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wireless range help

Discussion in 'Radio and Communications' started by somespirit, Jul 27, 2010.

  1. somespirit

    somespirit New Member

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    hi there- i have a 10mw module which quotes its range with a 2.1dBi antenna. it has an RP-SMA connector. i've seen that antennas are readily available at 3dBi, 5dBi, 7dBi ...

    simply put, what does this mean? guesswork would dictate that (since it's a dB scale), each 3dB of antenna 'gain' would double effective output power, but how is this possible without increasing 'actual' output power? (the antennas are unpowered). do these antennas make the emission pattern ovoid, increasing effective output at the expense of output axial to the antenna..? any help would be appreciated.
     
  2. JimB

    JimB Super Moderator Most Helpful Member

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    Welcome to the weasel world of antenna gain!

    Terminology
    dBi - is dB gain (or loss) with respect to an isotropic radiator.

    Isotropic radiator - a theoretical antenna which radiates equally in all directions, practical antennas cannot do this.


    So most practical antennas will have some gain wrt an isotropic radiator, in your case 2.1dB gain.

    Directional antennas have gain because they transmit most of the applied power in one direction, the greater the directivity the greater the gain.

    The antennas which you quote as having various gains, will all be bigger that the basic antenna, and somehow have directivity.
    Not knowing the shape of the antennas you are referring to, lets just make the assumption that the radiation pattern of the basic antenna is doughnut shaped, the doughnut is in a horizontal plane and the antenna is vertical through the centre of the doughnut. There is no radiation off the end of a simple wire antenna.

    The higher gain antennas will effectively "squash" the doughnut so that there is more gain in the horizontal plane and less at angles above and below the plane of the doughnut.

    Does that explanation help?

    JimB
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Gain from aerials is achieved by making the signal directional in some way - power is the same, it's just sent to where you want it more accurately.
     
  4. dave

    Dave New Member

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  5. somespirit

    somespirit New Member

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    yeah that makes perfect sense, many thanks! so any straight antenna will emit more perpendicular than axial, so will always have a little 'gain' in the perpendicular plane, compared to a theoretical omni.

    i noticed that the antennas get longer as the gain increases, so that makes sense- it 'squishes' more as it gets longer.

    now- i have much more experience with light radiation than i do with 'radio' radiation, but i am right in saying that 3dB of antenna gain is effectively doubling emission power in its "ideal" direction, which would give sqrt(2) signal boost at the destination? (in open-air). are there other factors which come into play?
     
    Last edited: Jul 27, 2010
  6. JimB

    JimB Super Moderator Most Helpful Member

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  7. lutful

    lutful New Member

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    I just wanted to add that while beam pattern of a "7dBi" gain rubber duck antenna will indeed look like a donut, a "7dBi" gain patch antenna will create something like a cone extending about 60 degrees.
     
  8. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Some antennas (verticals and co-linears: 1/2λ, 5/8λ, 3/4λ, J-Poles, Ground Planes, coaxial) exhibit gain in the horizontal plane but are omnidirectional.

    Some antennas, (Horiz. Dipole, Yagis, Rombics, V-beams, phased arrays, horns, dishes) have directivity in both vertical and horizontal planes and are unidirectional or multidirectional.
     
    Last edited: Aug 21, 2010
  9. koolguy

    koolguy Active Member

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    Please tell me some good point to have good reception of FM signal's from far transmitter in details.........!
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    1) A high quality home stereo FM tuner or a high quality car radio have very good sensitivity. A cheap clock radio has poor sensitivity.
    2) As discussed in this thread, a directional antenna has gain but it must be pointed at the station.
    3) If the signal is past the horizon then increasing the height of the antenna and amplifying it at the antenna (to cancel cable loss) will make better reception.
     
  11. koolguy

    koolguy Active Member

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    As I have Two Type of Antenna One is Long Horizontal ( 2.5 To 3 Feet approx ) Dipole directed antenna & other Circle type Antenna Of 1.5 foot, I Want To know What The Different B/W There Working As Circle one Have good TV signal not FM signal good & other Is Opposite Of It Pls, Tell some good more Point Of It.................!
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your dipole antenna is too small for FM frequencies. It works better at higher frequencies.
    A dipole antenna for FM radio frequencies from 88MHz to 108MHz is about 5.5 feet long. It has a small amount of gain that will probably not be noticed compared to a piece of wire.
    A yagi antenna with many reflectors is a very directional antenna with lots of gain.

    I have seen circular FM antennas with a noisy amplifier that overloads easily by strong local stations but I don't think they work properly.

    EDIT: A good Yagi antenna has many directors and only one reflector. Here is a photo.
     

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    Last edited: Aug 28, 2010
  13. RCinFLA

    RCinFLA Well-Known Member

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    2.1 dbi happens to be the directivity gain of a half wave dipole antenna, also known as 0 dbd, for gain relative to half wave dipole antenna.
     
  14. koolguy

    koolguy Active Member

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    A Dipole Antenna Of 5 foot (Or Near) Has 80 Km Range, On Connecting A // antenna above it, The range increase To 120 Km. In Fm SIGNAL Reception I found not so Clarity In Connecting Like This as The Transmitter is Near about 210Km Far, but In morning Signal strength is good during Day & Cloudy Day The signal Quality Decrease Why....? & Pls Tell Some More Point Regarding This.........!
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    In North America every radio station has a circle drawn on a map showing range for best reception. Hills and trees reduce the range.
    Here is a powerful FM radio station showing best reception within only 64.8km.
    A cheap clock radio would have good reception if it is about 6km away.
     

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  16. koolguy

    koolguy Active Member

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    I am from India in dehradun Here no private Fm station but signal from other states have less power that's i want to built strong antenna for it...!
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    42 years ago I bought the biggest and best TV and FM antenna that was made. I installed it on a mast so it was 10m high and above the surrounding houses and installed an electric rotator. Occasionally I received signals from hundreds of km away. I received TV stations on the same channel that were at different directions. It was a VHF and UHF Yagi antenna.

    FM is at 100MHz VHF frequencies that travel in a straight line and have almost no reflection from the ionosphere like happens with 1MHz AM radio signals at night. Therefore the curvature of the earth's planet attenuates the signals. A high antenna helps but the long cable has loss.
     
  18. koolguy

    koolguy Active Member

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    PLs tell what is clock cable, & what about the circle type antenna what is the name of it as it has more gain over dipole antenna for low frequency receiving channel of tv...!
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I have never heard about a "clock cable". What is it used for?
    A circular antenna is probably used for UHF and has a small gain at its caculated frequency because it is directional. A dipole antenna is also directional and has a small gain at its calculated frequency. They are both bi-directional.
     

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