January 11, 2023: New SO2R Controller Design

This page describes the original CTR2-Mini SO2R Controller I built back in July 2022. I’m currently working on a newer version of this controller. The concepts are still the same but the new controller will take advantage of the new features I added to CTR2-Mini+.

The new version will have a simpler interface and will be built in a conventional equipment case so it can sit on your radio or on a shelf instead of on your desk. Visit this blog post for the latest information on the redesign.

I’m sure you’ve seen the acronym SO2R thrown around in popular amateur radio publications, web sites, and manufacturers brochures. You may or may not know what it stands for. If you do, you probably only think it’s something contesters do to increase their scores.

SO2R stands for Single Operator Two Radios. One operator using two radios and two antennas, usually on separate bands. Contesters like to use this mode because it allows them to listen to two bands simultaneously while in ‘search and pounce’ mode. Each radio is on a separate antenna and usually the audio from one receiver is routed to the left headphone speaker and the other receiver is routed to the right. When they find another station calling CQ for the contest, they switch to that radio and make the contract. Sounds complicated, and it can be. Sounds mind blowing, and it can be that too!

Some manufactures build SO2R capabilities into their radios, most notably FlexRadio. But to get it, expect to pay a premium. You have to upgrade to the Flex 6600 radio to get true SO2R operation. The cheaper Flex 6400 doesn’t support separate antennas for each slice but ‘kind of’ gives you an SO2R experience.

Does that mean that those without Flex radios are just out of luck? No, of course not. Google SO2R and you’ll be flooded with SO2R options. What I’m going to describe in this post is how I implemented SO2R based on my CTR2-Mini radio controller.

What you may not know is that SO2R operation can be useful for non-contesters and radio experimenters too. For example…

  • What if you wanted to monitor 10 meters for an opening while you were working SSB on 20?
  • Or you want to listen to the same frequency on two radio/antenna sets so you could use the combination that worked the best for the station you want to talk to, and instantly switch to the other radio/antenna if conditions change.
  • Or you want to tune both radios at the same time while operating in different bands effectively combining two bands into one.
  • Or perhaps you have one late model radio with a spectrum display and wish your QRP rig or other older radio had one too.
  • Or maybe you have an HF rig and a VHF rig and just want to monitor them both and be able to switch between them but use with the same mic and headphone setup.
  • Or you want to be able to control your radio from somewhere else in the house, or in the world.

The CTR2-Mini SO2R system can solve all of these problems.


If you’ve been following this blog you already know about the CTR2 and CTR2-Mini radio controllers. CTR2 was designed to let you select one of 16 radios and choose from up to 8 antennas. It features DSP audio processing with audio and RF routing, but you can only use one radio at a time. CTR2-Mini was designed as a simpler solution for radio control. It too can control multiple radios but it has no audio management capabilities build in. However, the new CTR2-Mini Audio Controller can easily be added to your Mini to add audio management.

The CTR2-Mini SO2R Controller aims to solve some of the limitations inherent in both CTR2 and CTR2-Mini by bringing two radios together into a seamless user interface that allows you to monitor receive audio from both radios, enable Tx on one radio with a simple button press, provide support for two microphones, provide Tx and Rx audio gain adjustments, and automatically route your keys, mics, headphones, and USB audio to the selected radio at the push of a button. Two CTR2-Mini‘s are required in addition to two radios and two antennas. The Minis gives you independent control over each radio. In addition, the Minis can be linked together with either WiFi or Bluetooth LE so either Mini controls both radios. This linking feature provides additional features not available on other SO2R configurations.

To see what the SO2R controller can do, let’s look at the scenarios I listed above.

  • You want to monitor 10 meters for an opening while you are working SSB on 20:
    • Set Mini-A to your 20 meter operating frequency and mode and set Mini-B to 10 meters. Switch Mini-A’s Link Mode to Basic linking. Mini-B’s sync mode automatically follows.
    • Basic linking allows you to tune both radios independently but shares the keyer, keys, mics, headphones, USB audio, and function keys with both radios. 20 meters appears in your left headphone speaker and 10 meters appears in your right speaker.
    • Press the Tx Enable A button on the SO2R controller. Your key, mics, and function keys are now routed to Mini-A and on to Radio-A. From here you can operate Mini-A and Radio-A as normal. You can tune Mini-B/Radio-B around 10 meters and listen for activity while you operate on 20.
    • You can have the Keyer Speed window open on Mini-B and adjust the keyer speed on Mini-A on the fly while operating the Mini-A/Radio-A combination.
    • When you hear activity on 10 meters press the Enable Tx B button on the SO2R controller and instantly your keyer, key, mics, and function keys are routed to Mini-B and on to Radio-B. Tx Enable on Mini-A is disabled and it is enabled on Mini-B. You can now operate on 10 meters while still listening to 20.
    • You can mute 20 meters by turning off the Rx-A switch. When you do this, Radio-B’s audio appears in both headphone speakers.
    • You can open the Mode menu and Push or Pull the frequency and mode from one Mini/Radio to the other. This allows you to have your best radio on one Mini and an older radio (or even just a receiver) on the other Mini.
  • You want to listen to the same frequency on both radio/antenna sets so you can use the combination that works the best for the station you want to talk to, and instantly switch to the other radio/antenna if conditions change.
    • Switch Mini-A’s Link Mode to Basic+Freq. In this mode the basic operating features are synchronized between the Minis as described above. In addition, the frequency of both radio’s are synchronized so they’re operating on the same frequency. Radio modes can be set independently.
    • The receive audio from Radio-A and its antenna appear in the left headphone speaker and the receive audio from Radio-B and its antenna appear in the right headphone speaker.
    • A special Zero-Beat control in Mini-B’s Link menu allows you to fine-tune Radio-B’s frequency slightly higher or lower so that the frequency of the audio coming from both receivers is exactly the same. Yes, there is a difference!
    • You can tune both radios by tuning either Mini or either Radio. They will track each other.
      NOTE: If you’ve added a zero-beat offset to Mini-B the frequency on Radio-B will be offset by that amount.
    • When you want to work a station you simply press the Tx Enable button on the SO2R controller to enable one radio to transmit. Your keyer, key, mic, and function keys will be routed to that Mini and Radio.
    • If the received signal is better on one receiver simply turn off the other receiver’s Rx Audio switch on the SO2R controller.
    • The obvious question is, “Won’t you be listening to your transmitted signal on the offline Radio?” Well, yes you would, except in this mode, when you enable the transmitter on one Mini/Radio, the offline Mini/Radio’s receive frequency is automatically set 10 MHz above or below the signal. This offset is determined by the operating frequency. If you’re transmitting below 15 MHz the other receiver will be offset 10 MHz above your carrier. If you’re transmitting above 15 MHz the other receiver will be offset 10 MHz below your carrier.
    • The offline Mini/Radio returns to the carrier frequency when you turn off Tx Enable on the active Mini/Radio.
    • This has been an interesting mode to experiment with 🙂
  • You want to tune both radios at the same time while operating in different bands.
    • Tune Radio-A to start of the first band, say 7.000 MHz. Next, tune Radio-B to the start of the second band, say 14.000 MHz. Now set the Link Mode to Basic+Track on Mini-A and enable the link.
    • The Minis and their radios will now track each other exactly 7 MHz apart. Tune Radio-A to 7.05 MHz and Radio-B tunes to 14.05 MHz.
    • This mode allows you to work two bands as if they were one. When you hear a station you want to contact on one radio simply enable the transmitter on that radio.
    • NOTE: When Tx is enabled on either Mini/Radio, tracking is temporarily turned off and you can tune either radio to any frequency. Frequency tracking will be re-enabled when Tx Enable is turned off on both Minis.
  • You have one late model radio with a spectrum display and wish your QRP rig or other older radio had one too.
    • This is a side benefit of operating Basic+Freq Link Mode. Just setup this mode and tune both radios using either Mini or either Radio. You’ll be able to see the spectrum your QRP or older radio is operating in on your digital radio.
    • You can work stations using your QRP rig and instantly switch to the QRO rig if the band conditions change.
  • You have an HF rig and a VHF rig and just want to monitor them both and be able to switch between them using the same mic and headphone setup.
    • This is a good example of using the Basic Link Mode. Just set both radios up and tune them individually. Your key and mics follow the selected radio. Your VHF radio doesn’t even need to be CAT controlled.
  • You want to be able to control your radio from somewhere else in the house, or in the world.
    • It seems like everyone wants to do this now days, so why not?
    • Enable Link Mode Remote on Mini-A and it now becomes a remote controller for Mini-B and its radio. You can now take Mini-A anywhere in the Internet and connect it to Mini-B and your radio.
    • Of course it doesn’t do you a lot of good to just control your radio so you’ll need to connect the audio from the radio in your shack to an audio server like SonoBus. This allows you to transmit and receive audio on your cell phone or tablet. Just connect the USB Audio I/O from the SO2R controller to your shack computer and connect SonoBus to those digital audio streams.
    • On Mini-A set the [C] button to PTT.

In addition to all of the above, I have designed an antenna switch controller based on the Seeed Studio Wio Terminal and a rotary encoder similar to what I use in the Mini. It can control 2×4, 1×8, and 2×8 switch configurations. It will also link to Mini-A and Mini-B and provide auto antenna lockout and instant antenna swap.

Here’s a screen shot of the antenna switch controller display in 2×4 mode:


Maybe I’m alone in this universe, but maybe there’s one or two others out there (my son thinks there’s probably 10 of us) that are thinking “Cool, how do I get one?”. Well, the short answer is “you can’t, right now at least”. Once the QST article came out in September 2022 on CTR2-Mini I decided to redesign it and created the CTR2-Mini+. Once that was done I ended up enhancing the Mini’s firmware substantially and added additional radio protocols for Elecraft and Xiegu radios. I’m currently waiting for the (hopefully) final updated PCBs to arrive for the SO2R controller. Once the boards check out I’ll add the controller to the order form.

If you are interested in any of my projects I would love to hear from you! Even if you just have an idea to run by me. You can use the form on my Contact page or look me up on QRZ.com. I really enjoy talking with others to see what they’re doing or what they would like to see added to any of my projects.

73, Lynn, KU7Q