CTR2 is a modular system. You can start with controlling just one radio then add additional radios and antenna switches as your needs change.

The HMI board is always required as it is the brains of the system.

There are three display options. 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. The only time you would need a computer is if you want to run digital modes.

There is an alternative to a hardware display. The Nextion IDE contains a 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 additional external Radio I/O modules 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

In the near future an automatic RJ45 switch will be available that will replace the Auxiliary board and manual RJ45 switch. This switch will be presented in the Nov/Dec 2021 issue of QEX.


To 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 high quality manual antenna switches can be used.

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 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.


There will be three automatic switching options available for CTR2. These options automatically route up to 8 antennas to a common antenna port, route the common antenna port to multiple radios, and finally route the selected radio’s Radio I/O signals to the HMI. These options will be presented in a QEX article scheduled for the Nov/Dec 2021 issue. Once that issue goes to print I’ll publish more information and open ordering for them.

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