I have an ICOM handheld for backup.

I have a panel-mounted ICOM connector, ICOM part no. ANTSB.

The ANTSB is a splitter, in series with the coax that connects the COM-1
radio to its antenna. On the front of the ANTSB is a 3.5 mm female jack.
When a 3.5 mm male is plugged into this jack, the installed COM-1 is
disconnected from the antenna, and the 3.5 mm jack is now connected to
the antenna.

This requires a coax (50-ohm impedance) with a 3.5 mm male jack on one
end and a BNC connector on the other end. The BNC connects to the ICOM
and the 3.5 mm plugs into the panel-mount ANTSB; the ICOM is now using
the external COM antenna.

ICOM sells the ANTSB box with the four-foot coax as a kit. Phone call to
ICOM reveals that they do NOT sell the coax jumper separately, as a
replacement part.

I have tried everything I could think of -- avionics shops, Radio Shack,
Wentworth (used avionics), etc., all with no joy.

Does anyone know where I can find a four-foot 50-ohm coax (I believe it's
R17x/U, where x is either 4 or 9, I can't remember) with a BNC on one end
and a 3.5 mm mail on the other end?