Recent concepts for Multi/Multi, M/2 or SO2R station are based on monoband antennas. The reason is that you are by default operate on two bands simultaniously.

In case of SO2R stations you are not transmitting simultaniously by you are transmitting on lets say 40m while searching for mults on lets say 20m. What you need is the technical capability of your station that the single bands are decoupled so that reception is not affected by the nearby transmitting signal.

The most challenging case is to use a multiband antenna like your three or four band yagi antenna and use it for SO2R. That means you transmit with full legal power on 20m while using the same antenna to chase mults on 15m or 10m.

The way you do that is using band filters and a multiplexer.

I am using 4O3A bandpass filters at the moment. My own approach to homebrewed high-power bandpass filters is in the prototype phase.

I started this projekt in 2013 when there were no quadplexer (4-combiner outputs available)

The concept is based on the use of VA6AM, 4O3A or equivalent band filters. The filters show a neighboring band isolation of -55dB (typical). To protect the frontend of the receiving path you should avoid input levels above 0 dBm. Full legal power in Germany is 750 W. For the ease of calculation the following math is based on 1 kW of output power which corresponds to +60 dBm.

Lets assume that we transmit with 1 kW on 7010 kHz into the Yagi. The second receiver is tuned to 14020kHz and connected to the same antenna.

What we are doing is using a quadplexer with a certain frequency response and a bandpass filter to protect the receiver of the second radio tuned to 14020 kHz.

The bandpass filter has -55 dB attenuation on the neighboring band – resulting in +5 dBm at the second receivers input. This is still beyond the design goal of < 0 dBm plus safety margin.

So we add a multiplexer. The multiplexer is designed for  -25 dB on the neighboring band so that we end with -20 dBm at the second receivers input. This is equivalent to a 9 +55 dB signal.


In fact it should be named multiplexer or n-plexer, because you are able to add as many band segments as you need for your purposes.

The single bandpass elements are designed as coupled series circuits. The bandpass elements offer minimum attenuation in the passband and -25 dB attenuation on the neighboring bands.

Here is the frequency response of the quadplexer design:

The notches result from open ended outputs which are not present in this depth with the bandpass filters connected.

The attenuation [dB] with open outputs (including the notch effect!) currently is:

            80m        40m          20m          15m          10m

40m     – 26         – 0.2         – 43           – 49           – 45

20m     – 36         – 40          – 0.26        – 33           – 45

15m     – 41         – 47          – 23           – 0.26        – 27

10m     – 48         – 55          – 35           – 23           – 0.41

I will add the attenuation with real world conditions (bandpassfilter added) later.

Here ist the frequency response of the the quadplexer in combination with the 15 m bandpass filter:

The passband attenuation is -0.8 dB. 20m is at -82 dB and 10m is at -75 dB. The other bands are at < -100 dB.

The design of the quadplexer is simple. Capacitor coupled series circuits and proper shielding are used.

The components are Russian high power ceramic capacitors, and so far, toroid inductances using several T-157-17 cores. I choose toroid coils mainly for the ease of tuning which is crucial. Don´t start this project without having access to a network analyzer.

The use of air wound coils is still under investigation to improve the passband attenuation.

The design values are:

                       series L      series C        coupling C

40m                7.50 uH      75 pF            430 pF    

20m                3.75 uH      39 pF            220 pF

15m                3.21 uH      20 pF            150 pF

10m                2.81 uH      12 pF            110 pF 

The housing is made from a PCB, with the connections made by H-2000 flex coaxial line.

A german article (p.27) was published in the newsletter of the Bavarian Contest Club (BCC).

usefull links:

SO2R @ DF9LJ   2013   


SO2R @ DF9LJ 2022


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