I don't disagree with what Mmana says but, there is no antenna
simulation program that can give the definitive answer for how a
antenna will actually perform in a real installation.
In other words when a engineer plugs in his values when designing a
new product he can never just rely on the computer or that design as
the final product. If he is to give performance numbers on the product
he can never use the computed values. He must use the measured
values. The measured value is always the most direct and accurate way
to evaluate a product.
I did notice when I was playing with the parameters on that
antenna, that a small change in the coils or element lengths made a
fairly significant change in expected gain, swr, etc. Most center
loaded, or any shortened antenna is a little more touchy about tuning,
ground and proximity to metal surfaces than a 1/4 would be.
Thanks, was a good Friday project.