8291, I describe it this way in a letter to a friend.
Jim, there are some variations in AP models. Avanti originally built the AP a bit differently than the one made much later, but before they went out of business. Maybe Antenna Specialist made some changes. However, I’m not real sure if AS was ever actually involved with this antenna. There are some models out there that used a different mounting bracket design. I think they likely all work the same.
The AP is a funny bird and was designed to allow for maximum radiation at the highest point possible within the limitations for height by the FCC. It is basically up-side-down since the current node (feed point) is above the voltage node in the antenna. This, in no way, limits or improves its effectiveness at any height however. This issue is where all the erroneous BS comes from about this antenna regarding height.
The following is all take from an electrical point of view. Electrically the AP is, in principal, very similar to the voltage feed SE design in their beams. The AP too, uses a 1/4 wavelength stub as a feeder that works against a suitable ground. Here too, the mast or feed line is used as the ground element (like in dipole). This ground element extends down from the current node at the feed point in the mounting hub for some distance below the bottom horizontal orbital ring of the antenna. In addition, since the feeder and the ground element remain relatively parallel in the vertical plan, there is very little radiation generated in feeding the return side of the antenna a ½ wave element consisting of ¼ wave element at the bottom, a 1/8 wave element in the orbital loop at the voltage node, and at the top another ¼ wave element that is shortened by the use of a capacitance hat in the horizontal plane. Thus you have a 5/8 wave radiating element that is situated upside down just as if you hung a ½ wave dipole up side down.
This 1/4 wave section theoretically does not radiate due to equal and opposite energy forces working between the element and the ground. This arrangement creates a condition that cancels any radiation, an action that works similar to the actions of current in shielded coax.
This feeder is fed at the beginning tip end of the element (in the hub), therefore it typically will not show a 50 ohm resistance. It will be low like any direct fed 1/4 element. To compensate for this low input impedance the feeder is bowed out a bit from the ground affects of the mast or feed line. This has the affect of raising the feed point resistance. In this case it dose so at the resonance of the radiating element and this matches feed line, at 50 ohms. So, in affect this section is a fixed tuner for the antenna.
Note: It is my opinion that the transition in matching currents through this long and smooth element, do by design, create a condition of minimum reactance for the antenna. To me this has the affect of considerably reducing the bad affects by common mode currents on the feed line or mast and local RF interference. I won't get into the business of common mode currents here, but this is only one of the important aspects of why and how I feel this simple but complicated antenna works so well.
Ok, are you still with me? We’re down to the bottom end of the ¼ wave feeder. This point in the system is called a voltage node. Do you see now that the voltage node is below the current node in this antenna? This is the exact opposite of traditional vertical end fed antenna. At this point the antenna attaches to an orbital loop, 1/8 wavelength in size that surrounds the ground element in the antenna. From here on out to the tip of the top hat elements is the radiating element an inverted ½ wave element with a 1/8 wave element at the bottom in the horizontal plane. This element is comprised of four segments, the 1/8 wave orbital loop in the horizontal plane, another 1/4 element adjacent and parallel to the 1/4 wavelength feeder also in the vertical plane, and another shortened 1/4 wavelength top element that is a combination of 1/8 wave vertical and a 1/8 wave horizontal capacitance hat. This equals in total a physical 5/8 wavelength for those that want to argue measurements and characteristic wavelengths for antennas.
Regarding the horizontal notations above: This is a vertically polarized antenna, but the presence of some horizontal radiating elements, in my opinion, enhance, in particular, the DX qualities of this very efficient radiator. On the other hand it may be said that this also may cut down on some of the vertical ground wave action in the antenna as it may have some attenuating characteristics. This feature may also account for its quite noise floor operations. These factors may also be noted in many consumer accounts for the actions for this unique but very effective antenna. Just try it high up one day and watch it talk just as big a any, during skip.
Eddie
rodS80-00010+