Bandpass Filters for Portable Ops

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These are QRP rated 3 Pole Butterworth filters based on a design of Lew Gordon's, K4VX, published in QST September 1988. They feature simplicity, almost no insertion loss, and reasonable rejection on the other ham bands. They are intended for Field Day or other portable ops where antennas are close, and transmitters interfere with rigs on other bands. They are simple, easy to build, inexpensive, and perform well - all good QRP characteristics.

I tweaked Lew's inductor values to accommodate capacitors that I had on hand. Many thnks to Jim Kortge, K8IQY, for sweeping the filters to determine their real world performance. Notice that his spectrum analyzer plots dont look quite as stellar as Elsie's plots. The lowest insertion loss point is noted in the marker on the plots, and is off of the calculated qrp frequencies. And the top of the curve is drooping down frequency rather than being horizontal. In the lessons learned dept: Next time I'll try using trimmer capacitors, and then peaking the filters after building them. I think these will work ok in actual practice though, and i will give them a workout at the next multi rig portable outing.

If your rig needs some front end help during multi-op portable operations, one of these filters may be exactly what you need. I've created a family of 40M, 30M, and 20M versions for my portable use, and the passbands all look similarly good. Links to documentation for all the filters are listed below.

Elsie 20M Bandpass Curve
K8IQY 20M BPF Spectrum Analyzer Plot
Elsie 20M Rejection Per Band Table
Elsie 20M Filter in an Altoids
Elsie 30M Bandpass Curve
K8IQY 30M BPF Spectrum Analyzer Plot
Elsie 30M Rejection Per Band Table
Elsie 30M Filter in an Altoids
Elsie 40M Bandpass Curve
K8IQY 0M BPF Spectrum Analyzer Plot
Elsie 40M Rejection Per Band Table
Elsie 40M Filter in an Altoids
A family Photo
Schematic and Component Values
Typical Manhattan build in an Altoids
Downloadable Elsie file with which to experiment. Right click and Save Link As
Save it in the same folder where you have ELSIE installed, then launch ELSIE, choose Open Old, and then bpf_20m_3_pole_butterworth.lct.
All curves and schematics displayed here are screen captures from the student version of Elsie, an LC filter modeling program from Tonnesoft

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