Automatic Transmit Receive Switch

SwissUnit: Radio transceiver and SDR/second receiver on one antenna! New


Automatic antenna switch for one transceiver and one receiver on the same antenna.

The use of an SDR with a traditional HF receive is a nice enhancement of any amateur radio station. With suitable software the transceiver can be controlled by the SDR interface. This brings new life to any older radios by adding an up-to-date waterfall feature. But often you face the question, where to get the antenna signal from. Not every transceiver offers an antenna monitor output. What to do?

This is exactly the issue the Swiss made "SwissUnit" addresses. This is the combination of an antenna splitter with an antenna relay and the required controller electronics. When receiving the antenna signal is split up to bith - the transceiver and the SDR receiver. You can use the received signal on both systems, on the radio and on the computer. When switching to transmit mode the relay cuts off the SDR branch of the splitter and the antenna is directly connected to the transceiver. This protects the receiver input of the SDR. As a result you get a very comfortable system with up-to-date SDR reception and waterfall, and your trusted transmitter from the existing radio.

At the same time as switching the HF signal, the SwissUnit also switches an audio signal. This is used to cut off the audio signal coming from the SDR via the computer soundcard to the speaker system, to prevent audio feedback when transmitting. The supplied switching electronics box takes the PTT signal and supply voltage from the transceiver and switches the relays when going to transmit. The signaling is very simple, a cable can be homemade within a few minutes. Practically any transceiver can be used.

SwissUnit antenna relay & cables

The HF relay and splitter is a qualit product with excellent specifications, which can switch up to 100W power. The splitter is specified up to 400 MHz, but can be used up to the 70cm band without too much losses. Connectors for antenna and transceiver: N-jacks, for second receiver/SDR: BNC. See below for fitting cables. The control cable from the transceiver uses a 4 pin rund Foster type connector, as it is found on microphones as well.

There are two versions available. With the model POWER the PTT input is internally optically isolated. One grounded PTT output on the device is required. Most devices have such an open-Collector output either on the ACC jack or separately. Usually you can also tap the operating voltage for the transceiver (12V) from the ACC or tuner jack.
The model SWITCH requires a potential-free relay contact in the transceiver for the PTT output. Optical isolation is of course not required here.
There are two versions of the control cable TRX-Swissunit: one pre-wired cable for FT-991 as well as a cable without a transceiver-side plug for self-wiring. Cables for other transceivers are currently in preparation, please feel free to ask us about them!
Making your own cables is easy though, since there are only up to 4 wires needed.

Here you can find extensive developer notes and documentation of this project:

  • Shipment includes:
  • Swissunit POWER or SWITCH
  • Antenna splitter with control cable
Order No.
SWISSUNIT.SW SwissUnit model 'Switch'
SWISSUNIT.PO SwissUnit model 'Power'
SWK.FT991 Connector cable for FT-991 25.00 € Buy now In stock, shipped in 1 or two days.
SWK.OPEN Connector cable with open end
40390.01 Cable 1m BNC/SMA 20.50 € Buy now In stock, shipped in 1 or two days.
42007.04 4-pole plug for self-wiring of the DC/PTT cable 2.50 € Buy now In stock, shipped in 1 or two days.
Info freight charges
SwissUnit SwissUnit with DX-Patrol SDR

MFJ-1707: Automatic Antenna Switch

MFJ-1707 Automatic antenna switch

Sometimes it is necessary to use separate antennas for receive and transmit. For example you want to use an efficient vertical on 80m for transmit, but a directional beverage antenna for receive. The MFJ-1707 automatic antenna switch serves exactly this purpose. Just connect your transceiver, receive and transmit antenna as well as power - and off you go! The built-in HF sensor (HF vox) immediately switches from receive to transmit antenna. Instead of the HF vox a cable can be used for PTT keying. After releasig the PTT the switch reverts to the receive antenna after an adjustable delay time (hold time). This delay protects for example a very sensitive receive amplifier. An additional switching contact signals the state of the switch, useful for blocking other devices or. The MFJ-1707 automatic antenna switch can be used with any mode. Max. power is 200 W SSB PEP. A 12V DC supply is required, you can use the normal power supply from your transceiver. Size of the switch: 101 × 66 × 38mm.

Order No.
MFJ-1707 Autom. Rx/Tx Antenna Switch 102.40 € Buy now In stock, shipped in 1 or two days.
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TRSWITCH: Automatic Transmit/Receive Switch up to 160MHz


In case you use a scanner or a software defined receiver (SDR) for reception, and you want to transmit with your normal transceiver, you need an antenna switch. Such a device must switch the antenna immediately on transmission.

TRSWITCH does just that. The switch has a connection for the antenna, and two connectors for the receiver and the transmitter. The switching is actuated automatically (RF Vox, from 10mW on) or with a PTT line.

TRSWITCH has an additional AF switch built in: when using an external speaker the receiver is muted, instead the CW sidetone is audible through the speaker. The receiver connector is grounded during treansmission, further it is secured with a lightning protector. Without operating voltage the antenna is always switched to the transmitter (or transceiver).

TRSWITCH wotk from DC to 160MHz, the insertion loss at 160 MHz is 0.2 dB. Max. transmit power is 100W, the device requires 13.8V at 200mA.

  • Impedance: 50 Ω
  • Frequency range: DC - 160MHz
  • Max. TX power: 100W
  • Switching level: <10mW (1.8MHz-30MHz)
  • Turn-on time: <20ms
  • Turn-off delay: <190ms
  • Insertion loss: <0.3 dB @144MHz (0.2 dB typ.)
  • Crosstalk attenuation: > 38 dB @144MHz (40 dB typ.)
  • Power supply: 13.8V DC 200mA

Order No.
TRSWITCH 108.00 € Buy now In stock, shipped in 1 or two days.
Info freight charges
Download: Manual TRSWITCH (PDF, EN, 600KB)