I am in the process of optimizing a model of a ZS6BKW antenna using AutoEZ and EzNec6+. I have also been testing an actual physical antenna built to the dimensions that came out of the optimizer, and am trying to account for a small discrepancy between the model SWR vs frequency results and actual measurements of the antenna in-situ, using my RigExpert AA-600. All other aspects of the antenna design remaining constant (Apex Height, Apex angle, Out-of-plane angle, LadderLine Velocity Factor, LadderLine Impedance, series capacitor), I was using the optimizer to find the optimum Wire leg lengths and LadderLine length to get the SWR50 as low as possible on several HF bands simultaneously. I got a solution, built it, and measured it... After seeing a small discrepancy between the modeled SWR and measured SWR plots, it occurred to me that I might not have been building what the modeler specified, especially the length of the LadderLine matching section. Shown in the picture is the transition from LadderLine to coax through a Common Mode Choke CMC. I am using a CMC built from the Wireman 8331 kit. Everything is housed inside a PVC channel cut from Vinyl fence material. The LadderLine is clamped to the Vinyl, and exits right. The 50 Ohm feed is the PL259 on the left. The RG303 coax inside the CMC connects to the LadderLine at the terminal strip. The optimum LadderLine length predicted by the modeler was 40ft 6in, which is what I thought I built, shown in the picture. Where do you think the transition from 50 Ohm coax to the LadderLine actually is? I initially thought that the transition is at the terminal strip, but it is otherwise. The modeled antenna behaves like the physical antenna when I tell the modeler that the LadderLine is 41ft 3in long, approximately in the middle of the CMC. Can you see why? After I accounted for this in the simulation, the agreement between the model and the physical antenna measurement is dead-nuts on! I plan to post the results of the modeling, the optimization process, and how I built the actual antenna in this forum.