Add SDR Capabilities to your Radio — Part 2

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In my last post, I discussed how to add an SDR to your station using the built-in capabilities of your radio. If your radio doesn’t have this fancy antenna switching and selection feature, you can still integrate an SDR pan adapter into your shack using the handy MFJ-1708B or MFJ-1708B-SDRS devices. These devices can be used to allow your SDR to share the same antenna as your transceiver while protecting it from transmitted signals. It does this in one of two ways (or a combination).

  1. Key down input. Transceivers have a line that is used to trigger external amplifiers called key down or key out line. Usually, this line goes to ground when the transmitter is transmitting. (You should read your radio’s manual to verify this.) If you connect this device to this radio output then it will disconnect the SDR while you transmit.
  2. The device also has an RF sensing circuit that will disconnect the SDR from the antenna if it detects the transceiver transmitting.

Because these units use relays you might want to have a slight delay before reconnecting the SDR to the antenna after a transmission. That delay time can be selected by turning the “DELAY” control near the left edge.

The unit is simple to configure: connect power (12v) and your radio key out signal to “CTRL”,  then connect your transceiver, SDR, and antenna connections to the proper ports. Finally, adjust the delay control until it works for your style of operating. That’s it!

Here’s Pascal, VA2PV, with a quick overview of an earlier version of these devices.
Click the video above to see Pascal describe these devices
Note that Pascal is reviewing one of the earlier models of this device. MFJ made some improvements to these units almost immediately after they began shipping. Be sure to get the one with a “B” after the numbers to ensure you get the new-and-improved model.

MFJ actually makes two versions of the new model: one with a regular SO-239 connector for the SDR, and one with an SMA connector for the SDR. The SMA version is shown below.
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SMA version of the unit

Amazon has some nice patch cables for very little money to complete the project.
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SMA patch cables

Note: If you do have an external amplifier be sure to put the MFJ unit between your transceiver and the amplifier!