Hey LC! You write the following, "your upper vertical radiator is insulated from the upper mast mounting bracket right?"
I think you will find the upper element of this antenna is in fact directly and electrically connected to the upper mast mounting bracket. There is no insulation at this area even though the device being used looks like an insulator. This is shown as part 24 at level A in the patent. I try to describe this assembly below.
In the newer model AP a device similar to a long nut used with all-thread stock is housed within an assembly attached to one end of the mounting hub. You see this as an insulator. The threaded ends of the bottom and the top elements are screwed into this long nut and are necessarily jammed against a small bolt projecting thru the sides of this nut---which physically secures this unit to the hub bracket. Thus this assembly also makes for a direct electrical connection between all of these parts. I think you will find the top element is not actually insulated from the hub bracket as may be suggested by your comment.
In the original Avanti this area secured the top and bottom elements with the bottom tubing being compressed inside the top tubing in a direct manner, metal to metal inside a split mounting hub assembly. This mounting hub bracket consisted of two parts, a split bracket assembly that bolts together using compression to secure the elements. Results were the same in both cases.
If you are interested in more ideas about the possible thinking at Avanti on the AP project and you have access to the Patent, then look at Patents with numbers 2118496, 2185729, 3104638, that they used. Click on the Pat Num link at the top of page and enter one of these numbers for the reference.
