[TowerTalk] Yagi antenna fundamentals

[Al Williams]

I am trying to understand why the so-called reflector has to be longer than the driven element and the so-called director has to be shorter than the driven element for the standard configuration Yagi. I have been reading antenna books and also trying to rationalize what is going on.  Secondly, I am trying to rationalize why the longer element is called a reflector and the shorter element a director.  I posted this to Towertalk perhaps two years ago and only got simplistic (?) answers.  Hopefully some of the great gurus here can provide an explanation.

I think that I understand the fundamental principle of the Yagi i.e. the induced currents in the parasitic elements cause a re-radiation in all directions.  For compass bearing points around the array, the currents in all of the elements are vectorially added.  In some directions the vectors will add, but in other directions the vectors will subtract.
  
What bothers me is that the currents at the end of the elements are low so I would think that the ends would not have much effect on the pattern?

I think that I have the answer for my second question.  For a resonant driven element, a longer parasitic element will have vectors on the side facing the driven element that will add to the vectors from the driven element.  Hence the "reflector" appears to reflect the wave.  For a shorter element the vectors on the side away from the driven element will add to the vectors from the driven element.  Hence the "director" appears to direct the wave.

k7puc