A Mechanical TR Switch you can build

From QST May 2007

This design is intended to be used with a transceiver that is equipped with a jack for controlling a linear amplifier. (most do) Here is the key text from the QST article.  For the full text see pp 51/52 of the May issue.

"...if you don't have a spare jack, or don't want to mess inside your transceiver, the amplifier contacts could be used instead to flip an external antenna relay.  On receive, that relay could hook the antenna to both receivers and on transmit, only to the transceiver.  A better way is to use a two-way cable TV splitter on the receiver path.  Cable TV splitters work pretty well on HF and this will provide a proper load to the antenna and isolate the two receivers at a loss of about 4dB - not usually a problem at HF.  Another pair of contacts could be used to mute the receiver during transmit.  If you get it right, the relay can also ground the input to the splitter in transmit as shown in Figure 2 to further protect the second receiver.

Note that Figure 2 includes a second relay.  This is a fail-safe device device so that if there is a loss of power to the relay circuitry, the unit will result in putting the transceiver output directly into the two-way coupler and second receiver - resulting in exactly what you didn't want to happen."