CTR2 was designed from the ground up as a modular system so you only need to build or buy what you need now with the option to easily expand it in the future. The HMI with a Display and Auxiliary-Option 1 board come fully assembled, programmed, and tested and will give you all the features of CTR2 with one radio. This is a perfect solution for a one radio/one antenna station.

Single radio CTR2 system
HMI and Auxiliary-Option 1 boards

This configuration gives you all the operating features of CTR2 and it gives you the framework you need to add additional functionality in the future. Not shown in the diagram above is the USB connection to your PC. This connection carries digital audio and a virtual serial port so your digital mode apps like WSJT-X have access to your radio without the need of additional sound cards, audio coupling, or keying circuits.

An additional feature offered by the CTR2 system is the ability to use Node-RED in place of the dedicated touchscreen. Node-RED allows you to easily connect to and control CTR2 from any modern web browser, including the one on your smartphone.

Do you have two or more antennas and one radio? No problem. Add the Antenna Switch Controller and an remote antenna switch to your system and now you can switch multiple antennas to your radio. The HMI can even select a different antenna automatically when you switch bands and it allows you to select more than one antenna in case you’re using phased arrays.

Single radio with multiple antennas
HMI, Auxiliary-Option 1, and Antenna Switch Controller boards

Maybe in the future you’ll add another radio to your fleet. No problem. Just open jumpers JP5 through JP10 on the Auxiliary board (to disable the onboard Radio I/O module) and add RJ45 jack J4 to the board. Next, build two Radio I/O modules and connect them to the switched ports of a manual RJ45 switch. Connect the common port on the switch to the new RJ45 jack on the Auxiliary board and you’re ready to go. With this configuration you can easily control two or more radios.

CTR2 for multiple radios
HMI and Auxiliary-Option 2 boards with manual RJ45 switch and one Radio I/O module

For maximum convenience CTR2 offers options to automate your station. This is especially important if you intend on operating remotely using the Node-RED web based interface. These options give you the ability to route the correct antenna to the correct radio, then route that radio’s I/O to the HMI by just selecting a radio in the HMI. To do this requires an Antenna Switch Controller, a Radio Antenna Switch Controller, and one or more RJ45 Switch boards. The antenna switch controllers operate electrically operated remote antenna switches. These can be either home-brew or commercial units such as the DX Engineering RR8B-HP.

HMI and CTR2 RJ45-Master Switch with one Radio I/O module
HMI, CTR2 RJ45-Master Switch, and Radio Antenna Switch Controller with one Radio I/O module
HMI, CTR2 RJ45-Master Switch, Radio Antenna Switch Controller, and Antenna Switch Controller with one Radio I/O module

At the present time, CTR2 doesn’t offer automatic control of antenna tuners or linear amps so full remote operations isn’t available “out of the box”. Quite frankly, there are just too many auxiliary devices for CTR2 to manage them all. Fortunately, there is an active Node-RED ham radio group at groups.io. Members of that group are creating flows for just about anything you can think of. These flows could easily be added to the CTR2 flows to create a custom web page for your station.

The HMI board is always required as it is the brains of the system. In addition you’ll need one of the display options described below and one Radio I/O interface for each radio you want to control.


There are four display options available. CTR2 was designed around the 3.5″ and 5″ Nextion Enhanced displays. These are self-contained HMIs in their own right and manage the user interface. To use one of these displays you’ll need a display interface board to convert RS232 to 3.3 V logic and break out the four wires for the encoder. Using the Nextion display makes CTR2 completely self-contained and allows you to locate your radio control console anywhere it’s convenient. I keep my display next to my paddles. The only time you would need a computer is if you want to run digital modes.

3.5″ Display Console 5″ Display Console

There are two alternatives to a hardware display.

The first option is a low-cost user interface using Node-RED. You can read more about Node-RED here. Flows created in Node-RED can be viewed on any recent browser, including those on IOS and Android devices. Couple the CTR2 Node-RED flows with SonoBus, a light-weight, low-latency, high-performance two-way audio transport application and you have the makings of decent remote station operation. You can read more about my Node-RED flows here, and download them here. Even if you don’t have the HMI hardware the CTR2 Node-RED flows will give you an opportunity to test drive many of the features.

The second option is to use the Nextion IDE simulator that allows you to run a simulated Nextion display on your Windows PC. The Nextion IDE doesn’t support Mac or Linux PCs. I use the simulator to debug the Nextion code but you can also use it in place of an actual hardware display. I have a how-to post here if you’re interested in trying this. While the simulator works OK, you’ll probably find the diagnostic displays running along with it annoying.


There are several options to connect CTR2 to your radio(s).

For single radio control or manually switched multiple radios add an Auxiliary board to the HMI. It mounts to the HMI board using 25mm standoffs and connects to it with a short 10-conductor ribbon cable as shown in the lead photo.

The Auxiliary board has two options. Option 1 provides a radio interface for one radio. Option 2 adapts the Radio I/O ribbon cable from the HMI to a RJ45 jack that can connect to a manual RJ45 switch. Option 2 requires an external Radio I/O module to interface to each radio.

Auxiliary board with Option 1 installed

Option 1

When Option 1 is ordered you’ll get a fully populated and tested Auxiliary board as shown above. Option 1 provides an onboard Radio I/O interface for one radio. Just connect your radio to the 12-pin terminal strip. This configuration gives you all the benefits of CTR2 for a single radio.

Single Radio Control

Option 2

Auxiliary Option 2 provides just a10-pin ribbon connector to RJ45 jack adapter (only J3 and J4 are installed). This allows you to connect the RJ45 jack on the Auxiliary board to a manual RJ45 switch using CAT5 cable so you can manually switch between multiple radios. These switches are available in 2-port, 3-port, and 4-port configurations. You can daisy-chain multiple switches together to add more radios. Option 2 requires one external Radio I/O module for each radio.

WARNING: The signals between the HMI and the Radio I/O modules are analog, not Ethernet. Ethernet hubs, switches, and routers CANNOT be used to switch Radio I/O signals.

Multiple Radio Control


When you add more than one radio to CTR2 you’ll need a method of routing your antenna(s) to the selected radio. A simple patch panel or a high quality manual antenna switch can be used as shown above.

Resist the temptation to use cheap antenna switches to route your antenna to multiple radios!

The reason for this is simple. Cheap antenna switches have low port isolation, sometimes less than 30 dB. This means that power transmitted into the selected port will bleed over into the other ports at levels that may damage your offline radios’ receivers.

Let’s assume you’re transmitting a 100 watt CW signal into a cheap 2-port antenna switch that has 30 dB of isolation between ports. 100 watts = +50 dBm so with 30 dB of isolation, the receiver on the radio connected to the offline port of this switch sees +20 dBm, or 1/10 of a watt. This probably won’t destroy the receiver, but what if that switch only had 20 dB of isolation? The offline radio’s receiver would now see +30 dBm, or 1 watt of power. At this level, all bets are off. Your receiver will probably be damaged. The question is, do you know what the port isolation of your switch is? If you decide to use cheap switches, invest in a NanoVNA so at least you’ll know how much isolation you have. As a rule of thumb, I don’t like more than +10 dBm (10 milliwatts) of power on my receiver’s antenna port.

High quality antenna switch manufactures will publish their port isolation figures. Yes, they cost more, but consider the cost of your radio equipment. Depending on your transmit power you may want to buy a switch with 50 to 70 dB port isolation.

I have multiple antennas, do I need an expensive antenna switch to switch between them?

The answer is “that depends”. The difference between switching a common antenna between multiple radios and switching multiple antennas to a common port (or a single radio) is that low port isolation isn’t going to hurt the offline antennas like it would offline receivers. However, low port isolation will affect the tuning of your antennas because now you have additional loads (offline antennas) in parallel with your selected antenna. A low quality switch with 20 dB of isolation means that 1/100th of the impedance of each offline antenna is presented in parallel to the online antenna. This isn’t going to make a lot of difference to your transmitter, but the offline antennas will be radiating the power that gets to them and that may affect the radiation pattern of your selected antenna to some degree. Signals received by the offline antenna(s) will also mix with the online antenna’s signals. They may be out of phase or noisier which can cause distortion in the received signal. At the end of the day, you make the decision that works best for you.


Automating your station control for two or more radios and/or antennas requires at two or three separate functions.

  1. Route the desired antenna (if you have two or more antennas) to a common antenna port or tuner.
  2. Route the common antenna port or tuner to the selected radio (if you have two or more radios).
  3. Route the audio and control signals (radio I/O) from the selected radio to the HMI for processing.

Antenna Switch Controller

If you have more than one antenna you’ll need the ability route one (or more) of them to a common antenna port. At this point an antenna tuner can be installed if needed. The Antenna Switch Controller, or ASC for short, does not perform this task directly. Instead, it controls a remote antenna switch that does the actual RF routing. The ASC has several options that allow it to work with most remote antenna switches on the market.

The ASC also gives the HMI the ability to select antennas based on the selected band. It also can select more than one antenna if you’re using phased arrays and it can disconnect all antennas with the touch of a single button if necessary.

The ASC has several options that allow it to be used with most of the remote antenna switches on the market or a homebrew antenna switch.

Antenna Switch Controller (requires a remote antenna switch)

Radio Antenna Switch Controller

Regardless of the number of antennas you have, if you have multiple radios you’ll need to distribute a common antenna port to your radios. (I have 2 antennas, a dummy load, and 8 radios connected in my CTR2 installation.) Installing multiple high quality manual antenna switches to switch this many radios can be cost prohibitive. A patch panel made more sense but I knew I would forget to change the antenna when switching to a new radio. The Radio Antenna Switch Controller, or RASC for short, eliminates the need to remember which radio is patched to the antenna. It also gives you more station control if you plan on using Node-RED for remote control.

The RASC has several options that allow it to be used with most of the remote antenna switches on the market. I use a DX Engineering RR8B-HP switch to get the 70 dB of port isolation I wanted between radio ports.

Radio Antenna Switch Controller (required a remote antenna switch)

Automatic RJ45 Switch

One of the things I noticed right off after getting CTR2 running with the manual RJ45 switch was that while it worked just fine, I always forgot to change the RJ45 switch when changing radios. So even though the HMI presented the radio I had selected it was still connected to and controlling the radio still selected on the RJ45 switch. I set out to solve this irritating problem so I designed and built an automatic RJ45 switch.

This switch is nothing more that four DPDT relays on each Radio I/O port controlled by the HMI. DPDT relays were chosen for their simplicity and low cost. When you select a radio in the HMI it commands the switch to select a set of four relays to pick up. This routes the Radio I/O signals on that port to the Radio I/O port on the HMI. I designed the the RJ45 Switch to route up to 16 ports. It is modular with four ports on each board. You only build or buy the number of boards you need and can add additional boards at any time as your station expands.

RJ45 Master Switch

So what do I use?

My CTR2 system is shown in the lead photo. It is capable of controlling 8 antennas and 12 Radio I/O ports. The boards stack using M3 25mm+6mm male/female standoffs. The acrylic enclosure was cut on my CNC router. I can cut custom acrylic panels for any combination of boards for an additional cost.

I home-brewed an antenna switch to switch my dummy load and up to seven antennas using a cheap 8-port Arduino relay board. The relays on this board can handle around 100 watts of power and I ended up with a little over 50 dB of port isolation with the construction techniques I used so I’m happy with it to switch my antennas. Let me know if you’d like to see more info on this switch in the comments.

Homebrew Antenna Switch

To switch the common antenna output of my home-brew switch to my radios I use the DX Engineering RR8B-HP switch. This switch has 70 dB of port isolation so I don’t need to worry about frying my Flex 6400 or FTdx101D receivers. The switch just lays on my floor next to my operating desk. Yes, it’s pricey, but not nearly as pricey as my radios.

DX Engineering’s RR8B-HP 8-Port Antenna Switch

I hope this gives you an idea of what’s possible with this system. It can be a no-frills HMI for one radio and one antenna or a full-blown radio control environment for 16 radios and 8 antennas. It’s your choice!

To make it simpler for those that don’t want to build their own CTR2 system, I also offer complete, plug & play systems that are pre-configured, programmed, and tested. You can read more about these systems in this blog post.

CTR2 is a growing environment. The Auxiliary board’s Radio I/O features were just recently added and I have a few ideas for a few other modules the may be added in the future. If you have ideas for extending CTR2‘s functionality I’d love to hear them. Leave them in the comments below.