160 meter inverted L radials

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KG4DYN, Jan 8, 2021.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    Anticipating a question about reusing the same set of radials as modeled for 160m in post #19 above on 80m (leading to a dual band antenna with two back-to-back inverted-Ls sort of like a "fan-dipole"), without changing the model, here are the required wire lengths to achieve resonance on 80m:
    upload_2021-1-11_7-15-49.png

    This might require a bit of tweaking in the model by adding the second inverted-L in inverse parallel with the first. The optimizer in AutoEz could work out the small expected interaction...
     
  2. KG4DYN

    KG4DYN Ham Member QRZ Page

    Thank you. Having never operated on 160 I was thinking about centering the band at 1.9 mhz. That would be a length L of about 132 feet. After surveying the trees and using some existing dipoles with marked coax, I think I can get the vertical section from 50-60 feet up. It looks like I have plenty of room for 4 raised radials, although a couple might have to be bent around trees. Installing the radials 8-10 feet high shouldn’t be a problem. I can stand in the back of my Mule to reach them. I am planing to use a Wireman 1:1 current balun at the feed point and maybe a couple of resistors to bleed off static.
     
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    I suggested 8ft for the radial height O only to clear deer antlers, and so that a casual person would not touch the radials while you are transmitting (RF burn?).
    I will run the model with 0=8ft, M=30ft to 80ft in 5ft steps, K=55 - O = 47ft, and let it find L with F=1.9MHz. Think of the process as being an On-line calculator, but I have to run the model for you... ;)

    This tells you how long L needs to be for resonance at different radial lengths:
    upload_2021-1-11_10-0-53.png

    This shows that if you make the radials 35ft, you do not need a remote tuner, just direct coax feed to get a low swr.
    upload_2021-1-11_10-1-52.png

    This shows the expected usable bandwidth, which isn't much... You may wish to reconsider the design frequency...
    upload_2021-1-11_10-43-56.png

    Did you see 132 ft anywhere????
     
  4. KI4IO

    KI4IO Ham Member QRZ Page

    From an older ARRL Antenna Handbook:
    "...Experiments show that the ground system consisting of only 15 radial wires need not be more than .1 wavelength long,
    while the system consisting of 113 radials is still effective out to .5 wavelength.........however if 0.1 wavelength is as long
    as the radials can be, there is little point in using more than 15 of them."

    This always made my head hurt...

    More recent editions of the handbook talk about radials in terms of "how much wire do you have available?". If you only
    have a small amount of wire, then the radials should be installed where the ground losses are the highest, near the base
    of the antenna. "The first priority is to reduce the losses close to the base. As more wire becomes available and the losses
    close to the base have been reduced, then reducing the losses further out becomes useful."

    Of course this applies to radials on/near the ground.

    Jerry
    KI4IO
    Warrenton, VA
     
  5. KG4DYN

    KG4DYN Ham Member QRZ Page

    No sir, I don’t see 132 anywhere. I’m confused. I got the 130 feet from some others recipe. I’ll study the information you sent more. Thank you!
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    When most folk calculate, they start with the wire length, and then try to determine how many, how high, and how long should the radials be.

    I rather pointedly show an alternative procedure, where I start with the radials, and ask what length of wire brings the system to resonance... Even better, I show how to tweak the design so that the final feedpoint Z is close to 50 + j0..., instead of some thing near 25 Ohms, which then requires a matching network...

    For casual 160m operation (or DX, digital, or CW), you are likely to get more out of an 160m antenna that resonates lower in the band... Let me know and I can rerun it when I get near my "modeling computer". Without putting a remote, automatic tuner right at the feedpoint, it is tough to design a 160m antenna that covers the entire band...
     
  7. KG4DYN

    KG4DYN Ham Member QRZ Page

    This is a different approach. So what would it look like at 1.85 MHz? Also, would the K length be better higher? I have a nice white oak that I think I can get close to 60 feet for K. Using shorter radials to would make the installation simpler.
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    upload_2021-1-12_6-42-38.png

    [​IMG]

    upload_2021-1-12_6-44-21.png

    If the radials are 8ft agl, and the 90degree bend is at 60ft agl, then K=52.

    It radiates slightly better toward the side opposite the horizontal wire.
    upload_2021-1-12_6-58-15.png

    upload_2021-1-12_7-2-18.png
     
    Last edited: Jan 12, 2021
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    If you can actually make K=60 (i.e. put the wire corner at 68ft agl), then the optimum lengths for 1.85MHz are L=172.75ft and M=40.5ft. The patterns change only slightly....
     
  10. KG4DYN

    KG4DYN Ham Member QRZ Page

    Sorry for the slow reply, work stuff is getting in the way of the important stuff. I have some 14 gauge insulated wire that I’m planning to use. What’s the best way to increase bandwidth? Coil at the feed point?
     

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