End Fed but not Half Wave

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KN4LGM, Jul 17, 2019.

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  1. KN4LGM

    KN4LGM Ham Member QRZ Page

    I am trying to conceive a 1/16 wave vertical for 640m and I was wondering if anyone has heard of such a thing, and if so what would the impedance transformer ratio be. If not, does anyone know how to find that ratio?
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    EzNec?
     
  3. KN4LGM

    KN4LGM Ham Member QRZ Page

    I am familiar with it but I do not know how to find this variable.
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    This sounds like a short vertical with a capacitance hat on top and a huge tuner at the base.

    What are you planing to use for radials/ground system?
     
    KU3X and K7TRF like this.
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Once you go below 1/4 wavelength you're talking about a shortened antenna much like what folks use in mobile installations. You don't normally match an antenna like that with a transformer the way you might for an EFHW. A shortened 1/4 wave antenna which does not need to be vertical in orientation is typically fed by taking care of two things:

    - Load the antenna to become 1/4 wave resonant at or near your operating frequency with loading coils, cap hats, linear loading or some combination
    - Match the resulting resonant antenna which could me done with a shunt matching coil a simple matching transformer (e.g. 1:2 or 1:1.5 impedance ratio) or other methods including L network or T network matching networks. Depending on your ground losses which are driven by the quality of your ground radial or counterpoise system you might not need any matching at all as it will be hard to deploy a really low impedance RF ground at that frequency.

    For something like what you describe a combo of capacitive end loading and a fairly large base loading inductor would probably do the trick. The resulting antenna would be basically a heavily loaded quarter wavelength antenna and will need ground radials/counterpoise.
     
  6. KN4LGM

    KN4LGM Ham Member QRZ Page

    10-20 50 foot wires.
    Ok, I'll probably do that, but out of curiosity does, anyone have any other ideas.
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    A cursory run using EzNec:
    First, the antenna conductors: a 127ft tall, 6in diameter pipe, standing on an insulator, with 16ea. #12awg radials, each 50ft long. A top hat, consisting of 8ea down-ward sloped (~45degree) #12awg radials, each 67.4ft long, using rope to reach the rest of the way to the ground.
    The feedpoint is between the near-ground radials and the vertical. Here is a picture, and the feedpoint impedance:
    640m.png v640.png
    Click to enlarge.

    As expected, the antenna is very low resistance and hugely capacitive.
    It would be a bit better to add a loading coil about 1/2 to 3/4 of the way up the vertical. That would increase the current along the vertical element.
     
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    There are substantial differences between an EFHW and an extremely short antenna which limit options. The EFHW is half wave resonant on it's lowest frequency band and is electrically longer than that on all other higher frequency bands. That means the real portion of the feed point impedance is very high on those bands and on the bands for which it is not resonant the reactive portion can be somewhat high as well. A large impedance ratio transformer at the feed point will reduce those high impedances down to something near 50 ohms providing a decent match on several or many bands. But the key is that the antenna is electrically long relative to quarter wave resonance on all of the supported bands.

    But an antenna that's 1/16th of a wavelength long is very short compared to the first resonance at 1/4 wavelength. That means the real portion of the feed point impedance is very low relative to 50 ohms and the reactive portion is a relatively high capacitive reactance. So unlike an electrically long antenna it can't really be brought to a good match with just a transformer. You could use a step up transformer to try to improve the match to the real portion of the feed point impedance but that would make the reactive portion even higher and you'd end up with a terrible match. If you tried a step down transformer to try to minimize the impact of that high capacitive reactance you'd drive the real portion of the feed point impedance even lower which makes things worse.

    There aren't really a lot of options for an electrically short antenna. The best approach is to find a way to bring it to, or close to, quarter wave resonance with some form of loading and then match the resulting impedance. Then you'll have a loaded but shortened quarter wave radiator which means it will need to work against a good RF return system such as radials.

    Even in the case of a small transmitting loop antenna you really need to resonate the system with a capacitor across the tuning gap and then find a way to match the resulting low feed point impedance. That's certainly an option you could explore but it would take a very large loop and a fairly big resonating capacitor for the 640m band.
     
    KN4LGM likes this.
  9. KN4LGM

    KN4LGM Ham Member QRZ Page

    Thank you, this is very informative, now I understand.
     
  10. KN4LGM

    KN4LGM Ham Member QRZ Page

    "a 127ft tall, 6in diameter pipe":(... I'll try wire instead of pipe:)
     

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