Pages

▼

Saturday, April 14, 2018

Low Pass Filter for the 10m & 11m Bands

Another one from the archives :o). This is a design for a low pass filter for 29 MHz and would be suitable for anyone on the 28 MHz or 27 MHz bands who needs a filter to reduce harmonics. This is especially valid for anyone using a low powered radio to drive an external amplifier. e.g. Amplifying  4w to say 50-100w.


Coils...12 SWG wire. 6 turns. 8mm internal diameter. Coil length 19mm.

Capacitors...High Voltage ceramic or Mica. 110pf Cap is 100pf and 10pf in parallel.

A suitable enclosure might be a diecast box with the filter built on copper clad board. Note the screen which can be made from the pcb material as well. If you can live with a slightly lower performance in terms of filtering, you can omit the internal screens but make sure you put it in a metal box.
at

2 comments:

  1. PeteAugust 18, 2020 at 4:59 AM

    Perfect. Thanks for sharing.

    ReplyDelete
  2. SteveMay 14, 2026 at 2:12 PM

    I built this, but it did not turn out great. The thing has an SWR of over 3 with a dummyload after it on both bands (both on a NanoVNA and a transceiver. The dummyload has 1.0). The coils initially measured 0.33 instead of 0.27uH, and did thus not reach the 19mm length, so I dragged them apart a bit, but it did not help.
    Anything sticking out as bad construction wise here?
    https://postimg.cc/gallery/mMyd1vV

    ReplyDelete