# A Novel Wideband Splitter for a Four-Element Antenna Array
## 3 Proposed Antenna Array Splitter  
The proposed splitter was designed for the frequency band of 650 MHz and is based on a pair of transmission lines. The first transmission line is asymmetrical and its length is a quarter wavelength of the medium operating frequency. Therefore, the electrical lengths of transmission lines should be 90\({}^{\circ}\). The second line is symmetrical and has the same length as the asymmetrical transmission line. The characteristic impedance of the asymmetrical transmission line \(Z_{0a}\) is the same as the characteristic impedance of the used RF system \(Z_{0}\), which is normally 50 \(\Omega\) or 75 \(\Omega\). The characteristic impedance of the symmetrical transmission line \(Z_{0s}\) is in the range between 1.6 \(\times\)\(Z_{0}\) and 3.2 \(\times\)\(Z_{0}\). The proposed circuit of the antenna array splitter is shown in Figure 1.  
The first part of the proposed wideband splitter consists of a transmission line for input ports 1 and 2. These are connected to the first section of the symmetrical transmission line through coupling elements \(Z_{C1}\) and \(Z_{C2}\). Similarly, the second part of the proposed wideband splitter consists of a transmission line for input ports 3 and 4. These are connected to the second section of the symmetrical transmission line through coupling elements \(Z_{C3}\) and \(Z_{C4}\). The load of the symmetrical transmission line is a short circuit. In practice, it is desirable to realize this load as a non-inductive element with a maximum impedance of 10 \(\Omega\) in the whole working frequency range [1,3]. The coupling elements \(Z_{C1}\), \(Z_{C2}\), \(Z_{C3}\), and \(Z_{C4}\) are considered to be identical RF components that realize the appropriate coupling of the antenna array to the proposed splitter structure. Subsequently, the symmetrical transmission line feeds the asymmetrical transmission line. The ground point of the asymmetrical transmission line is connected directly to the first part of the symmetrical transmission line. The trace point of the asymmetrical transmission line is connected via coupling element \(Z_{Ca}\) to the second part of the symmetrical transmission line. Finally, the output of the asymmetrical transmission line feeds through the coupling element \(Z_{C5}\) to the output connector of the splitter. The central frequency of the proposed splitter depends primarily on the electrical length of the symmetrical transmission line, which should have exactly a quarter-wavelength at the central frequency. The bandwidth of the proposed splitter mainly depends on the characteristic impedance \(Z_{0s}\) of the symmetrical transmission line. From the structure of the proposed power splitter, it is obvious that bandwidth is determined by the input impedance of the symmetric transmission line, which ends with a short circuit. In theory, the input impedance will be infinitely large and its influence will be negligible. For the other frequencies, within the bandwidth, the input impedance is given by the following relationship:  
\[Z_{in}=j\times Z_{0s}\times\tan\left(\beta*l\right), \tag{1}\]  
where \(Z_{in}\) is the input impedance, \(Z_{0s}\) is the characteristic impedance, and \(\beta\) is the phase constant of the transmission line with length \(l\). From the equation, it is obvious that the higher the characteristic impedance of the symmetrical transmission line \(Z_{0s}\), the smaller the impact on the total impedance of the structure outside of the central frequency. Thus, the higher the value of the characteristic impedance of a symmetrical transmission line, the higher the bandwidth of the splitter and vice versa. However, the characteristic impedance is affected by the shielding cover [1, 2, 3, 4]. When a shielding cover is used, the characteristic impedance of a symmetrical transmission line will decrease. The energy efficiency of the proposed splitter depends mainly on the quality of the used transmission lines. The energy efficiency of the transmission lines is defined as the ratio of the input power to the output power.