Historical use of the term dBi as antenna gain

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KX4O, Feb 2, 2017.

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  1. KK5JY

    KK5JY Ham Member QRZ Page

    Newbies get separated from their money in a lot of different ways. So do people in the hobby for 50y or more.

    Regardless, you need a standard theoretical antenna that can't exist, so that you have a consistent basis for comparison. If you make your comparison anything else, (dipole, vertical, etc.), the basis for comparison becomes meaningless, because all those are physical antennas that have site-dependent behavior. A dipole in space is different than one 10' above the ground, and one made from copper is different from one made from aluminum. The ground in Oklahoma is different from the ground in three-land (mostly these days, ours just shakes more). Having a theoretical antenna that can serve as "zero" makes other antenna comparisons meaningful because they are compared to the exact same reference.
    KX4O likes this.
  2. KX4O

    KX4O Ham Member QRZ Page

    Your point is well taken, but keep in mind freespace is the unwritten requirement for these various reference antennas... such as the dipole in dBd. dBd is reasonably well defined to be a half-wave dipole in freespace. dBq is a little too antiquated to be useful for most. Of course dBi is a rock solid reference... the best of the bunch.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    dBd in the real world is higher than dBd in free space and it's real.

    Many manufacturers specify dBd antenna gain at a stated height above ground and assume "average" ground. That can be modeled, and it's real.

    A horizontal "dipole" can have several dB gain over a dipole (dBd) in free space, and at HF or even VHF nearly always does.
  4. KX4O

    KX4O Ham Member QRZ Page

    There cannot be two or more definitions of dBd or it is pointless. Various books on antennas and several operating standards agree.

    Name one so I can research how they might be using dBd and how they describe the environment. There are rules on the use of decibels referentially. First and foremost is leave nothing to the imagination. For a manufacturer to state an absolute gain using decibels the way you claim they might, they would have to say, for example, "7 dB (re dipole at 10m above average ground)". A reference gain isn't a reference without all variables specified.

    Yes, all true, but there is little doubt the dipole referenced by the term dBd is one situated in free space and is precisely 2.15 dBi to avoid the variations that would make comparisons impossible. The FCC agrees with this in determining ERP for example. TIA329-B-SEPT-1999 also assumes this.

    As you might have guessed by my OP, I am researching the history and use of decibels in the field of antennas. If you have an antenna textbook or antenna industry standard that suggests dBd involves some sort of real ground at some height, please share the title.

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    There aren't. But real "dBd" varies with installation. A horizontally polarized antenna suspended above Earth has gain (at some radiation angle above the horizon) over a theoretical free-space dipole, so indeed a "dipole" can have several dB gain over a free-space dipole. The exact same dipole oriented vertically doesn't. This is why a horizontally polarized beam starts out with gain even before it's focused. In "free space" a horizontal dipole and vertical dipole are equivalent. Installed above Earth, they're not.

    They normally use dBi (dBd + 2.15 dB) but do show how height above ground (for horizontal antennas) increases dBi gain. You can just subtract 2.15 dB from the dBi figures to correlate dBd. M2 has done this since their inception: http://www.m2inc.com and lists gain optimized by elevation above earth.

    An example is here: http://www.m2inc.com/amateur/20m6-125/

    I see Tennadyne does the same thing. They all should.

    Gain is often ~ 6dB higher than "free space" when the antenna is elevated 1WL.
  6. AI3V

    AI3V Ham Member QRZ Page

    Again Steve is wrong.

    Ground gain is possible, but certainly not guaranteed.

    Remember. For any given reflection there are 2 limits to what might happen

    1 both signals add perfectly and you get +6db on your computer screen

    2 both signals add imperfectly and you get a signal from +6 to minus infinity

    Nowhere ever except Sammy hammy talks about ground gain. It is laughable to think it can be predicted or counted upon except to null to infinity just at the spot the fire captain puts his ht down at.

    IMHO hammy descriptions that assume ground gain are designed to confuse.

    You want to talk ground gain? Show me the acres of copper sheet you measure it over.

    Google radio antenna Fresnel zone
  7. AI3V

    AI3V Ham Member QRZ Page

    While your researching look up Brewster's angle.

  8. AF7ON

    AF7ON Ham Member QRZ Page

    I am going to further muddy the waters. We shouldn't perhaps be talking about gain but instead directivity. Directivity and gain are the same when the antenna is lossless, which is approximately true for full-size antennas. It may not be true for any antennas that have reduced size or use loading in some form and it's definitely not true for trapped designs or any other antenna with a lossy feed system. Some trapped multi-band antennas are not much better than rotary dummy loads, whatever their directivity.

    KX4O likes this.
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    A gentle reminder that we need to consider the nulls that ground reflections cause, as well as the "gains". For example, look at the green trace here, for elevation angles in the range 10-15 degrees:


    That antenna at 80ft is really bad news if I want to work Europe. If I want to work both Europe and Oceania, maybe a lower height would be a better overall compromise.

    [The blue and red bars are the statistical frequency that signals from the two paths arrive at particular elevation angles]

    Steve G3TXQ
    KX4O likes this.
  10. AI3V

    AI3V Ham Member QRZ Page

    And this statistical distribution of angle of arrival is why we transmitted fleet broadcast signals over several different antenna types simultaneously.

    Its only a small joke when the answer to "which antenna" is "get both"


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