Re: T/R Switching


Good question.  I do a lot of fooling with different boatanchor matchups.  I built this RF sensing antenna switcher from Solid State Design for the Radio Amateur a few years ago that serves me well.  It has adjustable drop out, so it's semi-break-in.  But no provision for muting so I ride the AF & RF gains.

For little QRP RX and TX boards, I might just use an SPDT toggle switch to swap between TX and RX and maybe get those phones off my ears a bit before transmitting.

But like you said, you'd like to protect a more modern rig.  The old BAs were tough. With some kind of SPDT arrangement whether relay or toggle switch, you're never going to have the TX connected directly into the RX input.  Worst case, slow switching will have the TX deadheaded for a few ms but I don't know that you'd couple excess RF into the receiver doing that.  Maybe if you're on 80 or 40 you can afford to have the preamp off and even a bit of attenuation turned on for an extra measure of protection.  A bandaid fix for sure, I admit.  However, I have a K3 and if it senses excess RF into the receiver, it will start automatically taking such steps.

If your switch or relay had an extra pole, you could use it to ground the RX antenna input during transmit.  That still leaves you with that switching time vulnerability though.  The only thing I can think of for that is some kind of sequencer, like a microcontroller.  Or as others have said:

Something like the magic box would be the Cadillac solution, of course.


Nick, WA5BDU

On 2/13/2013 10:06 AM, Jim, ND9M / VQ9JC wrote:

As a recent joiner of 4SQRP, I'm looking forward to getting back into building my own stuff. The NS-40 seems to be a popular and well designed rig, and my plan is to use my FT-857 for the RX side. Otherwise, the 857 is my day-to-day rig, and I have to protect it. (For QRP use, I drop the drive down and measure the output on an external wattmeter.)
So here's a newbie query from an old time ham (me!) who should know the answer but doesn't: What is everyone else who uses separate TX and RX units doing to mute their RXs when the TXs are doing their thing? A rule of thumb was given me a few days ago that there must be at least 40 dB attenuation to protect the RX front end, but that's also going to depend on what the TX power is to start with.
My old Hallicrafters RXs could handle a bit of a pop at QRP levels when I did manual switching. I'm a bit leery however about subjecting the "newfangled" 857 to similar treatment. I never had a Dow key which would've done nicely for the boat anchors, but - assuming I could put my hands on one - would a Dow key with its mechanical relay have fast enough response time to protect the 857's front end?
What other alternatives are currently in play?
Jim, ND9M / VQ9JC

Join to automatically receive all group messages.