Antenna Conumdrum

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

ad: L-HROutlet
ad: l-rl
ad: L-Geochron
ad: L-MFJ
ad: abrind-2
ad: Left-3
ad: Left-2
  1. K7JEM

    K7JEM Ham Member QRZ Page

    GIGO!

    Your error is on line 2 of the calculation. You are using .84, I don't know where that is coming from. Certainly, the VF of a piece of insulated house wire is not .84, it is closer to .95 (as others have mentioned).

    Change that line to .95 and you get this:

    300 / 27.805MHz = 10.76m

    10.76m * 0.95 = 10.22m

    10.22 * .25WL = 2.555m for a quarter wave.

    2.555m * 39.37" = 100.59"

    That's pretty close to the 100" that it should be.
     
    KO7PAS likes this.
  2. KO7PAS

    KO7PAS Ham Member QRZ Page

    Thank you! I knew that, but for some reason it didn't click in my brain as that was the problem!!!!
     
  3. SWL37632

    SWL37632 QRZ Member

    When I started out with antennas, I proceeded just as you have with the free space vacuum equation and got the same kinds of results...very frustrating since the authoritative books say one thing and then you build it and it gives you different results.

    Bottomline: The final results are dependent on the unique installation and the materials used...it's 'cut and try' until you are satisfied with the results.
     
    SM0GLD and KO7PAS like this.
  4. N3DT

    N3DT Ham Member QRZ Page

    After you make your first try and you find the resonant spot, X-0, then use that length to recalculate your VF . It will be very close on the second try if not spot on.
     
    KO7PAS likes this.
  5. KO7PAS

    KO7PAS Ham Member QRZ Page

    So... by the time I got it tuned in... My math that I had figured out was much closer than what I believed it should have been.

    I was only off by less than an inch. I now have an antenna tuned to 28.3MHz with a 1.4:1 SWR. Now, I just have to figure out how to mount this beast! Just a smidge under 15' long. If you are wondering? I made a dipole for 28.3MHz.

    My math showed ~89.625" for the length... I'm 89.75" total antenna length.
     
    WA9UAA and KF4NU like this.
  6. KN6CSB

    KN6CSB Ham Member QRZ Page

    Don't get too hung up on SWR, because once you mount the dipole, depending on the final height and other factors like adjacent objects, the SWR will change.

    I haven't been at this too long, but by now I realize I'd rather have an antenna up 50 feet with 3:1 swr vs one at 20 feet with 1:1 swr.
     
    WE4E and PY2RAF like this.
  7. KO7PAS

    KO7PAS Ham Member QRZ Page

    Oh... I know. I still have to get this up in the air yet. and I know that things will change, hopefully for the better.
     
  8. SWL37632

    SWL37632 QRZ Member

    Are you going to mount this as a horizontal dipole or vertical dipole ?....For local/regional coverage it does make a difference.
     
  9. KF4NU

    KF4NU Ham Member QRZ Page

    Well done Patti :) Let us know how it does once you get it up
     
    KO7PAS likes this.
  10. WS9K

    WS9K Ham Member QRZ Page

    10 meters is open right now.

    slingshot
    20lb fishing line and sinker
    para-cord
    2 trees.

    :)
     
    M0AGP and KO7PAS like this.

Share This Page