Loaded Radials on an Elevated 43 ft. Vertical

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE7FXO, May 4, 2016.

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  1. KE7FXO

    KE7FXO Ham Member QRZ Page

    I'm not looking to get into the "virtues" (or lack thereof) about 43 ft. verticals in this thread. Just looking for some answers concerning the proper loading of radials that are currently 33 feet long (for 40 mtrs) to make them play better on 75/80 mtrs.

    The vertical portion itself is mounted at 20 feet above ground and is base loaded for 75/80 mtrs. I have a homebrew relay and coil combo that I switch in and out to make the antenna naturally resonate on 75/80. I also use a remote automatic antenna tuner at the base of the antenna, for minimal mismatch losses along the coax when I am using 10-40 meters and the 75/80 coil switched out.

    The system works great. But currently, I only have one 75 meter radial on the system, but I have four 40 meter radials and four 20 meter radials, and four 10 meter radials.

    Since I do not use 40 meters often, I'd like to add some loading coils to the 33 foot 40 meter radials to make them play better on 75/80. I'm not worried about exact precision tuning using the radials, since any mismatches are tuned out by the remote tuner at the feed point.

    But I would like to know what the proper length of additional wire I need when constructing loading coils for the 40 meter radials to turn them in 75/80 meter radials. I assume that the proper placement would be out towards the ends of the radials, but correct me if I am in error. And I do realize there are several factors at play, such as the diameter of the turns in the coil and distance between turns, etc...

    Could someone please give me some examples or a formula to follow when calculating a ballpark figure of extra length of wire I need to wind the coils with, given a particular form diameter and length.

    Again, I don't need to be exact, just in the ballpark. Although the current radials I am using are specifically cut for each ham band, they are not "tuned" radials, even though this is an elevated antenna. I use my remote tuner to do the precision work.

    I know that non loaded set of 75/80 meter radials should be approximately 61-66 feet + 3-5%. But what about adding a loading coil to a 33 foot radial to make it electrically 66 feet long?

    I am on a 6,000 sq. ft. postage stamp lot which is why I am stuck with a single vertical antenna, and why I can't simple go for full length 75/80 meter radials. I only have one direction where I was able to use a full length 75/80 radial. With my vertical radiator base loaded coil kicked in, that one radial was able to bring the antenna very close to a natural resonance on 75. But I'd like to get any additional benefit I can on 75/80, by turning my 40 meter radials into loaded 75/80's.

    And yes, I know that they are not as efficient as unloaded full length radials. But on an elevated vertical, it should help a bit. It wouldn't be as much an issue on a ground mounted vertical, where you can simply make all the radials the same length (or a little longer) than the vertical element.

    I'm trying to gain any additional efficiency as I possibly can, even if it is small, because I am currently forced to run barefoot, albeit a 200 watt radio.
     
    Last edited: May 4, 2016
  2. AA7QQ

    AA7QQ Ham Member QRZ Page

    I would think that copying the Alpha Delta DX-CC loaded 75/80 meter element would be a good starter.

    Ed
     
  3. KI5FJ

    KI5FJ Ham Member QRZ Page

    I suggest adding some linear loading.Insert a shorted stub ( 16 ft of window / ladder line) to each of the four 33 ft radials.
    73, Joe O, K I 5 F J NNNN
     
  4. W1VT

    W1VT Ham Member QRZ Page

  5. N3GSE

    N3GSE Ham Member QRZ Page

    KE7FXO - I have used this site for coil calculations. Maybe you will be able to figure out how to use it for your application.

    http://oldsite.rac.ca/en/amateur-radio/operating-technical/calculators/rf-coil.php

    Good luck,
    Joel - N3GSE
     
  6. KE7FXO

    KE7FXO Ham Member QRZ Page

    Thank you very much for your replies. I will investigate the info provided and see what I can come up with. Anyone else wanting to chime in, fell welcome to do so. 73's!
     
  7. KF6A

    KF6A Ham Member QRZ Page

    I used end loaded elevated radials (shaped like a T) on my 80m inv-L. Lengths arrived at through modeling to fit my former small california lot.
     
  8. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    There is almost certainly a better way to do this than to use loading coils.

    Linear loading or capacitive loading, as described by WV1T and KF6A respectively, will give less loss than coils.

    You do know that the radials can be formed into a spiral or any other pattern, as long as they don't overlap, right? There is a TIS I know of that uses a very short (in terms of wavelength) vertical and less than ten watts to transmit on the AM band. The FCC intended this type of station to be audible up and down a few miles of roadway, but this station can be heard twenty miles away, even after dark! (Obviously this means they are exceeding allowable field strength limits, but I bear them no ill-will, so I don't want to identify the station.) I asked the station engineer why he thought the set-up worked so well and he said, "I use 120 full size 1/4 wave radials." I asked him how he found the space to fit them all, and he said, "I laid them out in a spiral pattern."
     

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