The SO2R general diagram below shows main sections of the SO2R setup. This is my preference shown on a diagram below with two radios, two computers and two amplifiers . It is easier to manage each band from dedicated keyboards and in general the total setup is simpler when all separated.

  • For Low Power setup diagram RF Sections contains only transceivers without LP BPFs and Amplifiers. All other diagram sections are the same.

SO2R Diagram

With today’s modern equipment and software, it can be only one radio, one amplifier and one computer for SO2R setup. Some HAMs prefer one keyboard and some two but it is all a personal preference and will not change the whole idea of the SO2R operations.

  • HP BPF cooling fans can be turned on only when band chosen to run only two fans simultaneously to reduce the noise level. Voltage to each fan can be provided from a Switch Controller.
  • LP BPFs before the amplifier are optional but can be recommended to get harmonics level down. LP BPFs can be in separated boxes with some external switching or 6-band BPF set controlled by the same Switch Controller for a full automation.
  • For a Stacked tribanders and the maximum flexibility one Triplexer can be dedicated to each Tribander with a StackMatch and BPFs after the Triplexer. Please, see the CR6K diagram  for a general idea of such a setup.
  • Audio Switch can be very simple by design to allow to listen each radio in each ear but can be improved to allow switching audio between ears controlled by PTT (listen The RX radio while TX on another one).
  • Today’s transistor amplifiers allow to use only one Amplifier for SO2R setup because the amplifier can be switched between bands almost momentarily. Tube auto-tuning amplifiers are too slow for the next moment switching between bands.
  • Some of new SDR transceivers allow to use one transceiver for SO2R setup but it can make your setup and software alignments more complicated compare to a diagram shown above.

Tribander antenna with Triplexer and BPFs gives three fully independent bands which can be used in any station setup, such as SO2R, MS, M2 and MM. 

Today’s tribander is a full size element multi-band antenna with a performance closed to a mono-band antenna. A traditional tribander with LC traps is quite popular today as well. If you are limited by space to install a few towers it could be the only option to try a serious contesting in SO2R category.

  • Please, remember that no-trap full size tribanders are better compare to antennas with traps in term of harmonics interference from 20M TX to 10M receiver side.

If band isolation of the filter system of Triplexer and BPFs is a sufficient number than there is no interference between bands at all except the 2nd harmonics from 20M TX to 10M band RX.

Please, listen how Triplexer with BPFs sounds on most difficult bands, 20 and 10 meter bands. This is OR2F and ON5RA  as OT70OSB with their WRTC-2018 equipment (Low power BPFs with VA6AM HP Triplexer).

  • Channel L – FT-1000MP, Pout = 120W, QRG= 14216 KHz fixed
  • Channel R – Elecraft K3, Pout = 110W, QRG= S&P 10m band (CW + SSB)
  • After 03:38 from beginning of the file, 10M band switched to SSB and you can hear the harmonics noise increased.
  • They were able to work stations +- 5KHz from harmonics around 28432 KHz frequency.

10 and 20 meter bands audio file.


  • I am not considering a damage signal level to the radio as this number usually unknown and could be quite different.

Lets start with a radio blocking level. This level means that the radio blocked and no reception can be done.

As an example, for the main receiver of TS-590S and PREAMP OFF the worst case is +20dBm and with PREAMP ON it is +10dBm with OFFSET frequency >100KHz ( Up-Conversion).

  • Blocking level + 10dBm is 0.01watt or 0.7 volts signal level at the 2nd radio antenna port if the 1st one in SO2R setup is on TX mode.

Now, lets calculate the minimum band isolation with different output power levels with level of 0.7 volts going to the 2nd radio while 1st on a TX mode.

 At this level of 0.7 volts the second radio will be blocked while the first one transmitting, so we need a band isolation to get that level lower than that level from our Triplexer + BPFs system.

The required filter system band isolation at a transceiver blocking level:

  • 100 W   – 40dB   or  -30dB with PREAMP OFF 
  • 1500 W – 52dB
  • 3000 W – 55dB
  • If your filter system delivered only -40dB of the next band attenuation you cannot operate 100 watt two radios the same time.
  • Numbers above show that triplexer alone with isolation level of -30dB cannot provide a sufficient isolation level if used without BPFs for 100W radios.

How to define the comfortable level of band isolation when we can operate both radios the same time?

  • As per Rob Sherwood, NCØB measurements, IC-7300 and IC-7610 OVF (overflow) level is -10dBM  out of a pass-band signals. It means Triplexer+BPFs set MUST deliver a  better than that number to allow operate a new SDR radios.

I can assume that -13dBM or lower signal level can be handle by most of today’s radio. 

  • -13dBm is 0.05mW or 50mV on a radio antenna port.
  • This is 59 + 60dB on a radio S-meter and as it is a level from adjacent bands it is actually a very safe approach for calculation.

The numbers below are a very safe levels of a band isolation for using Tribander antenna with Triplexer and connected BPFs:

  • 100 W   – 66dB
  • 1500 W – 74dB
  • 3000 W – 77dB

So, for a low power setup Triplexer with BPFs system should deliver -66dB of adjacent band isolation to work with any available radios including SDRs.

  • Good filter system should be designed to meet those band isolation numbers with a minimum filter Insertion Loss and the best Return Loss.
  • It should be no trade-off  between higher IL or lower RL in favor of the maximum possible band isolation ( 100dB level of isolation is NOT a MUST thing to achieve). This approach will increase the filter system reliability with filter low loss and good SWR over entire band.

VA6AM High Power Triplexer with BPFs set is able to deliver better than -85dB isolation level.

  • For many today’s radios even -50-55dB of isolation between bands is a sufficient number to operate SO2R at 100 watts output power.
  • I am not considering a TX phase noise as if this noise at a high level it could be a limitation factor for the adjacent bands reception. Usually, this is a critical factor for the same band TX and RX  the same time.
  • VC3S used IC-7300 with a success with VA6AM Triplexer and  BPFs to setup a new Canadian M/S CQWW RTTY record. The filter system kept band isolation at a required level and IC-7300 receiver was not overloaded. Their short story and 3830 Result.

WRTC contest is a M2 category for participating teams. It means they use two bands simultaneously all the time and two switching approaches can be used.

Different filter systems can be used for the SO2R setup:

  • one set of BPFs switchable between radios using 2×6 antenna switch
  • fully independent two sets of BPFs switched by internal relays as required 

wrtc-1

I think the first approach has some benefits, such as simpler switching and as a result simpler way of replace any BPF if broken and less expensive.

The same time the first approach requires a 2 x 6 switch with good band isolation. Please, see some graphs and measurements. It is not a problem at all for low power setup but with a higher power it should be considered. 

For a high power setup, 2×6 switch could be a limitation factor for high band isolation and some other approach can be applied using 1×2 switches which can be designed with up to -90dB of a port isolation. I can say that if your 2×6 or 2×8 switch can deliver -75dB or better of band isolations for 28MHz it is a good switch to use.

Please, see below some of possible connection diagram. 1×2 switches installed a little different than  those A/B switches on NP2N diagram below allowing more band combinations for every radio in SO2R or M2 configuration.

SO2R (1)

Below is NP2N connection diagram using Top Ten Devices A/B switches and VA6AM high power triplexer and BPFs.

setup

I highly recommend that every 1×2 switch installed after BPFs contains 50Ohm resistor to connect unused band port with 50 Ohm load.

Top Ten Device A/B switch has open relay ports. George, W2VJN ran some test to see a difference between 50 Ohm loaded and open switch ports.

It is up to 7.7 dB better band isolation with 50 Ohm loads connected to every port.  

  • I would like to stress it out because Triplexer is a balanced filter system and required every port to be loaded for the best performance and reliability.
  • If triplexer used at its maximum power, 50 Ohm load to every unused port MUST be installed.  

TABLE 1. High Power Triplexer with BPFs – No back terminations on BPFs

BAND 10 15 20 40
10 -0.316 -89.3 -102.0  
15 -82.4 -0.409 -84.9  
20 -96.7 -89.3 -0.338 -69.8

 TABLE 2. High Power Triplexer with BPFs – 50 ohm back terminations on BPFs

BAND 10 15 20 40
10 0.316 -89.9 -109.7  
15 -82.6 -0.409 -85.6  
20 -100.1 -85.0 -0.338 -69.6