Topband: Modeling a BOG

[James Wolf]

I am trying to model a BOG (Beverage On or in the Ground).  I have access to
NEC4 and I am using the 4NEC2 interface.   
As soon as I drop the antenna below ground level, approx. 2 ft. without
errors,  I lose all indication of directivity except for a few dB.   I have
tried different lengths from 200 ft. to 800 ft.

If someone has experience modeling a BOG, can you provide suggestions, a
model?  NEC or EZNEC will do. 
The other thing I have not been able to understand is how the insulated wire
(IS) card works in NEC.  Does it only change the velocity factor or does it
actually isolate the wire from ground conductivity?
I have not been successful in getting the IS card working in 4NEC2 where the
program is happy with it.

Thanks,

Jim - KR9U