The simplest bandpass filters are Bill Sabin's filters found at the ARRL QEX download site from the file NBPF.ZIP. These filters are excellent and the 20 meter filter from this file is currently used in the receiver (as shown above).
The coils are ten turns #26, 240 degree span, with a T50-6 toroid core.
These filters will work with fewer parts than the Progressive Communication Receiver BPFs, and do not need careful adjustments. Sabin's filters can be wound, soldered and placed in the circuit ready to go.
This filter comes from the Progressive Communications Receiver (November 1981 QST & The ARRL 1986 Handbook). The values given are for 20 meters.
Designed for 50 ohm input and output. It is best tuned using a signal generator and any kind of power meter or oscilloscope. After the receiver is working, it can also be tuned using on-the-air signals. If you are only interested in one segment of the band, the 180 pf variable capacitors can be adjusted for maximum signal. The trimmer between the sections should be adjusted for approximately 2.3pf.
A set of bandpass filters that will provide a little better performance on 20 meters is illustrated in the book Solid State Design for the Radio Amateur, Hayward and DeMaw, ARRL Publication, "An Integrated Contest-Grade CW Station", Page 225. On Page 226, they go into considerable detail on the specifications and adjustments of their filters.
To find information on bandpass filters on the internet check out Bandpass and I.F. Filter Information at the QRP HomeBuilder web site. He has a very nice 20 meter and 30 meter bandpass filter using either capacitors or transformers for matching to 50 ohm input and output. Very good advice is given on building the filters.
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