Combline Filters With Increased Stopband and One-Sided Selectivity

The letter proposes two design methods to increase the stopband and one-sided selectivity of combline bandpass filters (BPF) with mixed couplings between adjacent resonators. One method allows us to expand the stopband of a combline BPF. It is based on the location patterns of special points along the length of
$\boldsymbol {\lambda }$
/4 resonator and the elimination of a number of periodically located resonant frequencies using them. This method is applicable to combline BPFs with simple and mixed couplings. The second method refers to BPF with mixed coupling between adjacent resonators that contains magnetic and electrical components
$\acute {K}_{i,i+ 1} = K_{mi,i_{+} 1}+ \, K_{e i,i+ 1}$
and generates a transmission zero
$f_{z i,i+1}$
(TZ), leading to increased selectivity. This method uses the representation of mixed coupling by admittance inverter (
$\hat{J}$
inverter) and consists of two stages. At the first stage, the absolute values
$\vert \acute {K}_{i,i+ 1}\vert $
, are determined. In this case,
$g$
-parameters of the low-frequency prototype are used. At the second stage, the components
$K_{m i,i+ 1}$
and
$K_{e i,i+ 1}$
are determined by using the
$f_{z i,i + 1}$
position. The method is applicable to
$N$
-order BPF. The experimental results are presented.