I know nec2 uses one set of currents for its calculations. I know that nec2 can handle induced currents, as is apparent with its ability to model yagi antennas, but I'm not sure how it would handle induced currents on a wire that already has currents present. I would think it would add the currents on said wire but I am not sure, and it is possible that it doesn't factor in induced currents on said wires with an existing current at all. I have also attempted to model a hairpin match once, unsuccessfully.
That being said, when I visualize the fields of the Vector design, I don't think you need two separate currents on a single wire to explain what is happening. It is complicated to explain, but currents in the basket area would add up to create two separate combined fields, one outside the basket, and one inside. These fields would be 180 degrees out of phase with each other, and would have minimal if any effect on each other. The outside field would be in phase with the upper section of the antenna, and the inside field is in phase with the part of the vertical that is inside the basket area. This scenario would have an effect that matches the animated picture that shockwave has provided, and without the need for "transmission line mode currents". Nec would have just as much trouble modeling this scenario as it attempts to use currents in place of RF fields to calculate gain. As such, it has no concept that an entire RF field is contained within the basket area of the antenna.
When it comes to transmission line mode currents, every time they are applied when I visualize the fields in the 3d nearfield space around the antenna, no matter which antenna is being talked about, I see a similar effect as what people say it takes two currents to achieve made by a single current. That being said I could be wrong, and am still learning and thinking on the topic.
Anyway, that is my take on what is happening with the antenna in question. I don't claim to be an expert, especially when it comes to nec, but this antenna has spurred more thinking and study of theory on my part than any other antenna out there... Unfortunately what I'm seeing happening with RF fields from this antenna in its nearfield is difficult for me to explain.
The DB
