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MFJ 1708SDR question

Posted: Fri May 10, 2019 9:13 pm
by NewYork
I have purchased a used RSPduo that came with a MFJ-1708SDR (not the B model). The more I search it seems the less I understand. I want to use the RSPduo as a pan adapter for a 100 W HF amateur transceiver. My concern is if the MFJ switch will provide adequate isolation for the SDR. The switch operates as designed via the rf sense or the control line using the transceiver's amplifier keying circuit.

I'd appreciate any input, hopefully from others who are using this switch in a similar arrangement. or those with knowledge about his switch with a SDR.

Everything appears to be functioning nominally. I am a little leery about pushing the 100 W button.

Thanks for reading this and hopefully for an answer...


Re: MFJ 1708SDR question

Posted: Sat May 11, 2019 10:30 pm
by vk3alb
Hi NewYork,

I don't have this switch and looking at the manuals (incl the B variant) I don't see any isolation specifications but I can offer some suggestions.

Easiest thing, ask MFJ what the isolation is for your unit. They should know but if you don't get a satisfactory answer you can do you own testing.

You can do this by turning the TX power down and doing a test transmission through the switch using another rig or your SDR as a measuring aid. With some simple math you can determine if your setup is safe for your SDR.

What follows requires that you have some degree of confidence that the 1708SDR switch is fit for purpose.

It has been mentioned here many times that the safe input level to the SDRplay is 0dBm. Since 100W = 50dBm it follows that you need to have at least 50dB isolation from the relay in your 1708SDR. At the receiver on HF, S9 = -73dBm. Please note the minus sign, this value is 73dB below 0dBm.

Hook up your switch according to the manual. Set up for 10W (40dBm) TX, leave the RX disconnected for a moment and run a test to be sure the unit switches correctly. Now hook up your receiver and transmit briefly. Let's say the result of your test is S9+20 (-53dBm) on your receiver. This is good. If you turn your TX to full power (100W = 50dBm) your RX signal will now be S9 + 30dB which is -43dBm at the input of your SDR - safe enough for your receiver.

Now the math tells us that your receive signal is 43 + 40 = 83 dB below your max TX output. You have determined the relay isolation in your 1708SDR is 83dB.

Re: MFJ 1708SDR question

Posted: Sun May 12, 2019 3:10 pm
by NewYork
Thanks, your suggestion is helpful. So far it does not look good for the MFJ-1708SDR. At 10 watts the SDRuno S meter is pegged at +60 over and the dBm reads -10.
I am hoping MFJ will return my customer service inquiry about the separation. If not then perhaps the "B" model is in order.

Am still hoping to hear from users who are actually using the switch.

73 NY

Re: MFJ 1708SDR question

Posted: Tue May 14, 2019 11:05 am
by vk3alb
Hi NewYork,

OK so 50dB isolation is probably about right for a single relay although I'm a little surprised it's not higher as the SDR input is grounded. I wonder what input you were using? The Hi-Z input picks up a lot of radiated signal.

One option you have is to add some attenuation into the SDR antenna line. Even though it feels wrong to do so and affects your MDS it will protect the RX. I'd start with 6dB and see how you go.

If you want to hack your 1708 you could add a second relay in line with the SDR. I reckon the added isolation would give you peace of mind.

Re: MFJ 1708SDR question

Posted: Thu May 23, 2019 7:52 pm
by Steve B
Do use a control line from your rig to the control input.

As long as the SDRuno display shows less than 0dBm during transmit you'll be OK. If necessary ramp up the power slowly so you can see the effect.

Have you read this app note for more background: ... onRev2.pdf