An electronics project - for the mechanic in you :-)
Drawing not to scale. Just for illustration.
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
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.
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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