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 To: , , [TowerTalk] Constructing and Using Loading Coils "Tim Holmes" Sun, 23 Nov 2003 01:50:02 -0500
 ```Hi Please excuse the cross post to several lists, but I am trying to achieve the largest response possible I am in the design phase of a project, and I think I need a bit of help and advice. I want to construct a tri-band rotatable dipole for 10-15-20 meters. I am planning to construct it out of PVC pipe (3/4 inch? Not sure yet) supporting wire elements. I wish to use loading coils to physically shorten the length of the elements. This construction method is based on the methods used to create the Homebrewed Buddipole backpack/portable antenna. This antenna however will be permanently mounted, and as such will be built much tougher (not using the collapsible whips etc). At this point I have chosen the following frequency center points: on 10 meters 28.4 MHz on 15 meters 21.375 MHz on 20 meters 14.287 MHz Using the standard 1/2 Wave dipole formula of 492/ frequency, I have obtained the following lengths for the dipoles for 10 meters - 17.32 ft for 15 meters - 23.02 ft for 20 meters - 34.44 ft Calculating the differences in velocity between free space and the typical conductors, and also capacitance added by insulators on the ends of a dipole yields the following classic formula 468/frequency, and returns the following values for 10 meters - 16.48 feet for 15 meters - 21.89 feet for 20 meters - 32.76 feet I would like this particular antenna to be no longer than the 10 meter length of 16.48 feet or 16 feet 5 3/4 inches. My question is how do I achieve this using loading coils, and where should they be placed. The construction of the coils is a fairly simple process of winding the proper amount of wire onto the proper diameter form, and I have done this in the past and had good success, but in those cases I was always working from someone else's plans, and I wish in this case to empirically derive the values that I will need. I have been unable to find any information on how to calculate the number of turns of wire of size X on a form of size Y to achieve resonance on a desired band, and this is what I am searching for, I also need to know how far out the elements to place the coils and how far apart they should be from each other. The problem of droop in the arms has already been solved, in that I will be using gusset wires to support the outer ends of the elements to minimize droop. There are two additional questions that I have, again related to my trying to learn this from the theoretical side. 1. How would one model the coils in MMANA or any other modeling software? and 2. What factors would have to be accounted for or what design changes would need to be made if said coils were to be used in a Yagi type antenna. I realize that I could easily purchase an antenna of this type, in fact this antenna is modeled somewhat on the cushcraft D3, however I am trying to learn some stuff as well as come up with a workable antenna. At this point I am open to all suggestions and comments. If I have made an error in my calculations etc, please correct me. Thanks Timothy A. Holmes W8TAH - Amateur Radio Extra Class Operator StormNet Chase 9 Certified Advanced Skywarn Spotter Assistant Emergency Coordinator - Medina County Web page: http://www.qsl.net/w8tah _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```