basic concepts on antenna 3dB (half power) & beamwidth, gain, directivity

Discussion in 'Wireless & RF Design' started by donut, Jun 22, 2012.

  1. donut

    Thread Starter Member

    May 23, 2012
    These trivial questions really helps in my basic understanding of antennas.

    Please confirm for me the following:

    1) The 3db (half power) of a antenna provides details about the antennas beamwidth (in degrees)

    2)narrower beamwidth means that the antenna has more gain, more gains is evident in more directivity

    3)wider beamwidth means than the antenna has less gain, less gain is evident in less directivity

    4) antenna beamwidth ≠ antenna bandwidth
  2. KL7AJ

    Senior Member

    Nov 4, 2008

    Hi Donut:

    What you say is generally true, if you ignore the EFFICIENCY of an antenna. Some very large antennas (for instance, the Beverage) actually sacrifice gain in order to get a very narrow beam. The Beverage is a very "quiet" antenna, but also very inefficient, contrary to some common conceptions.

    But...if we assume an antenna is 100% efficient, you are correct, higher gain always results in higher directivity, and vice versa.

  3. aprillove1719

    New Member

    Jul 17, 2012
    E9D: Directional antennas: gain; satellite antennas; antenna beamwidth;

    SWR bandwidth; antenna efficiency; shortened and mobile antennas; grounding;losses
  4. KL7AJ

    Senior Member

    Nov 4, 2008
    Here's a great analogy for looking at antenna gain, which demonstrates the NFL concept. No, not National Football League, but No Free Lunch.

    If you take a leather bean bag chair, it has a fixed amount of surface area. Think of this surface area as being the total power radiated from an antenna. Now, you can punch the bean bag into any shape you like, creating "lobes in one direction...but for every lobe you create, you also create a "dent" somewhere else. The surface area stays the same.
    This is also true for any antenna...all a high gain antenna can do is redistribute a given amount of power (surface area( in different directions. High gain in one direction always results in lower gain somewhere else.

    A simple concept, but one that's often overlooked.