Now and then we all have these “DUH” moments. Over 20 years ago I made a loaded 160M vertical with the loading coil at 33’ (the length of 2” aluminum pipe I had available at the time) and a whip on top for another 18’ (total height ~52’). Inductance was chosen to resonant the whole system at the low end of 160m. Works like a champ. Then I discovered that the antenna worked great on 40M as well. The “DUH” moment: 33’ is a quarter wave on 40M. The inductance was sufficient to isolate the lower 33’ section. Serendipity is a wonderful thing! Larry K5LJ
From: Rick Hiller via BVARC Sent: Sunday, July 10, 2022 6:17 PM To: BRAZOS VALLEY AMATEUR RADIO CLUB ; TDXS List Cc: Rick Hiller Subject: Re: [BVARC] Seeking Antenna Design Info Short answer to my design dilemma or "how to kill a hot Sunday afternoon" Focus is -- Shortened dual band dipoles that use a coil for loading a lowband wire and using the same coil to "trap" RF into the higher band, inner portion of the wire. [I was wondering how this worked, as JP had sent me a EFHW design using a single coil using this trick] The dual band antenna consists of a higher band antenna wire (i.e. 40 meters) with coils at the ends of this portion of the wire. Short end wires are added on the outside of the coils to get final resonance on the lower band. The coils play the part of low band loading coils and the part of "traps" for the higher band RF. Why? The coils are positioned at the ends of the center, 1/2 wl, higher band wire portion, which is a standing wave high voltage point and the XL of the coil is designed to be quite high to stop any high band RF flowing outside the high band inner wire...making a resonant dipole for 40 mx. When using the antenna as the longer 80 meter wire, the coil has less XL (XL = 2PI F L), and since the coil is within the standing wave sinusoidal curve, it acts as a loading coil. RF will continue to flow out to the very end of the antenna wire thus providing resonance at the lower frequency. The antenna, of course, has the benefit (to some) of being shorter, physically, due to the loading effect of the coils. The coils must have sufficient windings to produce the correct XL for each band and they must be positioned properly within the wire for this dual band system to work. References: --Alpha Delta' DX-CC and EE antennas use this design with their ISO-RES coils. --In Bill Orr''s Antenna Handbook 1979, he gives 2 example antennas -- 20/80 and 40/80, which I modeled and they work out just great -- nice dual band wires. Orr also comments that this dual function coil trick was first referenced in 1926 by the Bureau of Standards, but I could not find any such publication about it. --In Pop Com Feb 1988, W9INN built a 160 and 80 sloper vertical using this trick (thanks to Matt, KM5VI). So.......y'all probably knew all this, but I am still learning. Thanks for reading or deleting. I've had fun! Time for dinner..... 73...RH On Sun, Jul 10, 2022 at 11:38 AM Rick Hiller <[email protected]> wrote: I appreciate everyone's comments and direction. I knew if I pondered the question long enough and with the help of a few antenna craftsmen the answer has come to me. It's nice to have knowledgeable and interested friends. For others, discovery is a light going off or a bell dinging when you get the answer you seek. For me, most of the time, it is either a Homer Simpson "DUH" or a 2 x 4 to the side of the head. It's was no different this time. The answer to be told soon after a few EZNEC runs. Thanks for all of your comments. Regards...Rick W5RH On Sat, Jul 9, 2022 at 8:57 PM Rick Hiller <[email protected]> wrote: Loading coils used also as traps are my focus. Alpha Delta uses what they call an Iso-Res Isolation (trap) and Resonator (loading coil) in their antennas. Bill Orr mentions a few antennas in his books that use them, but they are cookbook examples for building and nothing on the design of the dual function coil. Can any one provide me with a book reference or a URL etc. that might provide clues as to the design, building and measurement of such devices? I realize that certain coils have a self-resonance and I can find info on that, but there are no references that apply this self-resonance to an antenna design. Contact me direct, if you wish. [email protected] More detail if you wish to read about it..............JP sent me a design of a shortened EFHW for 80 and 40 that uses a 110 uH coil near the end of the antenna wire that provides loading for 80 MX and traps 40 MX energy, thus making the antenna dual band. I modeled it in EZNEC based on the basic design, and lo and behold the antenna when doing an SWR sweep does indeed resonant on 80 and 40. Displaying the currents on the wire show that it is used at full length on 80 and stops at the coil on 40. I think I am missing something in my research. TNX for reading....any help appreciated. TNX ES 73....W5RH Rick Hiller e-mail: [email protected] choke Cell: 832-474-3713 Physical: 9031 Troulon Drive Houston, TX 77036 -- Rick Hiller e-mail: [email protected] Cell: 832-474-3713 Physical: 9031 Troulon Drive Houston, TX 77036 -- Rick Hiller e-mail: [email protected] Cell: 832-474-3713 Physical: 9031 Troulon Drive Houston, TX 77036 -------------------------------------------------------------------------------- ________________________________________________ Brazos Valley Amateur Radio Club BVARC mailing list [email protected] http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org Publicly available archives are available here: https://www.mail-archive.com/[email protected]/
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