Using the MFJ-1708 T/R switch?

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Old Ham
Posts: 4
Joined: Thu Jul 13, 2017 3:21 pm

Using the MFJ-1708 T/R switch?

Postby Old Ham » Tue Aug 01, 2017 11:04 pm

I'm posting this to both the SDRplay Community Forum and to SDRPlay Facebook, so I apologize in advance!

My goal is to interface my TS-480 with SDRPlay RSP1 (the later for its better receiver performance). I want to use my TS-480 for transmitting while obviously receiving on the SDRPlay. I purchased the MFJ-1708 T/R switch that many of you are familiar with to do this. The question is RF isolation between the receiver and transmitter port on the MFJ switch.

Does anyone have any knowledge or experience with a setup such as this to offer an opinion about overloading the receiver input with subsequent damage to the SDRPlay? I have successfully used 5 watt QRP levels and it seems to work ok. What about 100 watts? I'm very nervous about this. My modes of operation include CW so I need somewhat fast switching.

What do you think? Are there any suggestions to accomplish this? Thanks!

Hank/ke0cu
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vk3alb
Posts: 11
Joined: Sat Jul 01, 2017 11:43 pm

Re: Using the MFJ-1708 T/R switch?

Postby vk3alb » Wed Aug 02, 2017 7:57 am

Hi Hank,

The maximum continuous input to the RSP is 0dBm. 100W is +50dBm so your relay must have at least 50dB isolation to get you to the upper limit of what the RSP can handle. You need to satisfy yourself that the relay has the required isolation. I see the REC1 port is grounded during transmit so "maybe" that will be enough.

However, don't trust me. If I were you I would ask MFJ to tell you what the isolation is for their relay.

There's another thread about this.

viewtopic.php?t=1158
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Lou
VK3ALB

Old Ham
Posts: 4
Joined: Thu Jul 13, 2017 3:21 pm

Re: Using the MFJ-1708 T/R switch?

Postby Old Ham » Wed Aug 02, 2017 8:49 pm

Thanks again, Lou. Some kind of receiver guard, like the one from DX Engineering is probably what I will have to do. It's funny, though, the MFJ T/R switch grounds (or shorts, if you will) the output to the receive antenna. You'ld think that would be the end of it right there. :?
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vk3alb
Posts: 11
Joined: Sat Jul 01, 2017 11:43 pm

Re: Using the MFJ-1708 T/R switch?

Postby vk3alb » Thu Aug 03, 2017 2:33 am

Hi Hank,

Yes that could well be enough but if it is not then you may have a more difficult problem to solve. If you have the test gear you should measure what you get a REC1 with 100W TX.

If you can't get an answer from MFJ I would do that measurement, and, because I always consider "what if?", I would also add a second relay in line with REC1 to ensure that RSP was really isolated from the TX. Only a little extra work and a small cost compared to the replacement of your RSP.
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Lou
VK3ALB


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