Interdigital Filter

Interdigital Bandpass Filters
Joachim Köppen DF3GJ ... Kiel, July 2016

Some brief explanations

This tool is the JavaScript version of N6JH's BASIC program (HamRadio, Jan 1985, p.12) and its C++ version by WA4DSY (1997), which follow the FORTRAN program INTFIL by Rook and Taylor (1970).
The filter type is Chebychev (specify ripple in dB) or Butterworth (ripple=0).
Enter the design data in the text fields, and hit the Enter key or the compute button. If the bandwidth is too small or too large for the approximations used in the program, its text field appears yellow as a warning that the results are less reliable.
The program will happily design filters with as many rods as you might wish ... but be aware that the inevitable losses will not make it possible to build real filters with more than about 20 elements!
The plot shows the frequency response of the filter. The user may chose a different frequency span for the display.
The table below the plot summarizes all design parameters and the mechanical details of the filter

4 section interdigital bandpass filter

    Bandpass ripple = 0 dB
    Center frequency = 1400 MHz
    3 dB BW = 20 MHz
    Fractional BW = 0.014085
    Ripple BW = 0 MHz
    Cutoff frequencies = 1410 and 1430 MHz
    Filter Q = 71
    Est.unloaded Qu = 2060
    Insertion loss based on this Qu = 0.78 dB
    Delay at band center = 41.6 ns

Quarter wavelength 52.8 mm
Length of interior elements 47.4 mm
Length of end elements 47.5 mm
Rod height above ground plane 10 mm
Rod diameter 6 mm
End plates are 14 mm from center of end rod
Tap external lines up 3.1 mm from shorted end
Line impedances: End rod: 85, other: 87
Load impedance 50 Ohm

Dimensions (mm)

Element No.	distance from	distance to	G[k]	Q/Coup
	end plate	next element
end	0.0	14
1	14	31.7	0.76537	0.8409
2	45.7	34.6	1.8478	0.5412
3	80.3	31.7	1.8478	0.8409
4	112	14	0.76537	0.76537
end	126

Box inside dimensions [mm]:
    height 52.8
    width 126
    thickness 20