K9DP QRP Bandpass Filter


  • Designed to reject interference from other near by stations operating on different bands.
  • Lightweight and portable design, only 3.4 inches (85mm) in length and weighing just 23 grams. Perfect for Parks on the Air (POTA), Summits on the Air (SOTA) and Field Day.
  • Tested up to 20 watts with SSB, CW and digital FT8.
  • High quality C0G (NP0) capacitors and Micrometals toroids.
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The K9DP QRP Bandpass filter allows QRP (low-power) amateur radio operators to reduce or eliminate unwanted interference in their signals from other near by stations. Primary designed to be lightweight and portable. The filter is ideal for use in field operations such as Parks on the Air (POTA), Summits on the Air (SOTA), and Field Day. It has been tested at power levels up to 20 watts with SSB, CW, and digital modes. The female BNC version measures only 3.4 inches (85mm) in length and weighs just 23 grams. The male/female BNC version measures 3.7 inches (94mm) in length and weighs 35 grams. Made with high quality C0G(NP0) capacitors and Micrometals toroids. Its small size and low weight make it easy to take with you wherever you go.

These filters have a 50Ω impedance and are symmetrical, so it's not important which end is the input or output.

Caution: High SWR can damage the filter. The filter should be inserted inline between the transceiver and tuner, rather than between the tuner and antenna. If your radio has an internal tuner, be sure to use a resonant antenna or an antenna with a matching circuit, such as an EFHW (end fed half wave), in combination with the filter.

If you're interested in kit versions please let me know. (Building is a little more complicated than my BCI kits as it requires tuning with a VNA for best performance.)

Rejection Performance

Bandpass Filter 80 Meters 40 Meters 20 Meters 15 Meters 10 Meters
40m Filter (Blue) -20dB <-0.5dB -35dB -52dB -63dB
20m Filter (Green) -45dB -23dB <-0.5dB -24dB -38dB
15m Filter (Yellow) -53dB -36dB -5dB <-0.6dB -19dB

Additional information


10 Meters, 15 Meters, 20 Meters, 40 Meters, 80 Meters


Female BNCs, Male/Female BNCs


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