For Multi-Multi or SO2R operations,  quality bandpass filters are needed.

Not only  produced harmonics have to be rejected, but any below frequencies also, to minimize distortions produced by receiver itself in presence of strong outband signals.

SO2R filters

121 dB attenuation
  • Low cost
  • Easy to build
  • Alignement is a nightmare !

This filter has to be upgraded for SO2R operations

Suppose you already have a 1KW QRO producing 2’nd or 3’rd harmonics at -50dB over fundamental. Unwanted signal is +60dBm – 50dB = +10dBm … way too much, so where is the beef ?

The beef is where another cheap 80 dB high power low pass filter will retrofit power amplifiers. Yet , outband RX  2 notch filters  will squeeze out another precious decibels.

  • 5 Poles Bandpass filter+ 3 poles low pass filter.
  • About 0.5 dB insertion losses.
  • About 121 dB rejection at second harmonics.
  • 100 W continuous power.
  • Low heat dissipation.
  • SWR 1:1 – 1:1.4 for CW segment only.
  • T94 size toroids.
  • Quality silver mica capacitors.
  • Easy to build.
  • Not so easy to tune.
  • About 200 Euro total cost for 6 filters.

D.I.Y. or buy, that is the question !

31 January status update: new F/2 notch filter added

Example : For 20 meters band, F/2 = 7 Mhz
If transmitting in 40m and receive in 20m, transceiver second harmonic is at minus 120 dBm.
  • What happens with 40m carrier, aint it to big ? 50dBm (100w) - 80dB (20m filter) = -30dBm
  • This 30 dBm is too strong for Icom IC7300, and transceiver itself will start producing second harmonic due to intermodulations, internally !
  • Another 40 dB notch filter was added to knock off any remaining 40 meters signal to a safe zone below S9 inband, so listening in 20m and transmiting in 40m, total second harmonic should be heard at S1, maybe S2 level.

2 thoughts on “SO2R bandpass filters

  1. Hi Gabriel
    Interesting stuff with these filters. I have my station setup for SO2R with 2 x IC7610. I have some space here with seperate RX antennas about 100m away from the TX antennas. I just have filters on the RX side only, 5th order Chebyshev and these switch automatically with a band decoder. This results in virtually no interference, but I’m interested in your idea with band pass followed by low pass. I think this works better for you than the notch idea ? I might try this also on transmit side too with my other TX antennas that are in the same area. 73’s Kevin VK6LW

  2. Hi Kevin
    I seems the Notch filter gives great rejection for lower outbands but is quite unstable and narrow bandwidth.
    My icom 7300 give me headaches when dealing with “below” outband signals over S9+30 therefore somehow, if listening in 7 Mhz and transmit on 3.5, I have to reduce this signal even stronger not for harmonics but to prevent transceiver itself not to produce second harmonic. Low pass is used to reduce even better transmit harmonics, nothing special here while IC 7300 has good spurs levels.
    I did an extra 40 dB rejection using coax notch filters but they are too bulky. Bottom line, will take some time until I will develop cheap yet powerful Multi-Multi operation filters.
    73′ and good luck !

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