Inverted L - 80m/160m

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N5CM, Oct 27, 2018.

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  1. N5CM

    N5CM Ham Member QRZ Page

    Given the increase in activity on 160m, I'm getting the itch to build my antenna. I have a pulley in the air (suspended from 3 tall pines) at approximately 78' AGL and a 4x4 post with the top at 9' AGL. With some sag from the weight of the wire, I'm guessing the vertical portion of the radiator will be on the order of 65' to 68' long. The remainder will be as horizontal as I can make it, but I suspect it will droop/slope downward a bit.

    I would like the match to be capacitive on 160m. 234/f for 1.8 MHz gives me 130'. I'm thinking about a length of 140' for the radiator. I should be able to run 3 or 4 resonant elevated radials each for 80m and 160m. My station is 100 watts. Unfortunately, I cannot fan the radials out symmetrically (around the compass). Angular distribution for the 160m radials would probably be 180 degrees at best. I may be able to get a bit better angular distribution on the 80m radials but still not 360 degrees. I have 1,000 feet of #18 TFFN stranded to use for radials as well as other bits and pieces of wire that could be pressed into service.

    According to MFJ literature the 993 BRT provides up to 3,908 pf capacitance and 24.86 microHenry inductance.

    I do not have an antenna analyzer, and I'm not model savvy. I may be able to borrow an analyzer.

    I have 2 questions.

    1. Would an MFJ 993BRT remote tuner at the base of the antenna match this system on 80m and 160m?
    2. Does the diameter/thickness of all the elevated radials need to be the same? I'm guessing not.

    Thanks in advance.

    73 de KA5GEX John
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I modeled your proposed antenna: See if this correlates:


    Make any corrections to wire lengths and diameters, as needed. I will then resim, and post the feedpoint impedances.

    Pay particular attention to the radial layout. Note that the one radial under the horizontal leg (wire 5) is doing most of the heavy lifting (see current along it).
    Last edited: Oct 27, 2018
  3. N5CM

    N5CM Ham Member QRZ Page

    Mike, please run with current length & diameter wire.

    It appears that a match on 80m would be beyond the capability of the cited tuner.

    I have a 350' horizontal loop up 60' with which I've worked some reasonable DX on CW, but I have nothing for 160m. I was hoping to get both bands from the inverted L and perhaps improve my lot on 80m, but it doesn't look promising.


  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Is there any particular reason why a shorter inverted L wouldn't work. It would be always capacitive on 160m and always inductive on 80m? An L-network tuner should be able to handle either. It is usually best if it doesn't flip from +j to -j in the middle of a band.
    With the proposed lengths, on 80m, the antenna looks too much like an end-fed half-wave. That might be ok if you switch in an autotransformer between the remote tuner and the antenna only on 80m.
    KD6RF likes this.
  5. N5CM

    N5CM Ham Member QRZ Page

    The reason I was looking at something just over 1/4 wave on 160m was to be able to match it with a capacitor rather than an inductor. I am under the impression, perhaps flawed, that the use of a capacitor for the match may be more efficient for 160m.

    Decreasing the length would certainly make life easier as it would likely result in the horizontal portion being more horizontal and less droopy. I went out and measured from the post toward the tree to which I will run the horizontal part of the wire. I have about 40' of clear air before I get into the limbs. So, if we look at 65'+40' = 105', perhaps that would be preferable to my original design?

    Thank you for your time and help.

  6. WA7ARK

    WA7ARK Ham Member QRZ Page

  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here are the four corners:

    EZNEC+ ver. 6.0

    KA5GEX inverted L 10/27/2018 12:23:10 PM

    --------------- SOURCE DATA ---------------

    Frequency = 1.8 MHz

    Source 1 Voltage = 134.9 V at 80.13 deg.
    Current = 1 A at 0.0 deg.
    Impedance = 23.13 + J 132.9 ohms
    Power = 23.13 watts
    SWR (50 ohm system) = 17.851 (75 ohm system) = 13.667

    --------------- SOURCE DATA ---------------

    Frequency = 2 MHz

    Source 1 Voltage = 301.4 V at 83.77 deg.
    Current = 1 A at 0.0 deg.
    Impedance = 32.71 + J 299.6 ohms
    Power = 32.71 watts
    SWR (50 ohm system) = 57.050 (75 ohm system) = 39.293

    --------------- SOURCE DATA ---------------

    Frequency = 3.5 MHz

    Source 1 Voltage = 1359 V at -75.24 deg.
    Current = 1 A at 0.0 deg.
    Impedance = 346.2 - J 1314 ohms
    Power = 346.2 watts
    SWR (50 ohm system) > 100 (75 ohm system) = 71.339

    --------------- SOURCE DATA ---------------

    Frequency = 4 MHz

    Source 1 Voltage = 474.2 V at -72.89 deg.
    Current = 1 A at 0.0 deg.
    Impedance = 139.5 - J 453.2 ohms
    Power = 139.5 watts
    SWR (50 ohm system) = 32.565 (75 ohm system) = 21.984

    Taking what looks like the worst one:


    Piece of cake.
  8. N5CM

    N5CM Ham Member QRZ Page

    Mike, thanks for calculations, four corners, and the link.

    My original plan was to run the horizontal portion to the southwest. If I do that, I'll be limited to 40' of clear air.

    On further examination, it appears that I can run the horizontal northeast and likely get the 90' horizontal in clear air. The northeast run would require permission from my neighbor to shoot a line up into one of his pines. We get along fairly well, so I'll inquire about the possibility.

    Thanks again for your time and expertise!

    73, John
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    After seeing the patterns, it might not be any better on 3.8 MHz than a dipole.
    KA5GEXinvL Fields.gif
    Blue is 1.85MHz; Red 3.8MHz

    Maybe putting a trap in it for ~3.5MHz (near the corner) might suppress the high angle stuff on 80m (and shorten the horiz wire)?
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    So I added a trap (8.3uH,250pF) in the horizontal wire near the corner with the vertical wire, and shortened the horiz wire to 75 ft. Here are the patterns:
    KA5GEXinvL Fieldswtrap.gif

    Green is 3.8Mhz; Blue is 1.85MHz.

    Here is a combined plot of R and jX:

    Now you dont have to flip the L-network around when you switch bands.

    Here is the current state of the model:


    Note that I augmented the radials a bit.
    Last edited: Oct 27, 2018
    N5WVR and KD6RF like this.

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