# A Novel Wideband Splitter for a Four-Element Antenna Array
## 1 Introduction  
Power splitters are important RF (radio frequency) devices, which have been used in modern communication systems and antenna arrays [1; 2; 3; 4; 5]. In the field of antenna arrays, the power splitters are crucial elements, which have a significant impact on the properties of a complete antenna array. For this reason, it is necessary to design an antenna array splitter with a low insertion loss, an optimal impedance matching, and a desired bandwidth. The essence of the presented article is the novel design as well as the implementation of the broadband antenna array splitter that enables a significant increase of the antenna gain, i.e., an increase of the antenna gain by at least 3 dB, and an optimization of the antenna radiation pattern in a large bandwidth. The designed combiner is primarily intended for a broadband opposite-phase four-element antenna array. Such an antenna array consists of two pairs of elementary antennas. These two pairs are in anti-phase with each other. The first pair of antennas is connected to the first RF section, which is the RF transmission line for ports 1 and 2. The second pair of antennas is connected to the second RF section, which is the RF transmission line for ports 3 and 4. Ideally, the gain should increase fourfold. At the same time, the main lobe, defined as the half-power beam width (HPBW), should decrease fourfold. However, the real improvements of both parameters are primarily limited by the splitter unit of the antenna array [3]. In the case of a splitter based on two anti-phase parts, the phase error factor is the most important parameter. In practice, a suitable valueof the phase error factor is usually around 1% or less. Other important parameters that quantify splitter properties are the transmission coefficient and the reflection coefficient of individual anti-phase segments of the splitter. In practice, the suitable value of the transmission coefficient for a four-element antenna array splitter is between \(-6.1\) dB and \(-8.5\) dB. Furthermore, the suitable value of reflection is less than \(-20\) dB [1,4,6]. In this work, a novel concept of the broadband antenna array power splitter was applied. A power splitter that operates at a center frequency of 650 MHz with a fractional bandwidth of 84.3% was developed. Based on experiments performed with the manufactured prototype, a phase error factor of less than 0.65% in the working frequency range was achieved. The working frequency range is defined as a \(S_{11}<-20\) dB bandwidth concurrently on both transmission lines. Currently, the frequency band around 650 MHz is very attractive, since modern LTE and 5G communication technologies operate in this band. Moreover, the frequency band is also operated by DVB-T2 broadcasting systems. The described power splitter is therefore suitable for the abovementioned systems, where it could help cover more distant areas with better directional characteristics of a suitable antenna array. By using the proposed power splitter as a combiner, thanks to the principle of reciprocity, it is also possible to increase the power of the received signal in areas with limited coverage. Furthermore, the transmission coefficient of the prototype ranges from \(-6.8\) dB up to \(-8.4\) dB, while the reflection coefficient of the prototype ranges between \(-20\) dB and \(-26.5\) dB. Thus, the theoretical and measurement results are in good agreement and show good performances in the working frequency band.