Unfortunately, the top end is also affected by the horn for several reasons. A horn will have better efficiency towards the low end of its tuning, and in our case where we are trying to get a controlled narrow dispersion, that means a long horn with shallow taper which will unfortunately be tuned low. An initial faster flare taper will load the top end better, but it will still be neccessary to pick a driver that has increasing sensitivity with increasing frequency, which the 6ND410 does provide.
The other problem is reflections and path length differences in the throat causing cancellations, and this is where the phase plug comes into play.
A phase plug won't neccessarily reduce beaming, and if not designed properly, can make matters worse. Beaming is determined by the width of the throat - as the frequency rises, the wavelength becomes smaller in relation to the throat width and the horn will eventually lose control of the dispersion. As we are looking at a narrow dispersion horn anyway, I don't think beaming will be a problem at any frequency we are likely to get up to.
I'm hanging out to get hold of a couple of the 6ND410s to play with, but unfortunately I am being stuffed around severely by one of my customers, and it has impacted on my relationship with my supplier....... so I can't get hold of them just yet...
Cheers
Graeme