Hell Jeff,

I have found your excel sheet and did the model calculation. Your feed
have to be optimized. Also, some parts (surface patches) nearby the
feed, must be removed.

My model with surface patches:
Tin can diameter: 100 mm
Length: 135 mm
Lg/4: 44.452 mm
My model gets round about 7.7 dBi with the same dimensions. Calculated
total gain is 8.3 dBi, average gain test correction is 0.6 dBi. 8.3 -
0.6 = 7.7 dBi.

Model with wire grid (more accurate):
Total gain is 8.4 dBi without error (average gain test=1.0 (-0 dBi
correcting factor)).

Of course, with 198 mm diameter flare for the cantenna, you get 12 dBi
gain.
But the original cantenna can not be have 12 dBi!!!! It is a cheap
marketing issue.

Aziz