A few things about this antenna.
Their is no such thing as an "end fed" antenna. The best you can do is have a "near end fed". It is impossible to feed an antenna at its end without something accounting for the antenna's "other half". I also don't like how we use the words "other half", namely because it suggests that the other part of the antenna is acting like half of the antenna. In many cases it does very little, especially with some designs, but it must exist for the antenna to function. Even if you take a j-pole, which has a 1/4 wavelength matching stub feeding a 1/2 wavelength end fed antenna, the "other half" in this design is actually an imbalance in the stub, which causes the stub to radiate to some extent.
When it comes to this "other half" many people make many assumptions on how big it has to be. The 1/4 wavelength electrical length is the most common of these. However, for antennas that already include a matching system, these antennas don't need that specific length, and in fact, as little as a few inches will do. The size of this "other half" will affect the tuning of the antenna, but that is only because this "other half" is electrically part of the antenna. In the case of the a99 antenna, their is a built in "other half", namely the tube that is used to mount the antenna to the mast. In and of itself, this is more than enough of an "other half" for this antenna. It is possible to isolate this tube from the mast and choke the feed like immediately under this and have an antenna that works. This setup will work fine, aside from being about the worst place to put a choke on this antenna. (Note: when it comes to the size of the antenna's "other half" and matching sections tenability, not all matching sections can account for all sizes of antennas "other halves" so this may also be a limit for some antennas.)
When it comes to choke placement, putting a choke right at the feed point is, as I said above, about the worst place to put it. Chokes are current devices, and this being a half wavelength antenna, the area near the feed point is a high voltage/low current area of this antenna. A choke placed here will have very little (although still not no) effect on weather common mode currents form and how strong they are. The best place to put such a choke is electrically 1/4 wavelength down from the matching section. On this antenna this will mean using less than 9 feet of coax before the choke. Why less than 9 feet? There are two reasons for this. 1) Their is a foot of coax inside the mounting tube that you really should account for. 2) Unless the coax shielding is exposed to the outside elements the coax will have a velocity factor. This velocity factor will vary from one brand of coax to another, at least to some extent. Taking these two details into account, I would start with a little less than 8 feet of coax. Luckily you don't have to be exact, and positioning the choke off a foot or two will still be very effective.
Optionally, if you make a full set of near 1/4 wavelength radials, you won't need to worry about things like common mode currents forming on the mast or the feed line. 1/4 wavelength radials do an awesome job at dealing with common mode currents, as well as stabilize the antenna system as a whole. For antennas with 1/4 wavelength radials, things like isolating the mast and chokes are more or less redundant. Antennas that include a matching system don't need radials to tune and function, but in some ways said radials will benefit the antenna. If you used the antenna before adding such radials, and then after adding a set of radials you notice a change in performance, you had a common mode currents problem before putting the radials on.
The DB
