Idea: a third relay in line with the rig transmitting relay that will release simultaneously with or, if I'm right, before the antenna relay releases.
That will work, and that relay looks like a winner. When I had a Heath SB-220 I rigged a reed relay for the input and a vacuum relay for the output. At 1000+ W I needed to keep the vacuum relay closed until I was sure the input relay had dropped. I used diodes and the RC time constant of electrolytics across the relay coils.,
How did the diodes figure in?
My idea is the same - an electrolytic across the relay coil which - if I remember right, as I haven't played with relays in ages - will keep one relay energized longer after power is removed. Question: will an electrolytic across the relay coil also slow the energizing time of the relay when power is applied? I think not. (For an adequate power supply, it should be impossible for the charging current required by the capacitor to reduce the current flow through the relay coil at all, meaning there should be no change in the energizing time of the relay with or without a capacitor across the coil - right?) And also - I suppose at high keying rates the electrolytic across the relay coil might not have enough time to energize.
Enough talk - here's my schematic:
S1 is the operator's key. "To ant" is the transmit antenna; receive antenna (connected when S3a breaks) not shown.
Key down behavior: The rig is only keyed after both S2 and S4 close. S4 can only close after S3b closes. S3b closes basically simultaneously with S3a, the antenna. So S4 is closed and the rig is keyed only after the antenna is switched to transmit.
Key up behavior: C1 holds S3a/S3b energized briefly whereas S2 breaks contact more quickly. S2 breaking contact unkeys the transmitter (even though S4 is still energized). Some time later C1 discharges, S3a (and S3b/S4, but we don't care about that) breaks and disconnects the transmit antenna (and reconnects the receive antenna, not shown).
One other thing I realized - my rig has a built-in CW keyer (for paddle use), but with this relaying setup I can't use it. Oh, well.