Here are two model of the J-Pole with the Average Gain showing 1 in Free Space.
These model are over real Earth using Eznec Average soil condition so the model does not represent the situation for every installation. So, you mileage may vary.
Note: I added am image of the tabular currents for wire #8, the mast. You will notice all the current for each segment in this wire is a very low magnitude suggesting very little CMC flowing.
I added a choke to the top of the mast so I could connect the mast directly to the antenna in this case. The J-Pole antenna act very badly if Common Mode Currents on the mast or feed line. I these currents are not minimized, the skewing is terrrible and this is the worst case for the skewing that folks tend to attribute to the J-Pole in general. That said for this antenna...this bad situation should never be tolerated. Later I will post an image of the pattern with the choke removed to demonstrate the problem.
For convenience in my modeling only, I consider the mast as the feed line in this case and if there were even marginal currents flowing on the mast...it could easily be seen as a red line...similar to the currents noted on the radiator in the Antenna image.
I also added an image of page #40, from Henry HPSD, entitled "The Avanti Sigma4." My highlighted marks show his conclusion for the J-Pole compared to the Sigma4.
Excuse my typo's and other errors if any.
