# A Novel Wideband Splitter for a Four-Element Antenna Array
## 2 Related Work  
In this chapter, the state of the art and the results of the selected representative works in the field of RF splitters and dividers are presented and discussed. As mentioned above, the fundamental quantities that describe the properties of splitters and dividers are the transmission and reflection coefficients. From these fundamental characteristics, it is subsequently possible to derive additional quantities such as the flatness of the frequency response, insertion loss, and others. One of the parameters used for the comparison with the related studies is the flatness of the frequency response of the reflection coefficient. This parameter specifies the range of variation in the reflection coefficient of a device over a given frequency range. The passband flatness of the reflection coefficient is specified in \(\pm\)dB and is defined as the difference between the minimum and maximum values of the reflection coefficient in dB, measured over a given frequency range. The better the flatness of the reflection coefficient, i.e., the smaller positive number expressed in dB, the smaller the impedance changes over the considered frequency range. Smaller impedance changes in the passband result in a lower change of amplitude of the reflected power waves and standing waves. [PERSON] et al. [7] proposed a balanced-to-unbalanced micro-strip power divider that is based on branch lines with several stubs and a single resistor. The micro-strip power divider supports functions of power dividing, frequency selectivity, isolation between output ports, and common-mode suppression. The authors claim that it is possible to control the bandwidth of the proposed power divider and the maximum bandwidth is achieved when the common-mode suppression and differential-mode response are the same. Tests were performed on two prototypes, and the first one achieved a 1 dB fractional bandwidth of 7.7% with a 0.6 dB insertion loss, while the second achieved a 1 dB fractional bandwidth of 5% with a 0.7 dB insertion loss, with the isolation and common-mode suppression better than 15 dB and 20 dB, respectively. A compact wideband balanced-to-unbalanced out-of-phase power divider was proposed by [PERSON] et al. [8]. The proposed circuit consists of three pairs of cascaded coupled lined and a grounded resistor used for the isolation of the output, which resulted in good port matching, high output port isolation, and equal power division. The authors were able to achieve a fractional bandwidth of 37.2% and an insertion loss of 0.2 dB. [PERSON] et al. [9] proposed a four-port wideband power divider based on symmetrical transmission lines operating at 2.2 GHz with a fractional bandwidth of 89.1%. The minimum insertion loss of the proposed power divider was 3 dB and the reflection coefficient was less than \(-13\) dB. The flatness of the frequency response of the reflection flatness was up to \(\pm 25\) dB. An ultra-wideband power divider based on a symmetrical three-port circuit, which operates at 6.85 GHz with a fractional bandwidth of 110%, was proposed by [PERSON] and [PERSON] [10]. In the proposed design, the two output ports are properly linked with two identical step-impedance stubs and coupled lines. Based on the experimental measurements, the minimum insertion loss was 0.4 dB and the reflection coefficient was less than \(-12\) dB. Moreover, the flatness of the frequency response was up to \(\pm 25\) dB. A tunable balanced to unbalanced power divider that divides the power from a balanced input port to unbalanced output ports was proposed by [PERSON] in [11]. The divider was proposed to divide the power in a specific radio by changing the biasing voltage. The power division ratio was changed by tuning of the capacitance, which was realized by the varactor diode. Another three-port wideband power divider based on coupled lines operating at 2.3 GHz with a fractional bandwidth of 47% was proposed by [PERSON] et al. in [12]. In this case, the minimum insertion loss was 1 dB and the reflection coefficient was less than \(-13\) dB.