MFJ-1708BSDR SDR Antenna switch

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SDR and transceiver - but only one antenna? Then a combined splitter with switching relay helps.

An additional SDR receiver beside the actual transceiver offers many advantages. With the additional SDR you can comfortably observe a wide frequency range and retrofit any transceiver with spectrum display and waterfall display.

The MFJ-1708BSDR relay is looped directly into the antenna cable. While receiving the antenna signal is split between SDR and radio by a matched splitter. When transmitting, the output to the SDR is switched off to protect the sensitive receiver input. The transmit signal from the transceiver is switched directly to the antenna.

The MFJ-1708B-SDR uses four relays for this change-over, so an excellent isolation between SDR and transmitter is achieved. Switching between reception and transmission is either done automatically by an RF sensor in the MFJ-1708, or via a separate PTT line. Automatic switching via the HF sensor is useful if the radio does not have a PTT output. The PTT line has the advantage of reacting somewhat faster and simplifying integration with an amplifier.

The "Mute" output of the MFJ-1708B signals the transmission state, the polarity is switchable. This way further devices and switches can be controlled, for example the mute keying of another receiver. The release time of the relay is adjustable from 50 ms to 4 s. To operate the MFJ-1708B, 12V operating voltage is required, a supply line is included. All three connections (antenna, transceiver, SDR) are PL sockets (SO-239), PTT input and output is using a cinch socket. The MFJ-1708BSDR switch can be used up to 450 MHz.

Isolation (Tx) > 68dB @50 MHz, > 50 dB @ 300 MHz
Isolation (Rx) > 15dB
SWR: < 1.2:1 @ 50 MHz, < 1.8 @ 450 MHz


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Technical Data
Connector A PL Socket
Connector B PL Socket