Low-height 40m Inverted-L?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Nov 5, 2018.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    I have a buddy who would like to put up a 40meter antenna on a steel airport building. The roof area where the antenna can be placed is 29ft wide x 100's of feet long, but the vertical part of the antenna should be no more than 8ft tall, so as to keep it less than 30ft agl (22ft building height + 8ft antenna height).

    The first idea is a screw-driver mobile whip, using the steel roof as the ground plane. A much-larger-than-mobile capacitance hat could be added. That would allow easy qsy, and even multiple bands. Overall efficiency might be low?

    Another might be an inverted-L, fed against the steel roof, where the vertical part is only 8ft, but the horizontal part can be almost any length that makes sense... Likely to be more efficient, but radiate mostly high angle. A remote tuner is not out of the question...

    Other suggestions?
    Last edited: Nov 5, 2018
  2. KD6RF

    KD6RF XML Subscriber QRZ Page

    If you haven't seen it already, here is a classic article on an efficient cap-hatted top-loaded (probably should be more toward center loaded for slightly better efficiency) shorty 40 M vert "Classic Sevick Shortened Vertical Article"===> http://vtenn.com/Blog/?p=46


    BW is small, but may put the input Z within range of a base tuner for coverage of all or a large portion of the band.

    As you probably know, many radials is the other key to efficiency with short low-Z verts.
    Last edited: Nov 5, 2018
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  3. AA7QQ

    AA7QQ Ham Member QRZ Page

    Mount a screwdriver. I would think it will do better than a short L like you describe. You will also have all of the HF bands, too. Should work good.
    You may want to lay 4 radials to each corner and secure them. This will help insure that you have a good groundplane.

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  4. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    Remote tuner plus a loop around the entire perimeter, as close to the edge as practical and as high as possible.

  5. NH7RO

    NH7RO Ham Member QRZ Page

    The 10' Sevick experiment is exactly what came to my mind when I started reading this thread (TNX KD6RF for posting the pic and link). IIRC, that "little big" 40m antenna was only down a db or so from a full size ground plane. The key, of course is top-loading and a great radial field (hard to go overboard in that regard).

    That Sevick Shorty is what initially inspired me to use my Scorpion SA-680 atop a 6-foot T-post with a homebrew 60" circular cap hat, 1/2 copper radiator also about 60" tall, and five radials elevated 6 to 4 feet above ground (half wheel pattern is all I can install in the woods).

    This works FB and my CQWW SSB QSOs ranged fom Australia to Cyprus using 600 watts (not that anecdotal evidence is all that worthwhile---but it does show that this short vertical gets the job done reasonably well for me).

    I'd try a screwdriver and definitely DIY a big cap hat before trying an L; multiple bands and no tuner required.


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  6. WB3BEL

    WB3BEL Ham Member QRZ Page

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  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    To complete this project, I bought a used screwdriver mobile antenna at the recent Marana hamfest. I am in the process of converting it into a 8ft tall remotely-tuneable vertical antenna for 80 to 20 meters to be placed on the metal roof on an all steel aircraft hangar.

    The antenna will be 8ft tall, coax driven 18" up from the metal roof (to get the feedpoint out of snow). The bottom of the motorized coil will be 5ft above the metal roof. I can make the capacitance hat as large as I need to because the antenna will be subject only to light winds, not a car's slipstream.

    The vertical element is 2" od aluminum pipe, both below and above the coil. The top-hat will be 0.25" aluminum rods.

    Since I have no previous experience with a screw-driver antenna, I modeled the proposed antenna described above in EzNec, and that has generated some questions.


    It seems common to use a shunt coil across the feedpoint (in combination with re-tuning the main coil) to raise the feedpoint impedance to match 50Ohm coax. However, when I modeled it, the optimum shunt inductance is different on every band. How is this accomplished when switching from band-to-band?

    It seems that on 20m, I can make the top hat big enough so as to eliminate the need for the main coil, leaving only the shunt inductor. In fact, my first attempt (top hat consisting of 8ea 4ft radial rods) would not tune on 20m. I had to reduce the "area" (capacitance) to only 4ea 3ft radials in order to get the antenna to tune to 14.2MHz, which requires less than 1uH of inductance in the motorized coil.

    It looks like this will be a very effective antenna, especially on 40 and 20m. However, it looks like I will have to have two tuning motors, one inside the antenna, and the second to adjust the matching inductor. Alternatively, since I could get by with a fixed shunt inductor on each band, at least a band-switching relay matrix to select the shunt inductor per band.

    Could I use an auto-transformer (unUn) instead? Would that eliminate the second motor/relays?
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It's typically accomplished by finding a compromise solution that improves the match on most or all bands but doesn't optimize it for exactly 50 ohms on any given band. If you target something like 1.6:1 SWR on all supported bands rather than 1:1 on all bands you can usually find a workable shunt inductor.

    Here's a take on how to go about tuning the shunt inductor for multi-band antennas: http://www.k0bg.com/coil.html
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  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    The optimum shunt inductor for 75m is 1.85uH, 40m is 0.40uH, and 20m is 1.80uH. I will resim with a 1.0uH shunt inductor and see what SWR results...

    Needing a different shunt inductor per band might stem from my antenna being bigger, over a perfect, large ground, and a bigger than normal top-hat.
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, I suspect something in the 0.8 to 1.2 uH range would give you a reasonable compromise with some bands being slightly capacitive reactive once matched a some bands being slightly inductively reactive once matched but hopefully all bands being within a reasonable SWR range with the compromise inductor.

    Yeah, could be.

    The relatively high value of shunt inductance on 20m implies the antenna is pretty well matched without the extra beta match on 20m (z= 45.6 -j14.2 ohms) that's likely the result of the base antenna plus large cap hat being pretty close to 1/4 wavelength on that band. If you can't find a good compromise inductor for all bands you might just switch the shunt inductor out of circuit on 20m as you don't really need to improve the match on that band. A single relay switched open on 20m is easier than a second adjustable inductor.
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