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I recently put up a fan dipole for 80/40/20/10. I'm looking for a way to share this antenna with a receive-only SDR and my shack transceiver. Obviously I can just switch this antenna between the two manually, but I was hoping there was a way to have both hooked up at the same time without transmitting into my SDR.

Is there a practical, cost effective way to accomplish this? It would be great to have a waterfall display of the entire band while I'm going at it.

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  • $\begingroup$ Unless you are transmitting at microwatt QRP levels, you don't want to share an active antenna connection with receiver front-end circuitry that can get overloaded or blown out, especially if the bands overlap. $\endgroup$
    – hotpaw2
    Apr 8, 2014 at 0:09
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    $\begingroup$ An alternate solution some people take is to connect the SDR to the IF of the receiver, usually just before the filters. This way, you can get a nice wide waterfall from your SDR, but also can continue to tune with your radio, and have access to all its multiband capabilities and whatnot. Depending on how its done, you can also make it so transmitting with the SDR through the transceiver is possible. $\endgroup$
    – Phil Frost - W8II
    Apr 9, 2014 at 12:19

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Back in the olden days, before transceivers took over, the transmitter and receiver were separate units, and the receiver had to be protected from the transmitter. The solution then, as now, is an electronic T/R switch. One approach is an RF sensor that triggers a relay; the MFJ-1708 is one example. A more sophisticated approach uses an active element to isolate the receiver. Used to be a vacuum tube, and if you do a Google search you'll find lots of DIY TR switch projects for vintage radio fans.

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Without serious isolation, you will smoke the SDR the first time you transmit. Typical (i.e., cheap) coax switches suck at isolation. If you are lucky it'll be 30 dB down. That's still enough to smoke the SDR easily unless you are QRP.

The only way that comes to mind is to make a relay that drops out the SDR when you key up your rig. Most rigs have a signal line to key an amp up, you can use this signal to drive the relay. Basically the inverse of the amp keying circuit.

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  • $\begingroup$ Another relay that shorts out the antenna inputs to the receiver near the receiver might be a good idea. $\endgroup$
    – hotpaw2
    Apr 8, 2014 at 0:14
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For receiving, just a coaxial splitter should work, nothing fancy, just lead the antenna to both radios.

For transmitting, that is a different story, and bandpass filters are required, but on receive, interference should not be a problem.

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    $\begingroup$ A band-pass filter will not help at all unless the receiver is always on a different band than the transmitter, and this is unlikely to be the intended use here. $\endgroup$
    – Kevin Reid AG6YO ♦
    Apr 8, 2014 at 0:56
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    $\begingroup$ In this application, the bandpass filters are likely to be called a diplexer. $\endgroup$
    – Phil Frost - W8II
    Apr 8, 2014 at 11:16
  • $\begingroup$ sorry, my first answer, don't know what im talking about :) $\endgroup$
    – Skyler 440
    Apr 8, 2014 at 18:10

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