Posted by Marconi on 4/22/2006, 9:28:08 The following is all take from an electrical point of view. Electrically the AP uses a feeder that works against a suitable ground to feed the radiating antenna. Here the mast or feed line is used as the ground element (like in dipole). This ground element extends down from the current node near 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 (mast) remain relatively parallel in the vertical plan, there is very little radiation generated in feeding the return side of the antenna, consisting of a 1/8 wave loop at the voltage node at the bottom, another 1/4 wave element extending up from the loop at the bottom, 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 actually loaded near the top rather than near the bottom of the antenna. All of this process matches the feed line, at 50 ohms very smoothly and nicely in the Astro Plane antenna. The exact physical length of all antennas longer than ¼ wave length and that are end fed react this way and require some sort of matching device to be made purely resistive and resonant at the feed point. You will not find any other tuning or matching device in the AP. If you take close note at the actual radiating element it begins at the high voltage tip end of this tuner element where it attaches at the bottom hoop. This hoop begins the physical radiating element, 1/8 wavelength, attached to another ¼ wavelength element returning back up to the hub. Above the hub and directly connected to the bottom radiating element at the current node end in the hub you will find another ¼ wavelength radiator that is shortened. This shortened section uses a cap hat to complete this electrically resonant device at a physical length of 5/8 wave length. So it is referred to as a 5/8 wave antenna. Note: It is my opinion that this transition in matching currents through this long and smooth tuning element, the first ¼ wave element, is the key to good reports and is due to the genius in design. It creates a condition of minimum inductive reactance for the antenna to offset the additional load capacitance created by the excessively long radiator. The long radiator is the sole feature that makes for the possible increase in gain in these end-fed vertical radiators that are longer than the typical ½ wave element. To me this type of tuning and matching has the direct affect of considerably reducing the bad actions associated by common mode currents (CMC) on the feed line and/or mast that may develop additional local RF interference found in other GP antennas. This also has the affect of eliminating much vertical ground noises as well, but I won't get into the business of common mode currents here. But understand, CMC elimination is only one of the important aspects of why I feel this simple, but complicated little antenna works so well. If you will look at the Signal Engineering beam antennas, you will note a very similar tuner for their quad elements. You will see a similar feeder design to this simple, but very effective Astro Plane antenna. Maybe this is more than you wanted to know, but I have studied this antenna and hashed and rehashed this one out for over 20 years now, and this is the best we can come up with. 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 that polarity differences are known to make. This feature in the AP may also account for its quite noise floor operations as well; a factor, which more or less, stands alone by the many consumer accounts of the actions for this unique but very effective antenna. Just try this antenna high up there one day and watch it talk as big or bigger than any and for sure when DX is really rolling. Jim, I did not proof read this close. If I find errors, I will let you know. Marconi"
Message modified by board administrator 4/28/2006, 10:12:55
there are some variations in AP models. I think that Avanti originally built the AP a bit different than the one made much later that became popular. Maybe Antenna Specialists made some changes if they were ever involved with this project. However, I’m not real sure if AS was ever actually involved. There are some models out there that used a different mounting bracket design. I think they likely all work the same however.
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 (at the feed point) is above the voltage node at the bottom loop in the antenna. For me this does not impose a limit to effectiveness at any particular low height. This issue is there is a lot of the erroneous BS about this antenna regarding height and that it only working well when installed low to the earth.
This 1/4 wave section theoretically does not radiate due to equal and opposite energy forces working between parallel elements. This arrangement creates a condition that cancels out radiation, an action that works similar to the actions of current between the shielded and center conductor of your coax. This feeder is fed (in the hub at the feed point) at the beginning tip end of this element. Therefore it typically will not show us 50 ohm resistance at that point. It will be low, just like any end fed 1/4 element working against a suitable ground. To compensate for this low input impedance the feeder is bowed out a bit from the ground affects side as noted above with the mast and/or feed line running parallel and along the side of this tuning element. So, in affect this first section, 1/4 wave element is a fixed tuner for this antenna. This arching (mild bowing) of the elements has the affect of raising the feed point resistance. And in this case it does so at the resonance of the radiating element. The radiating element actually appears electrically as three 1/4 wave elements in order to be resonant. If this did not occur as a 3/4 wavelength radiator then no match at resonance would be possible. Even the end fed 1/2 wave radiator cannot be made to match and be resonant naturally, without a matcher of some sort.
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