposed calibration loop antenna consists of four arc-folded dipoles with dimensions of 52 mm × 52 mm × 1 mm, balun structures for suppressing common mode current, and four parasitic strips for impedance matching on each of the top and bottom planes. The experimental results show that the proposed calibration loop antenna has a small gain variation in the horizontal polarization less than 0.16 dB in a bandwidth of 20% (2.25-2.75 GHz), and its 10-dB return loss bandwidth is 18.2%. Also, the cross-polarization ratio in the operating bandwidth is greater than 24 dB. Along with the desired omnidirectional radiation pattern, the low profile and compact design further make the proposed calibration loop antenna a suitable selection for calibration in MPAMS.

I. Introduction

With the rapid development of Internet of Things, over-the-air (OTA) performance of a wireless device is critical to the effective operation of a wireless network [1]–[5]. Compared with a single probe antenna measurement system that has to switch different measured orientations with mechanical rotation, a multiprobe antenna measurement system (MPAMS) has a higher measurement speed by using the electrical switch [6]. Thus, in today’s OTA measurements [7], [8], MPAMS is widely used, where multiple probe antennas are designed to measure the radiation characteristics of the device under test (DUT) in different directions. However, because of the inconsistency of the absorbing materials combined with the difference in the gain of different probes, MPAMS requires calibration to ensure accuracy of the measurements. Fig. 1 illustrates the calibration process of a typical MPAMS. In the MPAMS, the probe antennas are placed in a ring, while the calibration antenna (CA) is placed in the center. The direction of the horizontal polarization is defined in parallel to the xoy plane, while the vertical polarization is in perpendicular to the xoy plane. Fig. 1.

Illustration of calibration of a typical MPAMS.