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Cell Phone Antenna — Parallax Forums

Cell Phone Antenna

william chanwilliam chan Posts: 1,326
edited 2009-06-09 01:54 in General Discussion
Hi,

Can I use a piece of wire to replace a 1800 Mhz cellular antenna?
Will it still work with reduced signal strength or will it totally not work?

Can the "piece of wire" antenna be soldered directly to the gsm module or a coaxial cable is mandatory?

Thanks.

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www.fd.com.my
www.mercedes.com.my

Comments

  • LeonLeon Posts: 7,620
    edited 2009-06-08 08:31
    If it's the correct length (1/4 wave) it should work.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • william chanwilliam chan Posts: 1,326
    edited 2009-06-08 12:25
    Then how come most tri-band or quad-band handphones work with seemingly the same internal antenna?

    Sorry for my ignorance...
    How long is a 1/4 wave antenna for 1800 Mhz? Would 1/2 wave give better signal?
    Should the ground be connected at all?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.fd.com.my
    www.mercedes.com.my
  • LeonLeon Posts: 7,620
    edited 2009-06-08 14:14
    Multi-band antennas can be made.

    Calculate the wavelength for 1800 MHz and divide it by 4.

    A 1/2 wave antenna has a high impedance at the feedpoint and won't work properly without a matching network.

    A 1/4 wave antenna requires a ground plane but the phone itself and the person holding it provide a ground that is sufficient for it to work reasonably well.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • Mike GreenMike Green Posts: 23,101
    edited 2009-06-08 14:37
    For multi-band antennas, you can have several 1/4 wave antennas (one for each band) connected together at the feed point (connector). The extra antennas are mostly inactive "off-frequency", but they do provide some additional capacitance, so the length of the active antenna may not be exactly 1/4 wavelength. This is commonly done in amateur radio where operation on one of several frequency bands is the norm.
  • william chanwilliam chan Posts: 1,326
    edited 2009-06-08 15:23
    I heard somewhere that using a coaxial cable that is the same length as the 1/4 wave antenna to connect the feed point to the antenna would boost the signal strength.
    They say it is due to the axial magnetic flux forming inside the coaxial cable that is suddenly exposed to air when it reaches the antenna.

    Is it true?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.fd.com.my
    www.mercedes.com.my
  • Mike GreenMike Green Posts: 23,101
    edited 2009-06-08 15:33
    No, it's not true. You can use 1/4 wave and 1/2 wave coax sections as impedance transformers. That can be why you might see a signal strength gain, but there are many ways to match impedance. The ARRL publishes antenna guides that discuss this. See http://www.arrl.org
  • LeonLeon Posts: 7,620
    edited 2009-06-09 01:54
    BTW, a 5/8 wave antenna is a 1/2 wave + matching section that will match a 50 ohm feeder, and give more gain than a 1/4 wave. It is actually an end-fed 1/2 wave dipole, and doesn't need a ground plane.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
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