Many of us have more than one antenna and we use a plethora of ways to select one for our radio. From simple patch panels to fully integrated remote antenna switch controllers your options are only limited by your budget.

Many remote antenna switch manufacturers offer controller options for their switches. Typically they use simple push buttons or rotary switches that do nothing more than assert voltage onto one of the control lines to the relays in the remote switch. There’s nothing wrong with these controllers. They’re simple and reliable. What’s not to like? Other controllers integrate with your radio and can switch antennas based on the band you select. Of course you need to own one of the radio’s they can interface with and the controller can come with a pretty hefty price tag.

With so many options already available, why would I go through the effort to reinvent the wheel? Well, for starters, I love designing and building gear. And I love sharing my projects with other like-minded people with the hope that at the very least I can inspire someone to design and/or build their own stuff. In my mind, building your own station accessories is one of the rewarding aspects of the hobby that’s unfortunately becoming a lost art.

Another reason I designed this switch is because I wanted to duplicate, if not expand, the functionality I built into the original CTR2 station controller for the CTR2-Mini controller. I’ve been working on a new SO2R controller and I wanted a way to control a dual-port antenna switch (i.e. two radios and multiple antennas). The original CTR2 antenna controller did not have that capability.

What I ended up with is the CTR2-Mini Antenna Switch Controller, referred to as the ASC in this document. It’s shown in the lead photo along with a 2×4 port antenna switch I built using two eBay 1×4 antenna switches. Before you go this route, see my comments in the Cautions About Antenna Switches section at the end of this post.

You can view a video of the CTR2-Mini ASC in action below…

ASC Features

Here’s a quick overview of the CTR2-Mini ASC features:

  • Standalone operation with the local encoder and function buttons
  • Remote operation from up to two CTR2-Minis, Node-RED, or a terminal
  • Supports a single port switch (1X) with up to 8 antennas (one radio + 8 antennas) or dual port (2X), 2 to 8 antennas (two radios + up to 8 antennas)
  • Color graphical display
  • Ready/Lock mode unlocks or locks the antenna switch
  • Active/Off mode activates the external antenna relays or opens them
  • User defined button – you choose what it does
  • A Return to Previous Antenna function is available in single port (1X) mode. This option allows you to toggle between the currently selected antenna and the previously selected antenna – great for comparing two antennas on the same receiver.
  • A Swap function in dual-port (2X) switch mode instantly swaps the two active antennas – great for comparing signals and noise on different antennas
  • Option jumpers for +12 VDC or Ground output to external antenna relays
  • Option jumper to allow EC2-5TNU or EC2-12TNU latching relays to be used when building the board (thank you global supply chain disruptions!)
  • Option jumper to have external relay power follow On/Off switch or remain On when ASC is Off
  • Encoder/Function buttons can be located remotely from the main control board
  • WiFi linking to a Mini provides the following features
    • Up to two Minis can control the switch remotely
    • WiFi and linking can be set to auto-connect on startup
    • Automatic Antenna Disable disconnects the offline antenna when one Mini/radio enters Tx Enable mode – this reduces RF into the offline radio when using less than ideal external antenna switches
    • Band-awareness automatically switches antennas based on the current band
    • Automatic switch locking when the radio is actively transmitting
  • Two board options are available
    • 1X model
      • For single radio, 8 antenna, remote switches like the DX Engineering DXE RR8B-HP switch
      • One control lead for each antenna relay
      • Relays are not interlocked
      • Relay LEDs are numbered sequentially from 1 to 8
      • Allows multiple antenna ports to be selected simultaneously for phased arrays
    • 2X model
      • For dual radio, 4 to 8 antenna, remote switches like the DX Engineering DXE RR2X-8B switch
      • Hardware interlock prevents one antenna from being selected for both radios
      • Multiple ports can be selected, but only one antenna from each A/B group (example: A1 and B1 cannot be selected but A1 and B2 can)
      • Relay LEDs are paired and are numbered A1B, A2B, A3B, and A4B
      • A port LEDs are red, B port LEDs are yellow
      • One 2X board controls 4 antenna ports (2×4). Add an expansion board to control eight antenna ports (2×8).

So what’s so special about this ASC?

Looking at the list above it’s pretty obvious that this little controller has a lot of features. It only requires a +12 VDC/1Amp power supply to be fully operational. The Wio Terminal is powered by the onboard 5 VDC regulator on the ASC so the USB port does not need to be connected to a PC unless yo want to use the Node-RED flow I developed for it.

It can control just about any remote antenna switch, such as the DX Engineering DXE series antenna switches. It’s ideal for controlling the DXE-RR8-HB single-input 8-port or DXE-RR2X8B dual-input 8-port switches. The only requirement is that your remote switch must accept a separate control line for each relay.

What makes this switch controller really special is that it seamlessly integrates into the CTR2-Mini ecosystem. Like the CTR2-Mini it’s based on, the CTR2-Mini ASC uses the Wio Terminal from Seeed Studios. It has a great little color display and a small assortment of buttons. It’s programmed using the Arduino C/C++ extensions so it’s extremely easy program. While you can use just the Wio’s buttons to control it, I’ve added a rotary encoder and function buttons to make it much easier to use.

The ASC shows the selected antenna(s) graphically. As mentioned, it can control single or dual-port antenna switches with up to 8 antenna ports. The arrangement on the screen changes with the configuration. Here’s a snapshot of it set up to control my SO2R setup with two radios and four antennas (2×4) and one another photo of it setup to control one radio and 8 antennas (1×8). Trust me, the display looks MUCH better than the photos I take of it!

2×4 Display
1×8 Display

Radio ports are color coded. The A port is red and the B port is yellow. The antenna selected for each port is also color coded to match the radio it’s routed to and there’s a line connecting the two. When controlling multiple antennas (phased arrays) the line connects to the primary (selected) antenna port. The other selected ports will be the same color without connecting lines.

If you look at the lead photo you see how the LEDs on the dual-port control board are arranged in four groups of two LEDs. The red LED is lit for the antenna selected for Port-A and the yellow or green LED is lit for Port-B’s antenna. These are bi-directional LED so the colors switch if you configure the controller to source (-) voltage to the remote antenna relays instead of (+) voltage. The LEDs are numbered 1 to 8 on the single-port control board.

The three buttons on top of the Wio Terminal have the following functions:

Ready indicates the switch is ready to operate. It’s below the [A] button to indicate that the [A] button controls this option. Press [A] to toggle the switch operate mode between Ready and Lock. In Lock mode no changes can be made to the switch. When linked to a Mini the switch automatically enters Lock mode when the transmitter is actively transmitting to prevent unintentional switching under load.

Active indicates the switch is Active (external antenna relays are active). It’s below the [B] button to indicate that this button controls this option. Press the [B] button to toggle the switch between Active and Off mode. In Off mode the the antenna relays are deactivated and the radio port(s) are open. In the Off mode all radio and antenna ports are displayed in grey.

The green antenna icon in the top-right indicates the ASC is connected to WiFi and is sending status updates to the Mini(s). In this case there are two Minis in SO2R configuration. The 32 next to the antenna icon in the 2×4 display represent the antenna selected for each radio. We’ll see these later on each Mini.

Swap is the user selected function for the [C] button in the 2×4 display above. (The [C] button defaults to bring up the A menu in the 1×8 configuration since there’s nothing to swap). The Swap feature instantly swaps the antennas on the two radio ports, i.e. antenna #2 moves to Radio-A and antenna #3 moves to Radio-B. Press it again to swap them back. The swap function can also be assigned to a function button.

Like the Mini, a rotary encoder is the primary method of navigating the menus on the ASC.

The encoder has the following functions:

  • Press it once to open the main menu then turn it to select an option. Press it again to select the option.
  • Press it and turn it to the LEFT to open the A port menu. This allows you to quickly change the antenna on the A radio.
  • Press it and turn it to the RIGHT to open the B port menu (on 2x switches). This allows you to quickly change the antenna on the B radio.

The encoder can be mounted on the enclosure along with the function buttons as shown in the lead photo or they can be mounted remotely. The function buttons utilize the same resistor ladder used in the Mini’s remote function keypad. These buttons are programmable. At my station I found that setting F1 and F2 to open the A and B switch menus, F3 to toggle Active/Off mode (Off opens all relays), and F4 to Swap antennas works out well.

The switch controller can be operated as a standalone device or as the following graphics indicate, it can be linked via WiFi to the Mini or Minis you are using to control your radios. Bluetooth linking IS NOT supported.

Single Mini Linked to the ASC

When a Mini is linked to the ASC an Ant Sw menu appears on its main menu. This menu allows you to remotely control the ASC from that Mini. You can also change the label for each antenna remotely. Linking the ASC the the Mini also gives you the option of automatically switching antenna ports based on the selected band and auto switch lock during active transmit.

Dual Minis Linked to ASC

When used in the SO2R configuration, two Minis can be linked to the ASC. Both Minis can control the ASC and each have their own band assignments. The antenna switch is automatically locked when either Mini/Radio enters active transmit mode.

When two Mini’s are used for SO2R operation the ASC can be set to automatically open the offline radio’s antenna port when you enable Tx on one of the Mini’s. This gives you an addition 25 to 30 dB of isolation between the active Tx and the offline Rx.

ASC Display on Mini-A

When the ASC is linked to the Mini-A its display will show the status of the antenna switch in the red icon at the top-right.

The At at the top-right tells us this Mini is Mini-A (the server). The link is established because the icon is green. The superscript ‘t’ tells us that Basic+Tracking Link mode is active. The green antenna icon indicates we are connected using WiFi.

To the left of the link mode icon the RED 3 icon also indicates that this is Mini-A, that it is linked to the ASC, and the #3 antenna is selected for Radio-A. Auto antenna selection based on band is enabled because the background of the Band label (20m in this example) is also RED, the same color as the antenna indicator.

ASC Display on Mini-B

The next screenshot shows Mini-B’s display.

The Bt at the top-right indicates this Mini is assigned as Mini-B (the client) and it is in Basic+Tracking Link mode. Mini-B’s Link mode is automatically set by Mini-A. The green antenna icon indicates we are connected via WiFi.

The yellow 2 icon next to the Bt icon indicates that this Mini is also linked to the CTR2-Antenna Switch Controller. This is done through Mini-A which is acting as a gateway for Mini-B. The #2 antenna is selected for Radio-B. Auto antenna selection based on band is not enabled because the background color on the Band label (20m) is the default color. If band sense is enabled the background of the Band label would be YELLOW.

But wait! There’s (always) more!

No, you don’t get two ASC units for the price of one by just paying shipping and handling (sorry for the lame joke).

A simple Node-RED flow provides a third option for controlling a standalone ASC without using any Minis. This is a single flow that can easily be added to any other Node-RED flows you may already be using. The control protocol is extremely simple and easy to implement so you can create your own flow or customize mine to fit your needs.

Here’s a screenshot of the Node-RED flow. It follows the same convention as the ASC display but there are not interconnection lines showing the routing. I don’t know how to do that on Node-RED.

To select an antenna, click the A or B-Port button first (it will turn green), then select the antenna. You can control the switch modes (Ready/Lock, Active/Off) and Swap antennas, and you can rename the antennas as needed.

A simple USB connection from the ASC to your Node-RED host provides the communications.

And if that isn’t enough…

Finally, the ASC supports a terminal interface. Just connect to the ASC’s USB serial port using any terminal program like Putty or Tera Term running on your PC and you can control all of the functions on the ASC from your PC’s keyboard. This is particularly handy when setting up the basic configuration of the ASC (antenna labels, the access point’s SSID and password, and the Mini’s IP address). These settings can be done with the rotary encoder but it’s tedious.

Here’s a screenshot of the terminal mode menu. Just hit the [Enter] key to open the main menu.


The design of the CTR2-Mini ASC is based on the CTR2-ASC design using the 4514 4-to-16 multiplexer and ULN2803A Darlington drivers. Unlike the CTR2-ASC I used 5 volt latching relays which seem to be in good supply right now. The 12 volt latching relays I used in CTR2-ASC are STILL out of stock and won’t be in stock until at least August 2022. The last batch in March sold out before I could order them, which is one of the reasons I discontinued the CTR2 controller product.

The multiplexer controls eight latching relays. Each relay has a set and reset coil. The relays are configured to control two banks of four antenna relays. The first bank routes one of four antenna ports to Port-A (i.e. to the A radio). The second bank routes the same four antenna ports to Port-B. When used with a dual-port remote antenna switch, this board controls four antenna ports. It can control 8 antenna ports on a single-port antenna switch.

One of the main requirements when switching a dual-port antenna switch is that you can NEVER allow the same relay on the A and B bank to operate at the same time. This would parallel your two radio’s antenna ports, and that would be bad. The ASC firmware prevents this from happening but as a fail-safe the A/B latching relays are wired such that if one relay is active it breaks the output path of the other relay, so it is physically impossible to operate both the A and B relays on the same antenna port at the same time.

But wait, this board only controls a 1×8 or 2×4 antenna switch. How do I control a 2×8 antenna switch? Simply add another controller board using the expansion port. On the second board, open jumpers JP1:1-2 and JP2:1-2 and add a solder bridge between JP1:2-3 and JP2:2-3. The ASC will recognize the second board and provide a 2×8 switch interface.

The remote antenna switch is connected to the ASC control board using a standard RS232 9-conductor serial cable (or two when configured for 2×8). A DB9-to-terminal board breakout can be used to wire the ASC to your antenna switch, or you can just cut the plug off the end of the cable and hardwire it to your switch.

For a single port, 8 antenna switch, wire pins 1 to 8 on the DB9 to relays 1 to 8 on the antenna switch. Pin 9 is common.

For a dual-port, 4 antenna switch, wire pins 1 to 4 to Port-A’s antenna relays 1 to 4, and pins 5 to 8 to Port-B’s antenna relays 1 to 4. Pin 9 is common to both ports.

If you use the expansion board to control a 2×8 switch connect a second DB9 cable to the DB9 socket on the expansion board and wire antenna relays 5 to 8 on Port-A to pins 1 to 4 on the expansion board’s DB9 cable and antenna relays 5 to 8 on Port-B to pins 5 to 8. Pin 9 is common to both ports.

There are jumpers on each board that allow you to source (+) or (-) polarity on the antenna relay select lines.


You can download the complete bill-of-material at the link below. I’ve also created a project notebook at You can order almost all of the parts needed to build the ASC, including the Wio Terminal if you need one, from the project notebook. Open the Mouser CTR2-Mini ASC v1 project notebook here.

If you’re building just one main board for 1×8 and 2×4 operation, order one set of parts, including the Wio Terminal (if you don’t already have one) for this project. If you’re building a 2×8 controller, order two sets of parts but only one Wio Terminal.

Three PC boards are required to build the ASC as shown here. The main PCB contains the 5 volt regulator, decoder logic, relays, etc. A small encoder board provides an easy way to connect the encoder and function buttons on the enclosure to the main board. The third board connects the main PCB to the Wio Terminal. The boards come with all of the SMT components pre-installed, you’ll just need to add the parts you order from the project notebook. In addition, I will throw in the single and dual-row pin headers you’ll need to complete the boards to build the interconnect cable to the Wio. I will not be offering complete part kits for this project but I can supply the acrylic enclosure shown in the lead photo if you want to duplicate my system. The enclosure comes with the knob, four momentary push button switches, rubber feet, and mounting hardware for the boards and Wio.

Cautions About Antenna Switches

If you intend on using 2×4 or 2×8 antenna switches make sure that you use remote antenna switches with the highest port isolation you can afford.

Note: If you’re planning on uses the Basic + Freq sync mode described here, I highly recommend NOT building your own antenna switch with the cheap eBay switches. They just don’t have enough isolation. Spend the money and get a good switch!

After all, you’re going to have transmitter power just one or two relay contacts away from the offline receiver. The offline receiver may be tuned to the transmitted signal so the lower the leakage the better. My personal limit is +7 dBm (about 5 mW) of RF power on the offline receiver’s front-end. This means that if you are transmitting 100 watts (+50 dBm) on one antenna port there needs to be at least 43 dB of isolation from that port to the receiver on the unselected radio port. The cheap eBay antenna switches I bought have a little less than 40 dB which just isn’t enough if you plan on running Basic + Freq sync mode.

As noted above, when using the antenna switch controller with two linked Minis you can enable the Minis to automatically disconnect the antenna on the unselected radio when you enable Tx on the other Mini. This gives you additional isolation for the offline Rx.

There are several lower cost antenna switches that you can build. KK1L has a 2×6 port antenna switch on his web site. It has around 60 dB of port isolation. The parts to build it are available at Mouser. The total cost is around $200 in 2020 dollars. There are a lot of commercial switches available. Just make sure you buy one that has individual lines to each relay. I’ve found the DX Engineering antenna switches to be high quality with high isolation.

Winding up…

I hope this provides a idea of what the ASC does and inspires you to think about building your own. When paired with a CTR2-Mini you get a complete station controller in a very small, and inexpensive, form factor.

If you have any questions or want to get on the order list please contact me. You can also email me from my QRZ listing. Also, please consider joining my Groups IO group at

73’s and happy building,

Lynn, KU7Q