Frank,

So NEC4 cannot calculate input impedance of the radial system and we
have almost reached a dead end.

Would it be possible to insert a loading coil ( 2.48 uH ) at the
bottom of the antenna to tune out its input reactance ( which is
what
my program does.)

Then repeat the efficiency calculation and tell me what you get.
----
Reg.

===================================
Frank,

Alternatively, or in addition to, you could shift frequency nearer to
8.3 MHz where the antenna is resonant and its input reactance is zero.

And again do the efficiency calculation. Tell me what the efficiency
is and the frequency. Also the antenna input resistance.

You will see I am desperately trying to localise the discrepancy in
efficiency. It is either in the radials or in the antenna. You should
also learn something about NEC4.
----
Reg.