The Yagi antenna I am modeling has a small tube, connected to the PL-239 connector, that is parallel to one of the antenna elements. What is it? Why isn't included in the model?
The simple answers are:
A gamma match is a transformer that couples the RF signal to the antenna. The transformer increases the antenna impedance to match the impedance of a feed line. It does not require a balanced feed line so it is particularly convenient when the feed line is coax.
The gamma match is created by using a short wire (or tube) in parallel with the driven element to create a short transmission line. By coupling to only one side of the driven element a unbalanced connection is created that eliminates the need for a balanced feed line or a balun.
The construction here includes a capacitor formed by the rod and the tube, but Paul McMahon, VK3DIP, points out, in his Playing with Matches article, that there is another form that does not include a capacitor.
The clamp, which forms a shunt between the tube and the driven element, is used to tune the gamma match. Moving it back and forth along the tube and driven element changes the length of the matching section. The tube extending beyond the clamp is inconsequential.
In section 3 of his article, Modeling the Un-modelable, L. B. Cebik, W4RNL highlights the gamma match as an example of real-life geometries that are difficult for the NEC2 modeling engine handle. Gamma matches are often constructed of wires and tubes that have diameters different from the other antenna elements. It becomes even more of a problem when those different diameters meet at an angle (90 degrees in the case of the shunt). So while a gamma match can be modeled in general (using wires of the same diameter) using an NEC2 engine, specific instances can not.
So I am not going to waste time even including a general gamma match in the Yagi model, though I will make sure that the signal source is connected to a wire segment that is offset from the center of the wire.