A coaxial antenna splitter is made for switching between two antennas using one single radio. A standard antenna splitter, for instance, from Albrecht looks like depicted in the following image.

They are heavy builds with a strong an heavy chassis as well as a very large knob that requires quite some torque in order to commute between the two antenna connectors marked A and B.

Operating the antenna commuter would require the device to be in the proximity of the operator which can be quite inconvenient depending on the complexity of the setup. It would be interesting to design some device around the antenna commuter that would be able to switch between the two antennas without having to do so manually. This document describes a build that would allow an operator to remotely commute between the two antennas.

• an ESP8266 or similar, we conventionally use WeMoS due to their small size, availability, low cost and overall stability,

• a servo motor capable of sufficient torque to push a copper blade inside the antenna commuter, rated at or digital; it is also possible to use a stepper motor, but a servo should be sufficient because the angle can be precisely controlled using the ESP8266,

• a buck converter stepdown from to , assumingly due to the rig being powered off a battery that is typical in cars

Antenna commuters are typically a heavy build due to the very strong RF Wattage that they are designed for. Typically, the switch could very well be replaced with a transitor switch, or anything similar and smaller but due to the high RF that can interfere with neighboring devices, the build of the antenna commuter will be leveraged and the switching will be designed to follow the large knob on top of the device.

Disassembling the Albrecht CX-201 is fairly easy, the bottom part being held in place by three screws that can be removed to reveal the inside of the Albrecht antenna commuter.

On the inside, we find a swivel that is connected to the knob on the outside of the Albrecht splitter through a hole, a spring and ball bearing that is meant to slide along the top of the chassis between two holes designed to house the ball and two copper or nickel blades or lamelles that are meant to be pushed into position by the switch. Intuitively, the ground is switched through the chassis of the Albrecht whilst the signal passes through the blades from one antenna connector to the output antenna connector.

The servo motor could be connected to the knob itself using some mechanism but since the Albrecht is already fully open, it might be wiser to throw away the entire swivel build, with the spring and ball bearing in favor of creating an axle that would be connected through the Albrecht chassis to the servo motor and would press or depress the blades. In doing so, a lot of friction is removed, given that the ball bearing is no longer removed such that the torque force required for the servo motor would be greatly reduced.

      +-------+          +---------+
      | Servo +----------+ ESP8266 |
      +---+---+          +----+----+
          |                   |
          |                   |
          |                   . manual flip switch?
          | spindle           . IoT MQTT Wireless?
          |                   . etc.
          |
  ANTENNA | ANTENNA
     A    |    B 
     +    |    +
 +---|---------|---+    
 |   \    |    |   |
 |    \   +    |<........ disconnected blade
 |     \   \   |   |
 |      \   \  |   |
 |       \     |   |
 +--------|--------+
          +
    ANTENNA INPUT
        (RTX)
          

By connecting the servo motor to the ESP8266 there will be a whole range of possibilities available to control the antenna commuter. A DPST switch might be suitable and conventional enough to make the connection between the two antennas given that rigs are typically mobile such that there might be no wireless available to control the switching via IoT or MQTT. Nevertheless, it is of course possible to connect the ESP8266 to a wireless network and automate the switching via Node-Red or similar as we have done for other projects.

The servo motor is mounted on top of the Albrecht antenna commuter with the small motor axle protruding through the hole left over by removing the knob on top of the commuter. Since not much torque is needed, the servo motor is glued to the chassis of the antenna switch using some two part glue such as JB weld.

On the inside of the antenna commuter, the small motor axle can be observed through the hole where the spindle switch used to be.

Luckily, the servo has an inner hole that allows a threaded screw to be inserted and fixed in place. The servo motor is delivered with a few plastic rotors that can be fixed on top of the motor. One of the plastic rotors has a trapezoid shape that can be cut down in order to use just one half of the plastic rotor that will be used to press and depress the blades thereby making the connection between the two antenna connectors.

However, a threaded screw is prepared along with a washer in order to match the height necessary to reach the two blades. The top part of the screw is sawed off letting only the main axle of the screw hold the washer on top of which the plastic rotor will be fixed. In order to make sure that the washer on the lower part of the axle will not move around, some JB weld is used to fix the washer in place.

While the JB weld glue is left to set, some code is written to ensure that the servo motor is at . will be used to correspond to one antenna coupling and will correspond o the other antenna port.

#include <Servo.h>
Servo myservo; 
 
void setup() {
  // put your setup code here, to run once:
  myservo.attach(D6);
  myservo.write(0); 
}
 
void loop() {
  // put your main code here, to run repeatedly:
  delay(60);
}

Using the Servo library, the code will first attach to D6 representing the pin marked "D6" on the WeMoS PCB and then myservo.write(0); is used just once to move the spindle of the servo motor to . Controlling the servo motor from now on will just be a task that will involve toggling between myservo.write(0) and myservo.write(45).

With the main shaft in place that connects the servo motor to the chassis and the plastic rotor blade in place calibrated to , the code is changed to:

#include <Servo.h>
Servo myservo; 
 
int o = true;
 
void setup() {
  // put your setup code here, to run once:
  myservo.attach(D6);
}
 
void loop() {
  // put your main code here, to run repeatedly:
  if(o == true) {
    myservo.write(130);
    delay(5000);
    o = false;
    return;
  }
  myservo.write(0);
  delay(5000);
  o = true;
}

that will commute between antennas every 5 seconds as can be observed in the following video.

All that remains is to decide whether to use a classic switch or some other mechanism to make the Albrecht commute between the two antenna ports.

Adding the WeMoS and a junction box to hold the circuitry, the project is now complete and ready to be used. It was decided to use a very basic on-off flip switch in order to commute between the A and B antennas. Ideally, the switch along with the long wires will be extended to some control panel next to the radio.

Now with an added flip switch, the code changes to:

#include <Servo.h>
Servo myservo;
 
#define SERVOS_PIN D6
#define SWITCH_PIN D7
 
void setup() {
  // put your setup code here, to run once:
  myservo.attach(SERVOS_PIN);
  pinMode(SWITCH_PIN, INPUT_PULLUP);
}
 
void loop() {
  bool state = digitalRead(SWITCH_PIN);
  if (state == LOW) {
    myservo.write(130);
    return;
  }
  myservo.write(0);
}

where:

• D7 is connected to the flip switch and the other end of the switch is connected to ground,
• D6 represents the servo motor control pin

The resulting schematic allows the servo to commute between the two SO-239 exit ports of the antenna commuter.

            +----------+                        +---------+
     +12V   |          |                        |         |
    ----+---+          +--+---------------------+         |
            | stepdown |  |                     |         |
        +---+          +--|-+-------------------+  WeMoS  |
        |   |          |  | |                   |   ESP   |
        |   +----------+  | | +-----------------+         |
        |               | | |                   |         |
        | GND           | | |        /          |         |
       ---              | +-|-------+  +--------+         |
        -               | | |        SW         |         |
                        | | |                   +---------+
                        +-+-+-+-+
                        | servo |
                        +--------

One interesting application of the antenna commuter is to allow the usage of an antenna tuner alongside a motorized antenna. Typically, for radios such as the FT-891, the tuner and the ATAS-120A are mutually exclusive and cannot be used together. Using the antenna commuter from this project, the output can be commuted between the FT-891 tuner an the ATAS-120A. Here is a sketch of what the setup looks like:

      
      ^ antenna
      |
      |
      |
      |
      +
     / \      automatic antenna switcher
 a1 +   + a2
    |   |
    +   |   
  tuner |  
    +   | 
    |   | 
    +---+
     \ /  dual antennas coupler (without balun)
      +
      |
    radio

where:

• the radio is an FT-891,
• the dual antenna coupler is a modification of a Zetagi "Dual Antennas Coupler",
• the tuner is the FC-50,
• the antenna is a motorized ATAS-120A

The Zetagi "Dual Antennas Coupler illustrated below:

is opened up and gutted entirely to replace the balun with a very simple straight-through connection with a copper wire beween all the three ports. In other words, the Zetagi now dumbly couples all the ports together without any additions which is necessary in order to feed both the ATAS-120A antenna and the FC-50 tuner the exact same signal.

With this setup, the operation of the ham goes like this:

• flip the switch to "straight-through",
• select ATAS from the radio menu and tune the ATAS-120A,
• power-cycle the radio and flip the switch to "tuner",
• select EXTERNAL from the radio menu and tune the FC-50

such that the combination of the two ATAS-120A and the FC-50 ensure the most minimal SWR possible.

One problem with this setup is that there is no feedback element such that using the switch will not guarantee that the antenna has been commuted based solely on the fact that the switch position has been changed. One idea would be to add strip contacts on the inside of the Albrecht antenna commuter that would ideally make contact once one of the commuter blades extends to the maximum with the strip connectors leading to the GPIO pins of the ESP8266. Nevertheless, for now, the project seems complete.