model for 100' vertical antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5VQ, Jul 26, 2021.

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  1. W5VQ

    W5VQ Premium Subscriber QRZ Page

    I have a vertical antenna 100' made of rohn 25G tower the base is insulated from ground. It is guyed with phillystrand non-conductive guy material. This antenna has 40 buried radials 130' long made of 10 stranded copper wire. I use a L-network at the base to match to 50 ohm cable. This antenna works great on 160m and 80m. Recently I tuned it for 40 meters, but it doesn't seem to perform as well as I thought it would. Just wonder if someone would be willing to model this so we could look at the angle of radiation and pattern?
    Thanks, Glenn W5VQ
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    upload_2021-7-26_6-7-58.png
    This does not include matching loss.

    One fix would be to add a parallel resonant trap tuned to 6.95MHz and place it at the 70ft point in the tower. I know that would be difficult, mechanically, but electrically it makes sense. That would slightly improve the performance on 160 and 80m due to the residual inductance of the trap acting as a loading coil mounted high:
    upload_2021-7-26_6-31-37.png
    You would have to redo your L networks.
     
    Last edited: Jul 26, 2021
    K0UO likes this.
  3. W5VQ

    W5VQ Premium Subscriber QRZ Page

    Thank you Michael I appreciate the work you did. This is what I needed to know. The antenna has always preformed well on 160 and 80m, but I see now why it's not up to par on 40m DX. Will have to think about the trap, it would be difficult to do.
    73 and thanks again
    Glenn W5VQ
     
  4. W9XMT

    W9XMT Ham Member QRZ Page

    Below is an analysis of an antenna system as described, using those exact dimensions. Soil conductivity for your region north of Dallas was set for 30 mS/m, as shown on the FCC M3 map.

    NEC4.2 is able to accurately analyze antenna systems using buried conductors. The relative gain it shows for low-angle, vertical plane radiation is significantly less than that system would produce on 160 and 80 meters (other things equal).

    [​IMG]
     
  5. W5VQ

    W5VQ Premium Subscriber QRZ Page

    Thanks Richard,
    I will have to try something else on 40m for DX. Thinking about a half square or bobtail.
    73
    Glenn W5VQ
     
  6. WA8OLN

    WA8OLN Ham Member QRZ Page

    Delta Loop
     
  7. KC7JNJ

    KC7JNJ Ham Member QRZ Page

    half squares intrigue me, I would like to hear if you get one going.
     
  8. M0AGP

    M0AGP Ham Member QRZ Page

    Sounds like one eighth of a four-square
     
  9. W5VQ

    W5VQ Premium Subscriber QRZ Page

    I have used the half square and the bobtail curtain in years past. Both are very good on 40 meters. Here is a link with great info on these antennas. https://www.angelfire.com/md/k3ky/page38.html
     
    M0AGP likes this.
  10. M0AGP

    M0AGP Ham Member QRZ Page

    Very interesting- I didn’t realize the bobtail curtain was voltage fed like an EFHW. It looks like a capital E that fell to the right. If a Texas rancher ham had invented it I bet it would have been called the “Lazy E” antenna.

    This gives me an idea: a “quarter-square”: it’s a voltage fed half wave inverted L!

    So it goes up a quarter wave and then over a quarter wave. The biggest advantage: no need for an extensive radial system as the max current point is up a quarter wave. Also less ground losses.

    Actually it’s just an EFHW in the shape of an inverted L. I guess it would naturally resonate about the same way as a classic EFHW… including on all the even harmonics.

    Hmmm… I’m planning an inverted L for 160m with elevated radials. If I’m not mistaken, for an 80m antenna I could just disconnect the radials (or even keep them?) and voltage feed the thing!It would then be an inv L EFHW on 80m… (or a quarter square :) )

    Will have to look at that pattern on 80m… probably some low angle from the vertical current and some NVIS from the horizontal current…
     

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