Largest bandwidth for all-pass filter with arbitrary group delay?

I'm trying to simulate the group delay of an Ultra-wideband antenna using the Arbitrary group delay filter specification object. The bandwidth for my antenna is 750MHz, and it's an LPA so it's got a non-linear group delay (kind of a monotonically decreasing group delay across the frequency band). I've played around with the Arbitrary group delay filter, and the highest filter I can design is about 75KHz, above which I get the "Poorly conditioned Hessian matrix" error. Does anyone know of a good way to simulate this group delay over such a large bandwidth?