SO2R Review

SO2R stands for Single Operator Two Radios. This mode is used by many contesters to increase their scores and in general become more productive during a contest by tending two radios. Most times the radios are on different bands, but they don’t have to be. Sometimes the second radio is just used in ‘monitored hot standby’ mode, ready to spring into action should the primary radio fail.

SO2R is also handy if you’re waiting for a band opening or want to be able to chose the best radio/antenna combination for a given QSO. I discuss these options in detail in the SO2R is Not Just for Contesters Anymore! blog post.

In SO2R mode two Minis are used to control two separate radios. Each Mini provides all its radio control features as if were all alone in the universe and each radio has its own antenna.

The SO2R controller in the center ties the Minis and common devices (mics, keys, headphones, etc.) together and provides audio connections to and from the radios via two small audio interface modules. The antenna port of each radio connects to an optional dual-port antenna switch that is under control of the CTR2-Mini Antenna Switch Controller. This switch allows any radio to be connected to any antenna but blocks connecting two radios to the same antenna (that would be bad).

The SO2R controller’s function is to route the common keys, PTT, microphone, and headphone audio to either Mini-A/Radio-A or Mini-B/Radio-B at the touch of a button. Transformer isolated audio from the selected radio is also routed to an external USB adapter which connects to your PC’s sound card for 3rd party programs such as WSJT-X.

Both Minis and the antenna switch controller are linked to each other via WiFi. The drawing below shows how everything is tied together.

SO2R Functional Diagram

CTR2-Mini SO2R Controller

The CTR2-Mini SO2R Controller is a pretty simple device. There are no processors, therefore, no firmware. Just old school switches, buttons, a few latching relays, and a couple of op amps. Most of the complicated circuits are SMT devices so the board house will do the SMT assembly. The only thing you’ll need to do is mount the through-hole components and assemble it.

The 3D rendering of the new CTR2-Mini SO2R Controller is shown above. The controls from left to right function as follows:


Press the [A] button to enable Tx on Mini-A. Press the [B] button to enable Tx on Mini-B. Pressing the [A] button (Tx Enab) on either Mini automatically switches the SO2R controller to that radio.


The controller powers the two Minis connected to it so this switch becomes the system power switch. Turning it off turns off both Minis and any other CTR2-Mini option modules in the system.


The MIC switch allows you to select from two microphones. The LED for the selected mic will light when the controller is powered up.


This control adjusts the gain on the A radio’s Line-Out Amp.


This control adjusts the gain on the B radio’s Line-Out Amp.


This control adjusts the gain of the A radio’s Line-In Amp.


This control adjusts the gain of the B radio’s Line-In Amp.

RX-A and RX-B Switches

These switches control the Rx audio routing to the headset. When both are on (up) radio A’s Rx audio is delivered to the left speaker on the headset and radio B’s Rx audio is delivered to the right speaker.

Turning either switch off (down) turns that receivers audio off and routes the audio from the other receiver to both headset speakers.

Turning both switches off (down) combines the audio from both receivers and delivers the combined audio to both headset speakers.

One question you may have is “Do I need the SO2R controller to operate two Mini’s and two radios?”

No, you don’t! The controller simply provides a convenient method power both Mini’s and to control the signals to and from the selected Mini/radio. You can use the link modes with just two standalone Minis. The Minis manage Tx Enable switching and the antenna switch without the SO2R controller. You can use a mixer board to manage the audio. You’ll also need a way of routing your keys and PTT to the selected radio which can easily be done with a couple of DPDT toggle switches. I designed the SO2R controller to put all these functions in one box.

Simplified Diagram

The schematic diagram for the controller is spread out over five pages so I created the simplified diagram below to provide a better idea of what’s going on inside the controller box. For clarity I left the antenna switch and it’s controller off.

Common Device Routing

The routing relays are shown in the Enable Mini-A position. These are latching relays. As you can see, the common devices (keypad, USB audio, remote PTT/straight key, paddles, mics, and headphones) route to Mini-A. Pressing the Enable Mini-B push button (not shown on this diagram) latches the routing relays into the Mini-B route which steers the common devices to Mini-B.

Pressing either enable switch automatically enables the Tx Enable option on the selected Mini. Press the same enable switch again to turn Tx Enable off.

Audio Routing

Audio routing is handled by DPDT toggle switches. The Mic Select switch routes the selected mic to the Tx-A and Tx-B Mic amps. Each amp has a level control. Mic bias can be enabled independently on each mic by adding J9 and J10 on the main board.

Receiver audio from each radio goes through buffer amplifiers. The Rx-A and Rx-B Vol controls allow you to set the receive level for each receiver. The Rx audio routing switches have four settings that are described above.

Winding down…

As you can see, there’s not a lot to the SO2R controller. But, when combined with two Minis and the Antenna Switch Controller a very capable SO2R control system can be created that allows you to use just about any two radios you have in your shack. You don’t have to be a contester to take advantage of this setup. Anyone with two radios and two or more antennas will find SO2R adds to their casual operating experience.

Let me know if you’re interested in building a SO2R controller. I can supply the front panel and main PCBs with SMT devices pre-installed for a very reasonable price. I’ll be creating a project file on Mouser with the necessary components you’ll need to buy to complete the controller.

73, Lynn, KU7Q