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MFJ-1708SDR review - Share your antenna between your transceiver and an SDR

Discussion in 'Amateur Radio News' started by VA2PV, Jan 21, 2018.

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  1. VA2PV

    VA2PV XML Subscriber QRZ Page

    With all the low-cost performance SDR available these days, we can have a lot of fun experimenting with it. But we need antenna, with the MFJ-1708SDR you can share your favorite HF/VHF/UHF antenna with an SDR.

    This arrangement allows you to simultaneously watch and listen. Then, when you are key up to transmit, the MFJ-1708SDR cuts off and grounds the SDR antenna line, providing your SDR with bullet-proof protection from damaging RF. It`s the perfect plug-and-play solution for upgrading your shack to wide-screen living color. MFJ-1708SDR uses the PTT signal from your rig to initiate the switching, but as a fail-safe measure, built-in RF sense circuitry will provide a measure of safety if the PTT connection is lost.

    In this video you will find a quick review and demonstration.

    If you like my video, please support by subscribing to my YouTube channel!

    Thanks for watching and 73.
    Pascal VA2PV

     

    Attached Files:

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  2. AI6US

    AI6US XML Subscriber QRZ Page

    Hello Pascal,

    Thank you for another great video.

    The MFJ-1708SDR model description states that the device allows the antenna to be shared between a transceiver and a SDR used as a panadapter. Does this mean that both the Radio and the SDR ports have signal in the receive mode (no PTT enabled)?

    I purchased the original MFJ-1708 model and found that it only allows the antenna signal to be shared on the Receive port. The Transmit port has no signal in the receive mode. This makes it impossible to use a SDR as a Panadapter and the transceiver as the receiver.

    Thank you for the clarification on the MFJ-1708SDR function!
     
  3. VA2PV

    VA2PV XML Subscriber QRZ Page

    Hi Brian,

    First, thanks for your comments and watching my videos.

    From what I read, it's the difference between the MFJ-1708 and the MFJ-1708SDR. I confirm that when the unit is turned on and there is no PTT, you have received both the radio and the SDR receiver. This was first checked on the multimeter, I should have shown it in the review as there was no propagation when I shoot the video.

    You can use it as an antenna port for your SDR or simply use it to feed a second receiver (RX ONLY). What's great is that this is done before entering your rig, for example, if you have a 40M and 80M dipole on the same feed line, you can listen to 80M on your HF radio and 40M on the SDR ...

    I hope this helps.

    73 Pascal VA2PV
     
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  4. KG2C

    KG2C XML Subscriber QRZ Page

    Hi Pascal,

    In the video you mention that if you want to run more than 200W you would place it between the rig and the amp. However since the rig's PTT is already connected to this unit, what would be triggering the amp? The non-SDR model had an Aux line which I think was for keying an amp when the switch is triggered but for some reason this model does not have it.
     
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    It is basically a antenna T/R switch.

    upload_2018-1-23_5-14-32.png

    Make sure your SDR Bias-T is turned off, Because the SDR gets shorted out.
     
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  6. K5KNM

    K5KNM Premium Subscriber QRZ Page

    Just use a RCA Phono plug Y cable and patch to the amp and this device.
     
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  7. KK5JY

    KK5JY Ham Member QRZ Page

    Looking at the schematics, the 1707 and the two 1708 variants appear to be the same device internally, just with different stenciling on the front panel, and the "mute/aux" jack missing on the 1708SDR.
     
    KD4LBR likes this.
  8. K1VCT

    K1VCT Ham Member QRZ Page

    Let me address this first:
    ".....Just use a RCA Phono plug Y cable and patch to the amp and this device......"

    No. Bad idea. Both the MFJ1708SDR and your amp are going to supply voltages (and in the case of the amp, not insignificant current) to be grounded when the transmit relay is engaged in the transceiver. Using a Y cable will mix those two power supplies. The better way is to use a diode (a 1N4007 should do just fine) to isolate those two supplies, disallowing them to "backfeed" into each other.

    The MFJ1707, 1708 and 1708SDR are all basically the same. The 1707 and 1708 are the same except for labeling (yet the 1707 is ten bucks more!). The 1708SR just has a wire soldered in place that shorts the transceiver input to the antenna. The transceiver is always connected to the antenna, and that allows it to receive audio while the SDR acts as a panadapter.

    The issue - there is a time lag as the SDR talks to the computer, the computer processes, and outputs to its audio system. The lag can be short, like maybe 1/4 second, or long as in several seconds. To get around that, the 1708SR keeps the transceiver connected as a receiver, so the SDR can be the panadapter. Remember, the 1708 is meant to combine separate receive / transmit hardware on one antenna, without the delay of the computer decoding.

    Depending on the delay in your receiver, between PTT send and RF envelope (as short as 3.5milliseconds in the case of some ICOMs... really fast!), you might wish to rig a little relay interface that uses your mic's PTT circuit to trigger the 1708SDR and the transceiver through a second, cascading relay. The 1708 and 1708SDR have an adjustable time delay one return switching, to account for any delay introduces with a system like I just described.
     
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  9. K1VCT

    K1VCT Ham Member QRZ Page

    I should also point out... you can make a 1708 into a 1708SDR in a few minutes time, by adding one one, preferably a rather heavy one to handle 200w.
     
  10. VA2PV

    VA2PV XML Subscriber QRZ Page

    K1VCT it's preferable to have some diode for isolation. Also, it's more important if you do not have a soft key installed on your amps. I key 5 devices with my TS-990 PTT, I made my own splitter with a diode installed on each port. I will check my archive as I have a diagram for this, somewhere... but it's very simple. 73 Pascal VA2PV
     
  11. VA2PV

    VA2PV XML Subscriber QRZ Page

    Hi Milen, the answer is to use a splitter, but you have to be careful depending on your setup. You're gonna find more information within this tread. 73 Pascal VA2PV
     
    Last edited: Jan 24, 2018
  12. K1VCT

    K1VCT Ham Member QRZ Page

    Just stick the cathodes toward the amp or 1708SDR
     
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  13. VA2PV

    VA2PV XML Subscriber QRZ Page

    I meant in my previous comments... I made a typo ;-)

    Thank you Charles for your post. 73

    K1VCT is right, it's preferable to have some diode for isolation. Also, it's more important if you do not have a soft key installed on your amps. I key 5 devices with my TS-990 PTT, I made my own splitter with a diode installed on each port. I will check my archive as I have a diagram for this, somewhere... but it's very simple. 73 Pascal VA2PV
     
  14. KK5JY

    KK5JY Ham Member QRZ Page

    If you look at the schematic (above), you will see that the PTT line for the MFJ device already has such a diode (D7). A quick look at the schematic of the amplifier to be used could verify that it also likely has a similar device in its PTT line.

    A local friend and I were looking at HR50 and KPA-500 schematics the other night, both of which show a blocking diode on their PTT lines, as well. Either (or both) of those amplifiers would work fine with a simple Y cable connected to this device also.

    What you describe is a valid concern, but modern device vendors are already including that feature.
    After doing the validation I described above, follow this advice:
     
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  15. KG5PIO

    KG5PIO Ham Member QRZ Page

    This video needed to be done, and I was about to do a simple one myself. This so much enhances rigs without a panadapter and even when you do like the 7300 like I have, enables the use a great software with spectrum scope on a monitor with smooth and quick responsive waveform/waterfall on screen and added benefit when not using CAT control to now have two independent receivers on any band at the same time. I did this on the original 1708 (with simple modification) and nice to see MFJ catch on and make a ready to use model. This has been the solution for folks who don't like the Icom remote software or other software that gives you a slower/ laggy spectrum/waterfall.

    If you have a SDR already doing nothing or get a cheap SDR and the MFJ1708SDR will bring you much more enjoyment and capabilities from your gear.
     
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