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Reviews Categories | Antenna Switching, Phasing, Controllers | MFJ 1708SDR Help

Reviews Summary for MFJ 1708SDR
Reviews: 1 Average rating: 5.0/5 MSRP: $$79.95 (2018)
Description: RF sensing antenna switch for SDR radios that share an antenna with a transceiver
Product is in production.
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K1VCT Rating: 5/5 Jan 9, 2018 14:58 Send this review to a friend
Worka well, with a few caveats  Time owned: 0 to 3 months
This is a fairly new addition to the MFJ line.

Some insight - the MFJ 1707 and the MFJ 1708 are EXACTLY THE SAME, except for the label. Why the 1708 is more expensive by ten bucks... ask Mr. Martin F. Jue!

Either of those models, and the newer MFJ 1708SDR can be used to switch out an SDR radio when sharing an antenna with a transceiver. The receiver has its antenna connection shorted to ground upon transmit, allowing the two radios to share one antenna.

You can "modify" the MFJ 1707 or MFJ 1708 to the SDR model very easily by removing the front plate and soldering a #14 wire between the relay contact and the SO239 connector.

Hint: get the MFJ 1708 and solder in one wire, and you've got the SDR model.

For the SDR version, MFJ recommends using the wired switching connection, connected to your rigs "SEND" circuit. If you need to share circuits, remember to use diodes to block the DC power from one circuit to the other. In other words, if you're using the MFJ 1708SDR and engaging an amplifier, and also engaging a mute switch on a receiver, they should all have diodes so that their respective DC supplies don't interact. The sensing circuit is still active, should you choose to use the MFJ 1708SDR as a plain 'ol MFJ 1708.

The SDR version of the MFJ 1708 has the transmitter permanently connected to the antenna. This is to allow the rig to receive while the DSR works as a panadapter. Many SDRs have their audio delayed by the connection to the computer, computer processing time, etc. By keeping the receive audio on the transceiver, you have real time, not delayed, audio.

The MFJ 1708SDR does not have the reverse keying output. Maybe somebody has a use for it, but a contact that goes "open" on transmit is not something I would use.

In the picture, there is a "green LED" where it says delay. Total BS. There is no LED. The green blob was edited into the picture. Where the green "delay" LED is situated in the picture, there is a hole for the "delay" adjustment.

Each of the models MFJ 1708, 1708 and 1708SDR have an adjustable delay that works with RF sensing or manual (wired) operation. There is a delay from zero to two seconds (or so) that keeps things in the transmit mode when the RF sense circuit, or manual operation, is no longer active.

The unit works as described by MFJ, and opening up the box revealed a decent construction quality. Not stupendously great, but decent and workable.

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