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Stepped_Impedance_Lowpass_Coax.php   21028 Bytes    09-11-2014 19:44:50

Stepped Impedance Coaxial Lowpass Filter Designer

An electronics project - for the mechanic in you :-)

N-Stecker Your browser does not support the HTML5 canvas tag. N-Kupplung

Drawing not to scale. Just for illustration.

Cutoff Frequency




Passband Ripple


First Element is

System Impedance


Form of the Coax

Dielectrics, rel. Permittivity

Tube inner Diameter / Width


Smaller Diameter (ind.)

       (High Impedance)

Larger Diameter (cap.)

       (Low Impedance)


Notes :

Soldering the connector is much easier, if you choose an odd number of elements with inductance at both ends. You may also add pieces of the system impedance (Section #0) as long as required. For mechanical stability, the minimum wire diameter is limited to 1.0 mm (0.039 inch). This also limits the maximum impedance.

In order to isolate the outer tube from the 'capacitances', we suggest the following materials which have an dielectric constant near 1 (air) : Plastic 1.1, Paper 1.2, ...

Assembly and Test

Stepped Impedance Coaxial Lowpass Filter

Stepped Impedance Coaxial Lowpass Filter - Assembly Stepped Impedance Coaxial Lowpass Filter - Assembly
Stepped Impedance Coaxial Lowpass Filter - Assembly Stepped Impedance Coaxial Lowpass Filter - Assembly
Stepped Impedance Coaxial Lowpass Filter - Assembly Stepped Impedance Coaxial Lowpass Filter - Assembly

For verification reasons, we designed a Butterworth lowpass, 7th order. Cutoff was 1190 MHz. We have chosen the Low Impedance to be 6 Ω resulting in 18 mm diameter, the High Impedance was 179 Ω resulting in almost 1 mm diameter. Therefore we could use standard copper rods which had to be cut only in length. The outer tube was made approx. 4 mm longer in order to have some space to solder the N-connectors. On one end of the tube, a hole was drilled, so that the inner conductor could be soldered after the assembly. As spacers, we glued 4 carton stripes.

The picture below shows the insertion loss, which was approx. 0.3 dB. It was observed, that the 3 dB corner frequency was at 960 MHz and not - as calculated - at 1190 MHz. As we found no error in the formulas it must be the material. We assume that the 'capacitors' were longer than ordered.

Stepped Impedance Coaxial Lowpass Filter

References • Further Reading • Brainfood

Sudipta Das, Dr. S.K.Chowdhury, “Design Simulation and Fabrication of Stepped Impedance Microstrip line Low Pass Filter for S-band Application using IE3D and MATLAB“ IJECT Vol. 3, Issue 1, Jan. - March 2012

Note of Thanks

We would like to thank Rob, K7QJ for his valuable feedback in order to make this calculator a 'handy' tool.

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t1 = 6577 d

t2 = 200 ms

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