DB, the main error I felt I made in my earlier 827 model with 55" inch radials, over real Earth, had the radiator physical length from the radials/feed point to the tip of the antenna set at 264.8" inches, and that is about 6700 mm as per the manual for overall length.
I think this overall length in the manual includes the bottom mounting bracket that I figure is about 15" inches long. So, if we subtract the 15" mount length it makes the radiator physical length equal 249.8" inches or 20.8' feet. However, without the added length for the mounting bracket and the added inductive length for the coil the model would not tune to resonance for me at 27.205 MHz.
This net length also does not include the inductor matching coil, which also adds some length back to this short radiator. DB, I didn't include the coil in my model and it looks as though you didn't either. So, in my case I apparently added <>15" inches back to the radiator length during my process of trying to tune the model to resonance and also show a good resistance.
Note: I don't think tuning is necessary for good results in modeling...as long as the Free Space Average Gain results for the model is close or equal to the value of 1.
Some folks may not understand this concept, but in such cases we do not need to try and match a model in order to get reasonable and workable results. The only problem I see is...we can't successfully scan the model for SWR or Bandwidth...two simple checks that folks can do with their antennas on their rigs at home.
Over time I've collected 827 dimensions at 27.205 MHz from several kind folks, and they were each different. Most of the errors were omitting the overlap for some tubing connections and showing the full lengths for the tubing instead of exposed lengths.
In my tuned model at 36' feet I used 264.8" inches for the radiator with an average tubing diameter of 0.80" inch, and the model shows nearly a perfect match.
I hope I didn't muck this up.
Below is a PDF file for my tuned 827 models with and without a mast for comparison.
I will fix this model with 4 x 108" radials and post them later for comparison to the model with 55" radials with and without masts.
i show here everything I did to the models to help with the tune, help eliminate Common Mode Currents, to scan the SWR Bandwidth Curves, noting the antenna resonant frequency, and overlays to compare the antenna patterns, gain, and maximum angle close to the horizon.
DB, if you don't mind, what were your lengths for the radiator and the 1/4 wavelength radials you used?
