Pipecap filters are one of the easiest 10 GHz filters to build. I found an interesting article by Paul Wade, W1GHZ, describing this filters and decided to build and measure one of them.
A pipecap filter for 10 GHz band can be built very easily using the following parts:
- A 15 mm diameter copper pipecap. - Two SMA connectors. - Two semiflexible 0.140" coaxial cables. - A piece of dual layer PCB boardFor 10 GHz band, Paul recomends the following filter dimmensions:
-pipecap diameter: 15mm (1/2") -probe lenght: 4mm -probe spacing: 9.5mm
Smaller probe lenght leads to sharper filter but losses increase. 4mm seems to be a good compromise lenght.
I have taken several pictures during filter construction, so I think that there is no need to describe the procedure using words.
Sadly I lost a closeup screnshot showing -3dB points, but the results are the following:
- Filter loss: 2.11 dB - Filter -3dB bandwidth: 142 MHz
I disassembled the filter with an air soldering station, reduced probe lenght to 2.5 mm, reassembled and tested again.
- Filter loss: 4.51 dB - Filter -3dB bandwidth: 53.75 MHz
We can see that reducing probe lenght leads to higher losses and narrower filter response. Interesting to note that at higher frequencies filter has no effect and works as a bypass, hi. It appears that probes are working as little antennas very close each other, inside the cavity. I have not found in the literature sweeps of 10 GHz filter so high in frequency, so if you find them please let me know. I would like to check this behaviour.
Copyright (c) 2013 Ramiro Aceves . Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".