1/2 wave dipole antenna gain increase design

Discussion in 'Wireless & RF Design' started by donut, Jun 20, 2012.

  1. donut

    Thread Starter Member

    May 23, 2012
    51
    0
    I understand the 1/2 wave dipole gain to be 2.14dBi. If I wanted to increase the gain how would I go by doing this?

    I know to increase the bandwidth of the dipole antenna you just increase the volume of the dipoles. But is increasing bandwidth increasing gain?
     
  2. TecknoTone

    New Member

    May 20, 2012
    21
    1
    In order to increase the gain of an antenna, you need to concentrate its effectiveness in the required direction. Two common methods are to concentrate horizontal beamwidth by adding reflectors and directors, thus creating a Yagi beam. The other is to stack more than one dipole vertically, as is done with co-linear types or stacked dipole arrays. This concentrates vertical beamwidth into an omnidirectional disc formation.

    Increasing bandwidth does not increase gain. In fact, generally in electronics, one can only be achieved to the detriment of the other.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
  4. donut

    Thread Starter Member

    May 23, 2012
    51
    0
    I saw a gain formula that says:

    G = (4∏ / λ^2) * Ae

    where Ae = effective aperture.

    So my point being is could I increase the gain of the antenna via the increase of Ae. If so what is Aperture and how can I increase it to increase the gain of my antenna.
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Effective aperture is a measure of the capability of an antenna to capture energy from an incident EM field.

    http://en.wikipedia.org/wiki/Antenna_aperture

    A particular antenna will have an certain effective aperture value. So a simple dipole has a given aperture which is dependent on the dipole physical structure. A dipole has a certain aperture and it has a certain gain relative to [say] an isotropic antenna. One can therefore compare one antenna with another in terms of the aperture or the gain. One can expect that there would be some other factor such as directivity being involved as well. That might be considered as a compromise - trading gain [aperture] and directivity. Not necessarily a 'bad' thing.

    So a Yagi-Uda antenna (for instance) has a higher gain [effective aperture] than a simple dipole. The Yagi-Uda has a higher directivity than a simple dipole.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,180
    1,800
    I think there is a fundamental misunderstanding here. The 2.14 dBi gain is relative to an iostropic radiator. That is all a dipole can do. You can't get a dipole to have more gain than that.

    If you want to go to other antenna type then you can get more gain relative to a dipole or to an isotropic radiator. What are you really trying to do? Try telling us that before we go off into the weeds.
     
  7. donut

    Thread Starter Member

    May 23, 2012
    51
    0
    I am just trying to learn more about dipole antennas. Im using the ARRL Antenna book to help me (edition 19).

    In chapter 2 (see attached) they explain that if the length of dipole is increased the larger the number of lobes into which the pattern splits and such the main lobe angle become smaller as the length of the antenna is increased.

    So that tells me that if I increase the length of my dipole, my main lobe angel becomes smaller, and this relates to more gain or directivity. At the cost of adding more lobes.

    So I was trying to confirm if you could indeed increase the gain of a dipole by asking the forum members what there thoughts were.

    Not sure if Im reading the text properly. Because the book says you can increase the gain of a dipole. But then I saw a video (see link below) that says the dipole will radiate at λ/2, not λ, 3λ/2, not 2λ, etc.

    http://www.youtube.com/watch?v=RF5r64fmFhU

    So Im just trying to establish the truth from a ton of information and ensure that I am interpreting correctly.
     
  8. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Well, not necessariy, because energy will go into those lobes, which will decrease directivity.

    However, if the energy in the dipole axis increases more than the loss in the lobes, well then you will increase gain.
     
    Last edited: Jun 28, 2012
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Keeping in mind the point that an antenna is just one [albeit important] part of a wireless system - are you thinking in terms of a transmitting or receiving antenna? I know from the perspective of reciprocity the gain is identical for either the Rx or Tx case - but in practice one must deal with issues such as the antenna impedance which is rather important for the Tx case. If you can't effectively "launch" the EM energy it doesn't matter how good the gain is.
     
  10. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    As a supplementary point one could attempt to determine the maximum available gain frequency for the 100' [30.48m] dipole.

    Depending on the antenna wire radius this turns out to be about 5dBi at ~12MHz with a 50mm wire [more like pipe] radius. With a 75Ω match requirement this would give a VSWR of >20. Not particularly helpful in a transmit situation. At about 4.8MHz the 75Ω match VSWR would be about 1.1.

    I've a attached a plot for the ~12MHz case.

    If I make the wire/pipe radius more practical [say 10mm] I obtain a VSWR(75Ω) of 1.05 at 4.75MHz. Sweeping the frequency I find the gain reaches 5.05dBi at 12.18MHz with a VSWR(75Ω) of 34.6. Again that VSWR level with a Tx system would be untenable.
     
    Last edited: Jun 28, 2012
  11. Papabravo

    Expert

    Feb 24, 2006
    10,180
    1,800
    Ah..part of the confusion is that each dipole will have a resonant frequency. This resonant frequency depends primarily on length. If you make the dipole longer the resonant frequency will go down. There is also a bandwidth to consider. Longer dipoles have a narrower bandwidth over which they will radiate effectively. Conversely, shorter dipoles have a wider bandwidth.

    There may be other modes of radiation than the λ/2 mode but they are less effective.

    Height above the ground is also a factor in radiating effectively. λ/4 is about the minimum height for a dipole.
     
  12. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    The maximum gain you can get by stretching a dipole is 5/4 wavelength (sometimes called an Extended Double Zepp). After this point you start getting multiple lobes. For a vertical antenna, the optimum length is 5/8 wave. In either case, the antenna is capacitive, and you need a loading inductor to bring it to resonance. The popular 43 foot vertical antenna is actually a 5/8 wave antenna on 20 meters.

    Eric
     
Loading...