5/8 wave antenna coil dimentions

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by JOHNL, Dec 16, 2010.

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  1. JOHNL

    JOHNL QRZ Member

    Im trying to build a 5/8 wave antenna out of an old CB antenna for the VHF band the problem is I cannot find out how to calculate the coil dimentions.I have tried out different wire sizes,number of turns and diameters and I have got good results on the swr meter but when the antenna is up and compared to a folded dipole at the same height the 5/8 is down at least 5 or more s-points.The swr is lower than 3 over a 15mhz range suggesting its nearer to a dummy load than an antenna.Fed with 50 ohm coax the braid connected to the bottom of the coil and the center connected to roughly mid way along the coil.Note I delibrately left out the design frequency because I want to find out the formula for calculating it.
    I have seen coil dimentions for higher and lower frequencys and have already tried scaled down/up versions.
    Apart from "scrapping" the project what else am I doing wrong?
    Any help much appreciated

  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    There's no one-step formula for calculating this, that's why you can't find one.

    The amount of inductance required (L) is equal to the amount of capacitance (C) in the antenna. So, first you need to know the capacitive reactance (Xc) of the antenna, then you can calculate the inductance based on L = XL/2pi*f.

    Once you know the required inductance you can go through a formulas to figure the variables for the coil itself; but there are on-line calculators for that, like this one:

  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    In re-reading your post I see you didn't discuss radials or a ground plane system.

    I hope you're using radials or a ground plane (like a car roof), otherwise you have half an antenna there, and not a whole antenna, which would greatly explain why it doesn't work well.:p
  4. AI3V

    AI3V Ham Member QRZ Page

    This will bring the antenna into resonance (resonance is defined where XC=XL). After you have resonated the antenna, you can now adjust the connection point (tap) of the center conductor to the coil to provide 50Ω resistive.

    The two adjustments will interact a bit.

    Some sort of a R-X bridge would be a great help, but a dip meter would work in a pinch for cut and try coil building

    Fixed it for you.

  5. K1DNR

    K1DNR Ham Member QRZ Page

    If you have no tools except an swr meter, you can find various articles online by people who have made them, e.g. http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=23868

    While this doesn't answer your question, it should get you in the ball park as far as a loading coil.

    Then you can find the best SWR by tapping the coils in different places or by trial and error.

    This is the official non-technical "what the heck is XL, XC and R" method.

    Not that I'm not interested in Antenna theory (I am) - but sometimes if you don't have the tools and need a quick/cheap project done, you just do it by experimentation.
  6. VK6YSF

    VK6YSF Ham Member QRZ Page

  7. W5DXP

    W5DXP Ham Member QRZ Page

  8. K7JEM

    K7JEM Ham Member QRZ Page

    All very good suggestions.

    I would interject that I disassembled a 5/8 wave loading coil from an A/S mobile antenna. It consisted of about 3 turns of #14 wire on about a 1 inch former. The wire was about 9" long.

    Since 5/8 antennas are non resonant, the overall length of the antenna is really not that critical, as long as the coil and antenna provide a good match.

    Build a coil, as described, then add a radiator about 48" (+ or -) and adjust the length until the SWR gets below 1.5 to one. With a good ground plane you should have a working antenna. The overall length might be 45 to 51 inches or so, but that won't really matter all that much as far as gain and angle of radiation.

  9. JOHNL

    JOHNL QRZ Member

    Thanks for the replies and yes it does have 3 ground radials a 1/4 wavelength long at the design frequency which is 70.200Mhz.They are mounted at the bottom of the antenna at 90 Degrees to the vertical radiator.
    To get it right it looks like its not as simple as one originally thought.Lots of reading and information and from the links posted it will keep me busy for a while.There is little or no antennas commercially available for the 70Mhz or 4meter band.
  10. VK6YSF

    VK6YSF Ham Member QRZ Page

    Loading coil

    While the 5/8 radiator can be trimmed at the final tweaking stage the most important feature of this antenna is to maintain a radiator length of slightly less than the 5/8 wave length, therefore the first component of the antenna to be adjusted is the loading coil.


    Peter VK6YSf

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