10 GHz PIPECAP FILTER

(2014-11-01)

Preface

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.

FIGURE 0: A PIPECAP FILTER

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 board

For 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.

Filter construction

I have taken several pictures during filter construction, so I think that there is no need to describe the procedure using words.

FIGURE 1: 15 mm DIAMETER COPPER PIPECAP AND DOUBLE SIDE PCB

FIGURE 2: INPUT AND OUTPUT SMA CONNECTORS AND SEMIFLEXIBLE COAXIAL CABLES

FIGURE 3: STRIPPING COAXIAL ENDS TO GET THE PROBES

FIGURE 4: READY TO BE SOLDERED

FIGURE 5: SOLDERING THE NUT TO PIPECAP TOP

Filter measurement

I have measured filter S21 using and old Marconi 10MHz - 20 GHz Microwave Test Set.

FIGURE 6: READY FOR MEASUREMENT ON THE MICROWAVE SCALAR ANALYZER

FIGURE 7: 500MHZ-20 GHZ SWEEP SHOWING S21 PARAMETER

FIGURE 8: 5-15 GHZ SWEEP S21

FIGURE 9: 8-12.5 GHZ SWEEP. S21=-2.11 dB

Sadly I lost a closeup screenshot showing -3dB points, but the results are the following:

- Filter loss: 2.11 dB
- Filter -3dB bandwidth: 142 MHz

Reducing probe lenght

I disassembled the filter with an air soldering station, reduced probe lenght to 2.5 mm, reassembled and tested again.

FIGURE 10: DISASSEMBLED PIPECAP FILTER TO REDUCE PROBE LENGHT TO 2.5 MM

FIGURE 11: 10-20 GHZ SWEEP WITH 2.5 MM LENGHT PROBES---

FIGURE 12: -3dB BANDWIDHT MEASUREMENT. S21=-4.5 dB

- 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.