160 meter inverted L radials

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

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  1. K7JOE

    K7JOE Ham Member QRZ Page

    Mike ! Thanks man. I need to put up another inv-L to do some comparisons (including shorter radials,and longer radiating element) !
    great stuff.
     
  2. 2E0ILY

    2E0ILY Ham Member QRZ Page


    I use a 3 relay switched tap loading coil up aloft at the corner feed point of my horizontal loop, co-ax fed, on 160m. It works OK and allows this high Q antenna to cover all of top band with a reasonable SWR. As you mentioned, I made the loop too short to allow added inductance to match it. None of the 4 taps is perfect save at one frequency of course, so a variable cap on a too long a loop appeals. But to remote adjust it aloft would be tricky due to such a contrivance being a lot heavier than a relay box and coil. What effect would having the tuning box at ground level, and the corner of the loop fed with either co-ax, window, or ladder line, and co-ax from the remote adjustable variable capacitor affair, back to the shack about 150 feet away? Thanks, a fascinating thread!
     
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    With overall wire length L=173.7ft, vertical part K=52ft, 4 ea radials length M=88.3ft, with radials O=8ft agl, and a variable capacitor in series with the wire just above the feedpoint, here is what capacitance value brings the system to resonance (and low Swr50) as a function of frequency:

    upload_2021-1-15_8-56-36.png

    This is the minimum Swr obtainable by varying the capacitor at each frequency:
    upload_2021-1-15_8-58-12.png

    This would require a variable capacitor with 500pF fully-meshed. It could also be done with a fixed capacitor of ~200pF in parallel with a 300pF variable. For a 100W transmitter, the capacitor would have to have plate spacing not to arc with ~750Vp-p.

    It is instructive to see the currents in the various wires if the antenna power is 100W:
    upload_2021-1-15_9-11-18.png
     
    Last edited: Jan 15, 2021
  4. WA9GON

    WA9GON Ham Member QRZ Page

    Reading from QRZ forums about inverted L antennas and came across your responses including using software information for 160 M and/or 80 M. I am interested in both of these bands. Basically you showed that using around 30 foot elevated radials and a much longer element works more efficiently than using a matching coil, etc.

    What I have here is a 60 foot tower with HF beams and I am thinking of an inverted-L for 160 M with feed point about 8 feet off the sandy ground going up about 45-50 feet bending towards the ground with the end about 15-20 feet off the ground.

    Would this work using 30-35 foot radials elevated about 8 feet above the ground?

    73, Gerry, WA9GON
     
  5. KG4DYN

    KG4DYN Ham Member QRZ Page

    This is some interesting information. I was attempting to follow some other guys recipes, but this has helped me think differently.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Responding to @WA9GON, I took the existing model and modified the parameters to include a down slope of the horizontal wire, which introduces a Sin/Cos to the coordinates of the wire end. I also "leaned" the vertical wire out from the tower so that it is as far from the tower as possible.

    I used to have up a 160m inverted-L just like this, except that I used my metal hangar roof as a substitute for the radials, and obviously I couldn't "tune" my radials, which gave my 160m inv-L a ~20 Ohm feedpoint resistance, which required an in-shack tuner...:( In my case, the total coax length between the tuner and feedpoint was only about 15ft.

    One thing I notice right away is that the vertical part of the inverted-L induces a lot of current into the tower due to its proximity, causing the tower to become part of the radiating structure, and making it much more complicated. This is due to the tower's size, and because the bottom end is grounded to earth. It might turn out that feeding the down-sloping wire against the tower as a 160m "sloper" might produce a useful antenna. In order to do that, I need to know more about what happens at the base of the tower (how it is 'grounded'), exactly how tall it is, and what is on the tower top (beams, rotor mast, etc).
     
  7. WA9GON

    WA9GON Ham Member QRZ Page

    Thank you, Mike, WA7ARK. I was afraid of something like the tower interacting.

    My tower is a Pirod at 60 feet high. Tower grounding consists of a 8-foot ground rod at each corner ( 4 corners ) of the concrete base about one foot from the concrete and another 8 foot ground rod is 7 feet further out from each ground rod near the concrete base. All 8 ground rods are interconnected by heavy gauge copper wire with each other around the tower base.

    Antennas: 20-10 meter beam with 14 elements and 24-foot boom at 61-foot height and above this beam is a 40-30 meter beam with 4 elements on a 14-foot boom at 70-foot height.

    Rotor mast is the length of the tower, as the prop pitch rotor is located near the bottom of the tower.

    Gerry, WA9GON
     
  8. W9XMT

    W9XMT Ham Member QRZ Page

    With a few more details and a fair amount of time, it would take software such as NEC4.2 or NEC5 to model your setup accurately.

    NEC2 is unable to model antenna systems with buried conductors.
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    As usual, Richard is trying to be helpful.

    Only naive antenna designers purposely build antennas that use the earth as part of the antenna. My goal when designing antennas is to minimize current flow into the earth.

    In the hands of a knowledgeable modeler, NEC2 is more than adequate to model this scenario, especially if the antenna is of the type that doesn't depend on currents in the earth in the first place. If you notice, I strive to keep radials as far from lossy earth as practical...

    I have NEC5.0, and would use it more if it didn't have such a primitive user interface. I use NEC5.0 to check antennas that have incidental Galvanic connections to earth, like Gerry's. The goal is to minimize how much current flows into the earth because that is where losses stem from... If you can minimize earth current, then it doesn't matter which NEC engine you use. In the far field, NEC2 uses the same SN ground loss model as NEC4.

    NEC5 is superior to both NEC2 and NEC4.2 in modeling actual Vertical antennas with buried radials, but that is not what we are doing here.
     
    AK5B and KA0HCP like this.
  10. W9XMT

    W9XMT Ham Member QRZ Page

    All of the installed ground rods described, used with the nearby, re-radiating, top-loaded tower are (necessarily) part of that antenna system.

    They must be included in the NEC model, if an accurate analysis of that system is required.
     

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