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Monopole antenna length.

Discussion in 'Radio and Wireless' started by rmdrmd, Jun 23, 2020.

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  1. rmdrmd

    rmdrmd

    13
    0
    Jun 14, 2013
    I'm after a little help.
    I want to add a very simple single wire antenna to a radio receiver.
    Is the correct length for the antenna one quarter of the wavelength?
    Is it as simple as that?

    The frequency is 27.145 MHz.
    From a calculator I see that makes the wavelength to be 11.044 meters.
    Does that mean the correct length of the single wire antenna should be 2.761 meters?
    Am I on the right track here?

    Would appreciate any advice.
    Cheers.
     
  2. davenn

    davenn Moderator

    13,585
    1,870
    Sep 5, 2009
    almost
    It's an electrical 1/4 wavelength at the frequency of interest

    300 / freq ( in MHz) = wavelength / 4 = 1/4 wavelength x 0.96 (velocity factor) = electrical 1/4 wavelength

    see above and do the math :)

    cheers
    Dave
     
  3. Nanren888

    Nanren888

    247
    50
    Nov 8, 2015
    With access to equipment some might start a little longerand trim the antenna down slowly while the SWR drops.

    Simple it is, but also rather long for most RC toys and applications.
    It may be that you can find a more compact design that is also simple.
     
  4. bertus

    bertus Moderator

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    Nov 8, 2019
  5. rmdrmd

    rmdrmd

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    Jun 14, 2013
    Thanks Dave.
    Why did you specifically nominate .96 as the velocity factor?
     
    Last edited: Jun 24, 2020
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Because electromagnetic waves travel more slowly on wires than in free air. See here.
     
  7. rmdrmd

    rmdrmd

    13
    0
    Jun 14, 2013
    Thanks to all of you for your assistance on this.
    I now have a simple antenna on my receiver (based on the advice received) and the effective range of the transmitter has gone up from only 10 meters to around 60 meters. More than adequate for the application.
    Cheers.
     
  8. davenn

    davenn Moderator

    13,585
    1,870
    Sep 5, 2009

    Thanks Harald :)

    @rmdrmd for a bare copper wire the vf ( velocity factor is approx 0.95-0.96
    if the wire has a plastic coating the vf can drop substantially down to around 0.55
    There are all sorts of numbers in between those extremes depending on the type of cladding
     
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