I have only one HF antenna and I wanted to share it between my Softrock Ensemble RX and my Kenwood TS-440s. The helpful folks on the Softrock40 mailing list pointed out that to avoid damaging the Ensemble I should disconnect it from the antenna when the TS-440s is transmitting.
This is what I came up with:
For the most part the parts are not critical and except for the enclosure and the DIN connector these were parts in my junk box.
The circuit provides more than 20mA to the drive the relay while staying under the 10mA limit on the signal provided by the TS-440s. An LED provides an indication when the relay is turned on.
And here are some photos of the final result:
Before final assembly.
Ready to close the top.
Ready to go.
The schematic is available below. The .sch file is for the free Open Source schematic editor, gschem, from the gEDA project.
Ideas for future versions
The helpful folks on the Softock40 mailing list provided some further analysis and suggestions. For example:
"I would think that switching the ground is normally not necessary.
I would double up the contacts, just switch the antenna. That way you have a
lower resistance and more reliable switch."