The Interlock is an equipment designed to be used by contesters where they need to check, supervise and limit, how many radios are in transmission at any given time.

There are contests where you can participate as a Multi/Single where a single signal may be present in an instant in the air; or Multi/2 where you can have a maximum of 2 signals simultaneously in the air.

The Interlock is responsible for overseeing just radios are in transmission and can block the radios that require it. For this it is necessary to connect 2 signals between each radio and Interlock: PTT and Inhibit. With the PTT the interlock detects which radio is in transmission through the Inhibit, the radio can be locked – in case it’s required. Radios as Yaesu or Elecraft (K3/K4) – that support this inhibit signal – are easy to interface with the Interlock, since they are Plug & Play.

Other radios do not exhibit this signal, can also be used, but in these cases should do pass the Key signal – CW – or microphone signal – SSB – through a box with relay; in stand-by the relay passes the signal to the radio, but when activated prevents the signal from the radio so then there will be no transmission. It’s available the Aux Box for those equipments that don’t include the inhibit signal as Kenwood or ICOM.

The interlock can act in the following environments:

  • Multi/1 o Multi/Single: Allowing only one active signal at any time.
  • Multi/2: Allowing a maximum of 2 simultaneous signals in 2 different bands at any time.
  • InBand: Allowing only one signal per band; It is only possible to have an active signal in the same band.
  • SO2R: A single operator with 2 radios. In this case, only one signal is allowed.
How does it work
The Interlock equipment supports 5 Radios (R1, R2, R3, R4 & R5) and can be programmed as: M/S or M/2. It monitors in real time only one (M/S) or two (M/2) signals are at the same time in the air. If it detects a violation, it will block immediately one of them by activating the inhibit signal and cause one of the radios remain without transmitting signal.

Yaesu radios or Elecraft (K3) include a signal called INHIBIT, which will be used by the Interlock to perform blocking. When radios that have this signal is used, the interface between Interlock and radios is very simple. Other radios that do not have this Inhibit signal can also be used, but in these cases we must pass in CW Mode the Key signal or in SSB Mode the Microphone signal by a relay box; In standby the relay lets the signal pass to the radio, but when activated, prevents the signal from entering the radio, so there will be no transmission on that radio. Two of the 5 Radios can be programmed as Inband Radios (R4 & R5) being associated with a Main Radio (R1, R2 or R3). Partners “Main” and “Inband” has an important feature: only one of them may be transmitting at the same time, for what has a high priority task that performs this oversight. Each connected Radio is programmed with a priority level (value 1-5) and Interlock uses this parameter to determine which radio “wins” and which radio “lost” in this case activating the inhibit. If it needs to inhibit a radio and both have the same priority, blocking is carried out by the criterion: “Last passed TX, First to be blocked “.

Interlock Monitor program

The Interlock includes a windows program called: IM that could be installed on any computer. In this way, each contest operator can monitor the status of the Team. Also any operator can change the priorities of the Interlock during the contest. The IM includes the “always on” feature, so the program will be all time Foreground. It’s compatible with Win-Test, N1MM, DXLog, etc.


Microcontroller: The Interlock uses a Flash microcontroller @ 48Mhz. 5 Radios supported: It supports till 5 Radios, 2 of them could be configured as InBands. Supervision and Monitoring:  A program called: IM is included with the product. It can be installed on any computer of the network. The program will monitor the status of all radios and will allow to the user to change the priorities of the Interlock. Response times: The average response time provided by the Interlock between the appearance of an active signal and the inhibit is less than 0.3 milliseconds (0.0003 seconds) USB native: The microcontroller includes a fully featured USB that is compliant with the USB Specification Revision 2.0. z. Bootloader: It includes a self programmability; it means that it can update itself a new firmware version. Any O.S. supported: Tested and working in Windows (XP, Vista, Win 7, 8 & 10), Linux and Mac OS X.

Yaesu / K3 cables

Those equipments include an Inhibit signal, so a simple cable between the Interlock 5R and the equipment is enought for all the control


Universal box that allows to use any equipment that does not include the Inhibit signal like for example Icom or Kenwood.

Antenna Switch AS2x2

This switch 2 antennas in 2 radios is perfect for operation with a second radio in the same or different bands. The Interlock 5R automatically manages the switch allowing the second radio to use the main antenna while switching the antenna 2 to the main radio.

Photo Gallery

Some of the possible schenarios are:

Customer Feedback

Jim K4QPL  @ VP5M:

Hello Pablo,

Just to let you know we used the EA4TX interlock and 2×2 antenna switch for the first time at VP5M in the 2020 ARRL DX SSB contest. The system worked great for both out of band and in band hardware control of the two K3’s with the 2×2 antenna outputs going to the SixPak. All of the operators were happy to avoid conflicts and the risk of accidentally putting two signals on the air at the same time.

The setup was easy and intuitive. I added the homebrew signalling box so the 2×2 was de-energized when not operating in-band and the status was clear from LED’s.

Excellent products and workmanship.

73, Jim, K4QPL  (VP5M)  

Interlock over IP during 2015-2016 IARU Contest (HQ)

During 2015-2016 IARU Contests, EF4HQ Team have used an Interlock HQ version. This version was operational via Internet and the Central Console was monitoring all Band/Modes (Slots) so only ONE signal peer slot was allowed. This is a short video of the Monitor program at the central site:

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  • Interlock 5R

    The Interlock 5R device ensures the compliance with Radio Contest rules for the participation in Multi-Operator categories:


    • Multi/Single: Only one transmitted signal on one band permitted.
    • Multi/2: Maximum of two transmitted signals on two different bands may be used at any time.
    • InBand: Only one signal allowed between a Radio and its associated InBand.


    The Interlock supports 5 Radios (R1, R2, R3, R4 & R5) and it can be configured as M/S or M/2 so the unit will monitor that only one (M/S) or two (M/2) signals are on Air.

    In case of rules violation, the unit will activate an Inhibit signal that will be used by the Radio (Yaesu/Elecraft) to STOP the transmision of the signal to the antenna.


    200,00 VAT not included Add to cart
  • Interlock – AuxBox

    Optional Interconection box for transceivers without Inhibit signal


    50,0055,00 VAT not included Select options This product has multiple variants. The options may be chosen on the product page
  • AS2x2


    The antenna Switch AS2x2 is a 2 Input Ports into 2 Outputs Ports, usefull for a contest station where you have a Main Radio (RUN) and a Multiplier Station (Inband) at the same band. In this way, the RUN station uses the main antenna(s) while the Inband station is looking for new multipliers or QSOs and uses an RX antenna located far from the main antenna. As soon as the Inband station passes to TX, the switch AS2x2 inverts the antennas, passing the Inband station to use the Main antenna while the RUN does not run out of antenna put to use the antenna of RX.

    The AS2x2 can be connected between the RUN Radio and the Linear Amplifier (in case a Amplifier is used) and between the Inband Radio and the RX antenna. As soon as the Inband Radio activates the PTT the AS2x2 will exchange the outputs, so the Inband will use the Linear Amplifier and the RX antenna will be assigned into the RUN Radio.

    The AS2x2 can be used combined with the Interlock 5R so in this case, the Interlock 5R will control automatically the antenna switching.




    60,00 VAT not included Add to cart

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