In order to verify these calculations / results, it is the best to build a filter and see how it behaves. We decided to do so and build a bandpass for ADS-B. We wanted to use 3 rods, center-frequency F = 1090 MHz and a bandwidth of B = 10 MHz. The Designer finally suggests the following values :

Interdigital Bandpass Filter, based on work of Jerry Hinshaw,
Shahrokh Monemzadeh (1985) and Dale Heatherington (1996).
www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.php
Javascript Version : 09. Jan 2014
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Design data for a 3 section interdigital bandpass filter.
Center Frequency : 1090 MHz
Passband Ripple : 0 dB
System Impedance : 50 Ohm
Cutoff Frequency : 1085 MHz and 1095 MHz
Bandwidth (3dB) : 10 MHz
Fractional Bandwidth : 9.2 MHz
Filter Q : 109
Estimated Qu : 2712.84
Loss, based on this Qu : 0.698 dB
Passband Delay : 63.662 ns
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Quarter Wavelength : 68.76 mm or 2.707 inch
Length interior Element : 61.15 mm or 2.407 inch
Length of end Element : 61.49 mm or 2.421 inch
Ground plane space : 30 mm or 1.181 inch
Rod Diameter : 8 mm or 0.315 inch
End plate to center of Rod : 15 mm or 0.591 inch
Tap to shorted End : 2.79 mm or 0.110 inch
Impedance end Rod : 88.549 Ohm
Impedance inner Rod : 93.734 Ohm
Impedance ext. line : 50.000 Ohm
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**** Dimensions, mm (inch) ****
#    End to Center    Center-Center    G[k]    Q/Coup
0    0.00 (0.000)
1    15.00 (0.591)    52.41 (2.063)    1.000    0.707
2    67.41 (2.654)    52.41 (2.063)    2.000    0.707
3    119.82 (4.717)    0.00 (0.000)    1.000    1.000
4    134.82 (5.308)   
**** Box inside dimensions ****
Height : 68.76 mm or 2.707 inch
Length : 134.82 mm or 5.308 inch
Depth : 30.00 mm or 1.181 inch


The case was milled from aluminium. The rods are made of copper. No surface coating was applied. Dimension Drawing Case. As the rods have been delivered slightly longer (they were all 62.5 mm), the center-frequency was slightly shifted downwards (1070 MHz). See amplitude response. As a a result, we decided to shorten just the middle rod, as the middle tuning screw was fully outside. After having done so, the bandpass developped its full beauty. See pictures below. (Yes, we know, that photographing the screen is not state of the art). It is to be assumed, that shortening the outer rods lowers the insertion loss further. It is advantageous to solder the taps before mounting the rods into the case. (heat transfer)

A second verification was done by my apprentice, who designed a 50 MHz Bandpass at 3 GHz. The report is attached below.

Andre (I assume from South Africa) sent the following pics :

Lars (I assume from Norway) sent the following pics :

Philipp, OE8JPQ (I assume from Austria) sent the following pics :

THANK YOU ALL FOR THAT COLOURFUL FEEDBACK !!!

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