Carl,
That's good information, thank you. The information manual that I have for the Narco lists 3 optional antennae and I forwarded those part numbers to the avionics shop. They said they would check to see if they had any of them in their inventory. They happened to mention that other antennae would work but they might not be "approved". If they can't come up with a suitable replacement, I'll go with your idea.
Thanks again.