By chapter 5 of my recently-mentioned book on antenna modeling I had enough knowledge to modify an existing EZNEC model of my Yagi to include the additional 10m element I added to it shortly after installing the antenna.
Luckily for me the base Force 12 C3 has been modeled and that file is included in the software CD that comes with the ARRL Antenna Handbook. All I had to do was add the new 10m reflector that converts the C3 into a C3E and then compare the before & after to see what my trouble and $99 bought me.
The verdict: on 10m, very little additional forward gain (.75 dB) but a significant amount of front-to-back (5.25 dB) and some (2 dB) front-to-side rejection.
Before & after plots of 15 and 20 meters showed absolutely no difference caused by the addition of the 10m reflector.
So far, so good.
C3 owners are aware that their antenna has gain and directivity on 12 meters but they may not be aware of the compromise made to this band if/when they upgrade their C3 to a C3E.
With the additional 10m element (a reflector) the C3E has 1.6 dB less gain, 5 dB less F/B rejection and 3.2 dB less F/S rejection on 12m.
As someone who uses this Yagi on all bands 10-20m, I think the additional 10m element is a disadvantage overall. I think I’ll remove it and instead add Force 12’s 40m rotatable dipole to the boom.
Who wants to buy a 10m reflector for their C3 – low mileage, RF only applied on Sundays, etc…