[TowerTalk] Ground Plane Model Problem

[K0SN]

Gentlemen:
I have been modeling a ground plane vertical antenna with elevated radials.
It's made of 3" aluminum tubing at a base height of about 15 feet.
My problem is EZNEC is reporting the feedpoint impedance to be
around 10 ohms, which disagrees with the conventional wisdom that
says it should be in the vicinity of 50-75 ohms. Any suggestions? Has
anyone encountered a similar problem while running EZNEC or another
modeling program? Any input would be appreciated.
Tom K0SN
Porterfield, WI