RF Sensing Switch
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RFI-EMI-GUY
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You might search past issues of QST and QEX. I recall a construction article of an TR switch like the MFJ/ Also TAPR has a product, but it appear low power.
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I was afraid of that. That's what my google-fu seemed to indicate. I'm not yet willing to open my radio and install a panadapter, so I may toss one of my mobile HWEF antennas out the window and us it only for my SDR and see how it goes. I was only trying to get an edge and do some spotting within the rules and maintain my alternate power status.
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I think Phil Salas wrote an article on such a switch. I had some questions as to the failsafe aspect of it. You could build your own easily. Just make sure you consider failure modes.
It's not "essential" but just a thought. I've never contested before (alright, I did WW Digi FT8 this year, which is very different) but thought it might be an advantage. Hind sight, I should have gone with the IC-7300 over the FT-450D.
I did find Phil's article: http://www.ad5x.com/images/Articles/SeperateTRantswitch.pdf
It's not exactly what I was looking for, but could be modified pretty easily by changing the relay. It may be a project for another time, since there are no longer stores to go pick up the parts from.
I did find Phil's article: http://www.ad5x.com/images/Articles/SeperateTRantswitch.pdf
It's not exactly what I was looking for, but could be modified pretty easily by changing the relay. It may be a project for another time, since there are no longer stores to go pick up the parts from.
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EDIT: Here is the article. I could not locate it searching for SDR because actually he made it for protecting a spectrum analyzer. There is no RF sensing here. You will have to cobble that from another design. Problem with RF sensing is that the RF will be present at the SDR receiver while waiting for the relay to disconnect it.
Give some thought on how you want to tackle that.
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That is a different article. I am sure he had one for an SDR panoramic receiver to share the antenna during RX and isolate during TX. I think he had a light bulb "fuse" or something. I was not confident of the protection it provided. Can you tap into your receivers first IF somehow and band scan from there?
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I could always use the amplifier controller port for keying to make it disconnect. That should work. I was going to see if my RLT-SDR had enough sensitivity to be useful, but my kids made my adapters disappear from my desk (my calculator also ended up downstairs), so that's frustrating. I was going to see if it was even worth the effort. 
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The RTL-SDR and similar really need an outboard RF preselector to be happy on any significant antenna system.
A simple 12v Dow Key relay with constant 12v to one side of the coil and the other side to the amp keying port on your radio will work fine and I've done it. You don't need the more complicated circuit from Phil. BTW Phil Salas is my neighbor a couple of doors down from my TX house. Haven't met him yet but someday I'll catch up with him.
I could always use the amplifier controller port for keying to make it disconnect. That should work. I was going to see if my RLT-SDR had enough sensitivity to be useful, but my kids made my adapters disappear from my desk (my calculator also ended up downstairs), so that's frustrating. I was going to see if it was even worth the effort.
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I think i remember Phil Salas from the GE/M/ACOM/COMNET/Ericsson era.
Do you mean it gets overloaded?one is the V3 and I was able to pick up a few strong HF stations with the basic extendable whip that came with it, but I could see it overloading with a real antenna.
That could work, but I'm not sure if I would have to tell it there is an amp attached, and what that would do to power output.
For a "ground out" or amplifier key out you would put constant 12v to one side of the relay coil and the other side goes to the radios amp key connection. During receive no current flows through the relay coil and when you transmit it grounds the floating side of the coil energizing the relay.
Under failure, that would leave the SDR connected at all times, correct. The schematic would only connect the SDR under power and under failure would be disconnected.
Or am I off base, it has been that kind of week.
The relay coil would be on a single pole, double throw RF relay. You wire the antenna to the common port of the relay, SDR to normally closed and transceiver to the normally open. During receive the antenna is routed to the SDR and when you transmit the antenna is routed to the transceiver. If power is turned off the antenna defaults to the SDR. You could also put a manual switch to the relay coil to energize it and route the antenna to the transceiver at all times if you want to receive on that.
I was trying to "duplicate" the MFJ, where it never switches off transceiver's receiving, but only protects the SDR. Basically making an inline panadapter without opening my transceiver. I think now I read the description wrong.
You could keep the receive path to the transceiver and only cut it to the SDR. It should be easy at HF but for VHF/UHF the loss might be too much. Just put a T adapter in line with the transceiver antenna and put the RF relay at the T feeding the SDR. Use the same wiring I mentioned before and during receive the SDR will be in parallel with the transceiver and when you transmit the SDR is disconnected. There will be some receive loss doing this but at HF it will go unnoticed.
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Nope:
If the radio is asserting ground during TX, the R1 lead should connect to the "ground" side of RLY1 and have no other ground connection..
The Phil Salas circuit I noted above will give you simultaneous reception on your radio and SDR. The BNC Marked SA is where your SDR goes. Also put a preselector in line there. Note the two 1n4003 diodes should be soldered back to back. It is unclear in the drawing. and he has a 100 ma fast blow fuse which is a better solution than light bulb in his other circuits. You will have to order several fuses * from internet. I would try to make the 100 Ohm resistor larger to the extent it does not degrade your SDR reception.
* The fuse will also protect the SDR against lightning discharges near the antenna. It may fail from time to time.
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