Topband: EZNEC Shunt Feed Model Gain

[k8gg at voyager.net]

Fellow Topbanders:

I made a model of a shunt fed Rohn 45G
tower with a 2 element short 40 meter beam on top.  I used 78 feet of
tower plus 12 feet of 2.375 inch diameter mast plus the actual dimensions
of the boom and elements for the beam. 
When I model it as
series fed at the base, all the data looks good, with a gain of about 1.5
dbi and a self resonance below 1600 KHz.
When I model it as a shunt
fed tower, base grounded, with 50 feet of 1.6 inch OD pipe about 27 inches
from the center of the tower as the shunt feed rod, the gain drops to
about 0 (zero) dbi.  The radiation pattern, etc. stays the same.
Is this a fault of EZNEC4, or does the shunt feed truely reduce the gain
of the antenna?

Thanks for your thoughts, 
73,    George