Analysis of Coupling Plate Effect in Closer Proximity of TEM Horn Antenna Cahit Karakuş S. Cihad Yurt Çağatay Aydın Doğu Çağdaş Atilla B. S. Yarman Istanbul Kültür University Istanbul, Turkey Istanbul University Istanbul, Turkey Istanbul University Istanbul, Turkey Istanbul University Istanbul, Turkey Istanbul University Istanbul, Turkey Abstract—In this paper, variation of the radiation pattern of TEM horn antenna is investigated. Ultra wide frequency band is achieved by electromagnetic coupling plate to back side of TEM horn antenna. Because of this coupling plate, radiation direction is changed depending on frequency. In closer proximity, coupling effect is analyzed using simulation results in different cases. Simulated TEM horn antenna radiates in ultra wide band range from   to  . Keywords- electromagnetic coupling plate; transverse electromagnetic (TEM) horn antenna; ultrawide-band (UWB) antenna; radiation direction. I. INTRODUCTION Specifically, transverse electromagnetic (TEM) horn antennas have been used as wideband antennas for various applications. This type of antenna has the advantages of wideband, no dispersion, unidirectional and easy construction [1]. The TEM horns are constant aperture antennas and make use of the propagation of TEM waves. Hence all frequencies generated at the horn throat will arrive at the aperture together, and a wideband pulse is transmitted from the aperture. This basic property makes TEM horn antennas suitable for ultra wideband (UWB) applications and broadband measurements [2]. In radar based techniques, wide bandwidth is required for the antenna to radiate a pulse with high fidelity over an ultra- wide frequency range [3]. The TEM horn antenna is a traveling-wave structure that consists of two conducting plates. To radiate electromagnetic waves, its flare angle, plate width and plate length must be properly determined. Tapering has been proposed for the distance between the two plates [4] or for both the distance between the two plates and the width of the plates [5]. TEM horn antennas are usually fed by a coaxial line. In order to match the broadband radiation impedance of a TEM horn (balanced structure) to that of an unbalanced coaxial line, a coupling plate is needed. Our goal is to design the coupling plate such that it covers the whole UWB range [3]. The characteristic impedance of the coupling plate gradually transits from that of the coaxial line to that of the balanced parallel plates. The coupling plate parameters influence significantly the impedance match of the antenna [3]. In this paper, we propose an ultra-wide band TEM horn antenna having coupling plate effect in closer proximity. The TEM horn antenna has been chosen due to its wide bandwidth, directional radiation pattern and low distortion. The antenna has been designed to operate in the UWB range covering between   to   with the coupling plate. The power is radiated via the front aperture of the antenna without coupling plate [3]. But direction of radiation is dramatically changed with lower edge of frequencies via coupling plate. The next section describes the design of TEM horn antenna with coupling plate. Further antenna realization and measurements are presented. Simulation results demonstrate the UWB impedance match and direction of radiation via the antenna aperture [3]. II. DESCRIPTION OF ANTENNA STRUCTURE AND DESIGN To radiate electromagnetic waves into the air, its flare angle, characteristic impedance variation between the two plates, plate length and width must be properly determined, since these parameters are the most important design factors to achieve the wideband characteristics of the TEM horn antenna. In this study, the design of the antenna starts with the idea of two parallel bow-tie plates which are diverging from feed point [2, 6, and 7]. This geometry is shown in Figure 1. In the illustration, the relative sizes of the antenna are exaggerated for clarity. The physical size of the feed and the frequency,  , (wavelength,  ) remain constant throughout all of the simulations. Keeping the physical size of the feed constant insures that any parasitic effects near the feed remain the same for the proposed antenna [8]. The antenna produces radiation patterns in different directions depending on operating frequency. In Figure 1, the sketch of designed antenna is shown with three different geometries. In Figure 1(a), antenna is seen without coupling plate. Additionally, it is given with coupling plate and front view of the antenna is depicted in Figure 1(b) and (c).