combline design
Combline filter if wide band then use Wenzel&Horton synthesis ,
case of narrow band use MYJ book (Mathaei) as a begining point, in practice narrow band combline filter are heavily dependent on size versus frequency. For small size low frequency they work nearly as Matheai equations(use Cristal for round rods design or Getsinger for rectangular bars, or better nowdays use the italian proffesors paper that give equations to solve easily the coupled lines dimensions).
The problem with narrow band comblines with big size (relative to frequency of use) is that you might design for a certain bandwidth, but measured data might show a much wider filter then your original design and simulation.
Look at paper that gives cure to that (Shafir,D.Swanson,S.Sharir in IEEE-MTT).
The basic problem original filter theory assumes TEM and coupled line of infinite lenght, in practice dimensions are finite and higher modes(evanesent waves) will propagete thru your structure, so you might need an EM solver, or a good technician for cut and try, usually after first prototype is built you can scale you design in order to be quite close to final result, in designing for such approach leave some room for tuning
To design such filter use CST,QW3D,HFSS, or Mode Matching software, use for starting point Mathaei data, or,PCfilt,Sfilsyn(comb),Parfil or Eagleware M-filter, all of them will do,or make your own Matlab file.