# An Optimization Design of Bi-log Hybrid Antenna with Taguchi's Method for EMI Measurements
## 1 Introduction  
Broadband antennas are widely used in electromagnetic interference (EMI) measurement systems. Many types of broadband antennas have been well-documented and applied for various aims, for example, biconical antenna, log-periodic dipole array (LPDA) antenna, biconical log-periodic dipole array (Bi-log) antenna, and horn antenna. Bi-log antennas are the most commonly used in practice tests and are used primarily as receiving antennas in radiation emission tests. The Bi-log antenna is a typical broadband antenna with complex geometry and is a hybrid antenna [1]. A hybrid antenna is defined as any constructed antenna that includes a combination of both broadband dipole elements and LPDA elements. Refs. [2; 3] evaluated the structure of Bi-log antennas in 15 elements to cover the frequency range of 30 MHz to 1 GHz, but their operation bandwidth might exhibit a limitation when compared with conventional antennas. The impact of a ground plane and mutual coupling on the antenna factor (AF) was investigated using the Bi-log antenna in [4]. The paper provided information on the AF, an important parameter for EMI measurement with a receiving antenna in an electromagnetic field. Additionally, it is also defined as a key factor for characterizing antennas. In recent years, many types of printed LPDA antennas have been developed to possess lightweight, low-cost, good directivity, and wide bandwidth properties [5; 6; 7; 8; 9]. Although the design of this antenna was uncomplicated, expanding it to cover broad frequency ranges and higher frequencies while maintaining a compact size could pose challenges. [PERSON] et al. [5] presented a compact printed log-periodic biconical dipole array antenna that achieves size reduction, and bandwidth enhancement from 0.5 GHz to 6 GHz,but its feeding technique may result from undesired radiation during measurements. In [6], a compact bowtie-shaped dipole element of printed LPDA antenna is proposed for ultra-wideband application. Its lowest operation frequency was 0.5 GHz, and its average gain was only 6 dBi. A sinusoidal curve LPDA antenna with dielectric loading has been proposed in [7]. It has a bandwidth of 0.2 GHz to 0.8 GHz and a gain of 4.5 dBi. According to [8], the study introduced a printed LPDA antenna with a dual-band dipole element. The antenna has a compact size in terms of axial length, from 0.5 GHz to 10 GHz. Although the antenna gain was between 3 and 6.01 dBi, forming an array of 25 antenna elements could be a challenging issue. A compact planar LPDA antenna operating from 0.55 to 9 GHz within top-loading techniques with 50 elements array was proposed [9]. Its radiation patterns were not pointed in the direction of light, and its antenna gain ranged from 2.48 to 7.89 dBi. Additionally, the bandwidth of these antennas is not wide enough to cover 30 MHz and may not be suitable for EMI measurement. All of these antenna structures were also implemented on a lossy substrate, which affected both impedance matching and radiation efficiency. The balanced-to-unbalanced (Balun) is typically supplied as a part of the antenna for matching the wideband impedance. The analysis of the infinite Balun feed structure in [10] was exaggerated due to antenna asymmetry, which was resolved by decreasing the size of the feed. For measurement applications, previous studies have extensively presented the causes of performance issues and problems of the broadband hybrid antenna in test environments of various sites [11; 12; 13; 14; 15]. Although these Bi-log antennas have been applied to achieve different purposes, little attention has been given to the evaluation of the comprehensive design of antenna optimization related to dimension, symmetry, and impedance match. The proposed structure in this paper is the combination of a bowtie and log-periodic to achieve a Bi-log hybrid antenna. The proposed antenna improves the reproducibility and consistency of a series of EMI measurements while significantly reducing measurement time.