Wednesday 6 March 2019


Protecting the RSP1A

I use the RSP1A primarily to provide me with a panadapter for the FT-897D (or even the Elecraft KX3). The trouble is, when the RSP1A is sharing the same antenna as the 897, I'm unable to transmit for fear of blowing the front-end of the RSP1A. It would take very little power to render it useless.

So the answer is to 'disconnect' the RSP1A from the antenna each time I wish to TX. That would be a little tedious if it were not for the MFJ-1708B SDR, which disconnects the RSP1A each time I press the Mic key on the FT-897D.

The unit works in two ways - one is by sensing RF and the other is by detecting a control signal from the FT897's PTT circuitry. The latter is the preferred method and it is achieved by purchasing a CAT cable which has an RCA Phono lead that is connected directly to the MFJ-1708B.

As soon as I key-up, the signal from the 897 tells the MFJ to disconnect the RSP and as soon as I drop the Mic, it reconnects the RSP to the antenna. There is an adjustable delay setting to allow you to decide when the re-connection is made.

The “B” series offers better isolation, achieved by additional relay isolation, and is specified up to 450MHz. An internal hybrid splitter reduces the antenna loading effect and provides isolation between the rig’s input and the SDR input. A TX LED on the front panel provides visual confirmation of correct operation when the transmitter is keyed.

The MFJ-1708B-SDR features 3 SO-239 connectors, the MFJ-1708B-SDR-S substitutes an SMA female connector for the SDR connection.

  • Product Code: Y8PCATLIN

CAT control AND linear switching with one clever cable!
One problem with the FT-8x7 series of transceivers is that the CAT interface and the PTT output share the same connector. This unique cable not only allows you to control your radio from your PC, but also provides a linear amplifier PTT switching output too!CAT/programming cable
This handy cable allows control of your Yaesu transceiver by your computer.
  • High quality screened (shielded) cable to reduce noise and RF interference
  • Every cable is individually assembled and tested in our UK workshop
  • Superior quality USB to serial chips for guaranteed reliability
  • Length approx 1.7m/5ft 6ins - long enough even if your PC is under your desk
  • Extend up to 5m by adding a USB extension cable
For example, you can use remote control software like Ham Radio Deluxe or edit and backup your memory channels easily with FTBasicMMO. Contest and logging software can fetch your operating frequency and mode direct from the radio. If you are working satellites, your computer can automatically track your transmit and receive frequencies and compensate for doppler shift.
One end of the cable has an 8-pin mini-DIN plug which connects to the radio:

FT-100 – Band Data pigtail
FT-817 – ACC socket
FT-857/897 – CAT/Linear socket

The other end has a USB plug which connects to the PC's USB port. The cable contains a USB to serial converter, so the radio appears as a COM (serial) port on the PC.
The standard cable uses the SiLabs CP2102 USB to serial chipset.
If you prefer the FTDI FT232RL chipset, select the premium version.
Both work with Windows up to version 10.
For more detailed information please read our USB chipset information page.
Please note that a CAT cable is not an audio interface for data-modes.
Linear amplifier keying/PTT/switching output
This cable also enables your Yaesu radio to control the transmit/receive switching of your linear amplifier. Essential if your linear doesn't have RF sensed switching. Even with amplifiers with RF switching, it eliminates the need for switching delays that often mean missing the first part of the other station's transmission - irritating at best, but disastrous while contesting or working fast CW!

The transceiver end of the cable has a short pigtail with a phono (RCA) socket. You will need a 
screened (shielded) phono (RCA) cable to connect to the Key, Tx or PTT input connector of your linear amplifier. 


Rob Ramsey said...

Thanks for posting this info! I just bought the MFJ SDR switch for use with my Yaesu FT-818. The cable you posted looks like a great solution for connecting these together. Thanks!

MadDogMcQ said...

You're very welcome Rob. Thanks for visiting the Blog.

73, Tom, M7MCQ.