My intention is to leave the top hat alone on this antenna and to also to build an Astrobeam.
I had an Astrobeam once and liked the front to back rejection. I am not quite sure if I can find the 35 ohm coax for the matching system or if it is really needed. I know some guys had a better match without it. Should have the beam ready soon, most of the parts are made.
Hey Starduster, sorry I put you to the trouble of posting the manuals. It was Wire Weasel that posted he had a A/S Starduster for sale, not an AstroPlane. You have the real macoy AstroPlane made by Avanti.
About your beam idea. What did you do about the dual RG59u transformer before when you had a AP Beam?
I don't have the beam, but I did modify my TopOne with a full 1/4 wave element in the top of the antenna and I did learn something during the process. Since you have questions, maybe what I experienced will help you as you setup you new AP beam. I have to admit however, all of this is speculation on my part, but it makes sense to me.
Here is what I found.
Idea #1. The beam manual says to make the top element about 77 5/8" long from the mounting plate stud on top of the mast to boom bracket and that is very short for a 1/4 wave element. That is all the material they supply for the top radiator however. For the full 1/4 wave element you see in the picture I posted earlier, I made my element 79.5" long from the AP hub to start with. I did this even though I thought it had to be short. I did add 2" for the beam adapter section called the plate that holds the top stud for the radiator however. I ended up with 79.5"
Below is the results I got with my analyzer scanning a full bandwidth curve. You will notice that the length suggested in the Avanti Manual is way short and thus the antenna resonance is up there almost to 28 mhz. You will notice that it makes a nice match starting at 27.605 mhz. It is very flat with a bandwidth about 60 channel wide and at a low SWR. There is a little reactance in the match however and that is probably why we see a 1.55:1 SWR at 27.205 mhz.
Now if you look down frequency to 27.205 mhz, where the antenna should be matched and resonant by the factory, you will see the resistive part of the match is approximately 33 ohms on my chart. My VA1 reads a few ohms low, so the feed point resistance was maybe closer to 38 ohms. I'm not accounting for the transformation or reactance here, so again this is just speculation. However, this factor might help explain why Avanti includes the 37.5 ohm jumper, to transform the feed point match back up to near 50 ohms on the beam.
My opinion would be however, if this is the way your beam might look with this short element, the very nice broad bandwidth should make working 11 meters do just fine if you could live with a 1.5 SWR.
Idea #2. There is another factor that has to be considered with multi-element antennas. It is more simple and may be more practical. When you place another element or two close to and parallel to the driven element, the affect might be to make the short driven element Avanti provides to look a bit longer electrically. If this is so, then it would help to bring the resonance down a bit. If this is what happens, and the SWR is lower, CB guys would go this route---and leave the jumper off. This is a very wide spaced beam and I don't think this response would be very remarkable, so the beam may work fine with or without the jumper.
edit: disregard Idea #2 for now, 'Doc raises a point that these words are not well thought out.
I have wondered for a long time why this beam had that 6' jumper in the kit, but I never had an idea why. After I added a full 1/4 wave element to my Top One, and I saw how it responded, being very short---this idea came to me.
Starduster, do you remember what happened before regarding this jumper?
