Homebrew BPF and comparison with W3NQN type.

Some results of comparative measurements between homebrew filters ( 5 pole ) and commercial W3NQN band filters, which have an excellent reputation on the market.

My homebrew filters were housed in an individual box, with SO-239 in/out, the toroids being maintained in position by liquid rubber glue.

The values of  C are according to 'MELITTA 2' table, the corresponding values for L  should be calculated with for example the Mini Ring Core calculator of DL5SWB and the corresponding tores used. I recommend to use the T106-6 type.

Resonance of each circuit to be adjusted with care by adjusting number of turns and spacing of windings. Proceed step by step, first roughly adjusting each LC-circuit on an impedance bridge, then inserting it into the  RF path, while monitoring the SWR / return loss.  Only when results are acceptable, add the next section, till the 5 are eventually in line. At the end, SWR in-band should be lower than 1.2.

Example of the filter for 40m band

Band filter for 10m band. 50 dB reject on 15m with homebrew filter vs 30dB for W3NQN, but inline loss 1.5 dB instead of 0,5dB


Band filter for 15m band. 55 dB reject on 20m with homebrew filter vs 38dB for W3NQN, 47 dB on 10m vs. 55dB. Inline loss 1.5 dB instead of 0,5dB


Band filter for 20m band. 55 dB reject on 40m with homebrew filter vs 47dB for W3NQN, same reject for 15m band


Band filter for 40m band. 54 dB reject on 80m with homebrew filter vs 48dB for W3NQN, same reject for 20m


Band filter for 80m band. 53 dB reject on 40m with homebrew filter vs 56dB for W3NQN.


Homebrew filters ( 5 pole ) are providing similar or even better results than W3NQN type, at the cost of a slight pass-through loss  (1.5 dB instead of 0.5dB for W3NQN).  This is not a disadvantage, considering that  the filters are inserted between transceiver and PA, as in general 50 to 60w drive are sufficient to run the PA up to full power.  The loss will off course be dissipated in heat, so 100w PEP is the maximum they can handle safely.

These filters can be easily combined a '5-pack'  with relay switch box, controlled by a band decoder. For ICOM the band decoder solution based on LM3914 works fine, there are several schematics to find on the www.