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Hy-Gain 18AVT/WB-A stub tube radial system clone

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N1FAM, Oct 8, 2011.

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  1. N1FAM

    N1FAM Ham Member QRZ Page

    I have an old Hy-Gain 18AVT/WB-A 80M through 10M vertical antenna that is roof mounted about 20-25 feet (two stories) above ground. I currently have four sets of five wire rotor cable as the radial system. The lengths are as described in the 18AVT manual. The fifth set is a second 40M length. I need to replace the radial system due to age. I was interested in cloning the Butternut RMK II roof mount radial system. This is the unit that uses 300 ohm twin lead notched at several places. I have seen a couple of write ups on this system but nothing definitive enough to reproduce the set. I have seen some dimensions but I still have a few questions. One question is that the end of the uncut twin lead leg is mated with the other twin lead leg 3' from the end. I am assuming that the 3' leg that continues on to the insulator Of the notched and longer leg) is not in play at any time, is that correct? Another question is, I was wondering if do I use the butternut dimensions or do I try to use the Hy-Gain manual dimensions which vary a bit. Hy-Gain gives you 37' for 40M, 18' 7.5" for 20M, 11' 10.5" for 15M and 9' 10" for 10M. The Butternut dimensions, when adding the legs together at the proper time vary from the above dimensions. Is the difference simply each manufacturer used different resonence frequencies? I also wondered if I could build a set of twin lead radials designed like the Butternut RMK II but cut to lengths for the 30, 17, and 12M bands. I was thinking that with a total of 8 radials (four like the RMK II and four for the 30, 17, and 12M bands, I could cover everything. I am trying to keep the number down because I will need to just drape some of the radial legs over the edge of the roof as well as I am trying to keep the antenna system discrete. The RMK II dimensions are in the later section of the Butternut Tech Notes found at this website. Any comments or advice regarding what I am planning to do would be appreciated.
  2. W9GB

    W9GB Ham Member QRZ Page

    Hy-Gain 18AVT/WB-A multi-band HF vertical (1/4-wave) MANUAL

    Let's TALK about WHAT you have and a bit of RADIO THEORY.

    1. Your Hy-Gain 18AVT is a QUARTER-WAVE Vertical antenna, that uses TRAPS (Hy-Gain Hi-Q traps) to achieve multi-band operation.
    Hy-Gain / MFJ does not make this exact antenna today (1960s Nebraska design) -- it was redesigned (change in capacity hat construction and traps) a few years ago into the AV-18AVQII.
    The 1/4-wavelength vertical antenna is sometimes referred to as as a MARCONI antenna, after the Italian inventor Guglielmo Marconi .

    2. The ground RADIALS for a 1/4-wavelegth vertical antenna are -- a 1/4-wavelength (10 meters for the 40 meter amatuer radio band), per theory.

    3. It is common to see the radials for some vertical antenna installations to be a bit longer than the 1/4-wavelength -- impedence (Z), ground losses ...
    SO I would recommend using the Hy-Gain stated radial lengths (e.g. 37 feet for 40 meters).

    An appropriate radial design for ROOF MOUNTS, is shown in the Hy-Gain manual (link above)

    Last edited: Oct 9, 2011
  3. N1FAM

    N1FAM Ham Member QRZ Page

    Thank you for the input. I have always been a nuts and bolts guy so the theory snows me some times. The link you provided is very similiar for the manual I have with my antenna and reflects the same dimensions we have been discussing. I have not calculated leg match ups yet, but can I use the twin lead four wire concept with notches to create the four radials?
  4. N1FAM

    N1FAM Ham Member QRZ Page

    I found this write up else where on the forum. Could I use a set up like this with any success or could I take the dimensions from the manual or link and come up with a corrected length set in this configuration? The set up is a Stub Tuned Elevated Radials clone of Butternut STR II Kit.

    A - - - - - - - - - - - - - - - - - - - 27' 8"- - - - - - - - - - - - - - - - - - - - - - - - - - - -B

    A- - - - - - - 15' 8"- - - - - - - - - - - - - - - C - - 3' 10"- - - - - - C - - - - - - -8' 4" - - - - - - - - - B - - -3' - - - - -

    “A & B “ denotes where the wires are connected. "C" denotes the two places the one wire was notched. Wire used was a 300 ohm twin lead.

    The 27’ 8” part of the upper wire plus the 3’ part sticking out the end makes a 40M radial. Probably will also work well on 15M.
    The 15’ 8” part of the lower wire is a 20M radial.
    The 8’4” part of the lower wire in conjunction with the corresponding part of the upper wire (27’ 8”) and the short circuit across the end makes a 10M quarter wave shorted stub which will act like a trap. That leaves 19’ 6” of the upper wire as a 10M radial.
  5. WA7KKP

    WA7KKP Ham Member QRZ Page

    I"ve had good luck with the 18AVT, but don't just set it to the manual dimensions -- you'll have to tune it, starting at 10 meters and tweaking each band on down to 80. Same goes with the radials -- although these are a bit less critical.

    I'd just use 5 conductor rotor cable, using each conductor on a separate band. Start with about 80 feet, and when you cut them to resonance, the two ends will diagonally 'fit' and you'll have two radials. Do this twice, and you'll have a pretty good groundplane.

    I've always used mine ground mounted (2" galvanized pipe about 2 foot into the ground). Even without radials, they do work well enough . . . here if you want radials, you'll need a lot more. I'd go to a farm supply store and get a spool or more of aluminum electric fence wire for radials. Doesn't cost nearly as much as copper, and corrosion is negligible.

    Gary WA7KKP
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