Antenna Conumdrum

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KO7PAS, Jun 4, 2021.

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  1. KO7PAS

    KO7PAS Ham Member QRZ Page

    So... When I do the math, taking into consideration velocity, I get a different answer than I was expecting. And, when I run the math the other way, I get a different number. I don't know what I could be doing different, but something isn't adding up.

    300 / 27.805MHz = 10.76m

    10.76m * 0.84/v my answer is 9.045m

    Looking good so far?

    9.045 * .25WL = 2.621m for a quarter wave.

    2.621m * 39.37" = 89.028"

    I know that for a dipole you have to have a matching leg. That said, though I trimmed my piece of wire to where I thought it would tune in, giving myself a bit of extra, it's way, way off. I don't give myself a lot of extra wire, but I do give myself a few extra inches (I'm sure you boys can relate - HAHAHA - Someone has to have a sense of humor here! :D ) That said... I have been doing this for a little bit now, but never encountered this type of conundrum.
     
    KF4NU likes this.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    492/f is true 1/2wl (f in MHz, length in feet)
    468/f is supposed to be the length of a resonant dipole near real earth.

    468/27.805 = 16.83ft.

    NEC2.0 says a dipole in free space made from lossless wire has to be 17.122ft long to be resonant. If the dipole is made from bare #14AWG Cu wire, H ft agl over average dirt, then it has to be adjusted per the following:
    upload_2021-6-3_16-38-6.png

    Required wire length will change if the wire is insulated, or if the wire gauge is different, or if the ground conductivity or dielectric constant changes.
     
  3. SWL37632

    SWL37632 QRZ Member

    Your first calc is for Free Space in a vacuum and doesn't take into account air and any dielectric effects and ground and proximity effects. ARK is correct in his additional inputs. As you have already surmised, additional wire that you can 'prune' to your desired frequency is a good option....don't get to worried if you are too short, you can always add some wire length as desired.
     
    KO7PAS likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ...wear high-heeled shoes.
     
    2E0CIT likes this.
  5. AG5CK

    AG5CK Premium Subscriber QRZ Page

    I'm thinking platform shoes with goldfish tank heels.
     
  6. KO7PAS

    KO7PAS Ham Member QRZ Page

    That's just it... When it should be a resonant frequency, it's way to high for that frequency, though I have added more wire to the end of it. I'm not sure what's going on with it... I'm thinking I need to be about 8' 6-5/8" for the length of it for that frequency. Yet, when I do the math, it's telling me that it's 89.28". I am calculating for the ground at roughly 30' (7-meters).
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "Measure with a micrometer, cut with a chain saw."
     
    M0AGP likes this.
  8. KF4NU

    KF4NU Ham Member QRZ Page

    Patti, I got your joke and loved it :D ARK is spot on. He did add that if the wire is insulated, that will change the formula as well. I've found over the years, that in general when using insulated wire, I make it 5% shorter.
    73's
    Mickey KF4NU
     
  9. KO7PAS

    KO7PAS Ham Member QRZ Page

    It is insulated... Just standard solid strand house wire.
     
    Last edited: Jun 4, 2021
  10. KO7PAS

    KO7PAS Ham Member QRZ Page

    Interesting quandary... It's coming in higher in frequency on the antenna than it should when it should be much lower.

    Just using basic numbers here... My math has it coming in at 90", but... I know that it should probably be closer to 100. If I shorten the antenna, that moves the antenna up in frequency. I need this antenna to go down in MHz. Wouldn't I add 5% since it's insulated?
     

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