I do appreciate the reference to the horn mouth paper. This is much like what I use in my text. I found some slightly different results and Johansen admits that it is known that the wavefronts do not adhere to the walls in many horns, but his analysis assumes that they do. This is a weak point in the paper.

In my text I do not assume this adhernce since I do the calculations using higher order modes which accounts for this effect. I would thus claim a slightly higher accuracy to my analysis than Johansens.

But I really am indebted to you for a coincidence that happened after I reviewed the Johansen paper. The next article in that journal is about using woven cloth as a HT screen, which I had also recently concluded to be supperior to a commercial screen for all the same reasons as in the paper. My new screen cost me $10 (bed sheet) and replaced an inferior $1200 perforated vynal screen! Can you beat that!