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Consider an omnidirectional antenna with X dBi gain. Intuitively I understand that the power that would have been dispersed in now-dead directions is directed around the antenna. What I don't get is that we find Yagi Uda antennas with gains around 10dB, but also omnidirectional antennas of the same gains.

What's the point in getting a Yagi Uda over an omnidirectional if it has the same gain? I thought directional antennas would redistribute the power from one direction to a privileged direction, so shouldn't we end up basically with a total gain of that of an equivalent omnidirectional + the directivity?

Otherwise I would expect a Yagi Uda of gain X to have a larger vertical beamwidth than an omnidirectional of the same gain, but it does not seem to be the case so I am puzzled.

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    $\begingroup$ Only the colinear omni has gains around 10db, the rest of the omni antennas are closer to 3-5db. The colinear gets it by flattening the torus into a disk. (And it's huge) Moxon (folded yagi) has a smaller back lobe and larger front lobe. A typical yagi gets less gain on the sides than on the back. $\endgroup$
    – user10489
    Aug 13 at 11:27
  • $\begingroup$ Can you edit your question to specify what type of omnidirectional antenna that you had in mind? $\endgroup$
    – Mike Waters
    Aug 14 at 13:10
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An antenna that radiates in all directions equally is called an isotropic antenna.

An omnidirectional antenna is one where the gain does not vary by azimuth. It does however vary by elevation. Such antennas are usually mounted such that their maximum gain is aimed at the horizon and are most useful when the terrain is flat. So an omnidirectional antenna with 10 dB of gain achieves that gain by sending less radiation at the ground and less at the sky.

Omnidirectional antennas are most often co-linear arrays. The more elements in the array, the more gain they can provide, and the more narrow their main lobe becomes. Paradoxically, less reputable manufacturers seem to have figured out some technique for increasing gain beyond what is physically possible for a given number of elements in the array.

A Yagi has gain that varies by elevation and azimuth. As with all antenna arrays, more elements means higher gain and a narrower lobe.

The idea with an omnidirectional antenna is your intended communication targets are moving around on relatively flat terrain. They won't be up in the sky and they won't be in the ground. But noise sources tend to be in similar places. So given a Yagi and an omnidirectional with equal gain, the Yagi will often perform better because it excludes more noise sources, however a Yagi can only be used if you know the direction of your intended target.

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  • $\begingroup$ Thanks Phil Frost. I find your last paragraph particularly interesting: does that mean that Yagi Udas only have an advantage over omnidirectional antennas of the same max gain when receiving? Also, I was actually wondering how we could have the same output power between directional and omnidirectional antennas for the same power in, whereas the power density (dividing by solid angles) was higher for directional antennas but I think I just got it: Yagi Udas also radiate quite a lot of power backwards, the power is just wasted by destructive interference. Am I right? $\endgroup$ Aug 13 at 21:17
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    $\begingroup$ No, power is not "wasted" by destructive interference. That power doesn't exist, it is actually distributed elsewhere. And people just live with the yagi backlobe. The yagi concentrates its power by not sending it sideways, and sending less backwards than forwards, but frequently not much less. $\endgroup$
    – user10489
    Aug 14 at 1:00
  • $\begingroup$ @MisterMystère Yes, a Yagi vs an omni of equal gain would have equal performance (neglecting perhaps interference you might be causing for others, and assuming they are both aimed ideally) on transmit. As for your second question, remember omnidirectional != isotropic. As onmidirectional gain goes up, the pattern is "squished" into a narrower range of elevation. Perhaps "omniazimuthal" would be a better name. $\endgroup$ Aug 14 at 2:50
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Geometry.

Consider a dish versus a cone. You could make a (dinner platter) dish and a (ice cream) cone with the exact same volume. But with a dish, the edges would be radially omidirectional from the center. However with a cone, the base would be be pointed directionally from the point of the cone. And both still having the exact same volume.

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  • $\begingroup$ But in that case the gain of Yagi Udas should naturally be higher in the privileged direction than omnidirectional antennas, in average by as much as the front-back ratio. But it's not the case. $\endgroup$ Aug 13 at 21:15
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    $\begingroup$ The average gain of an antenna with the same aperture as another will be the same. Nobody cares about average gain. $\endgroup$
    – user10489
    Aug 14 at 1:02
  • $\begingroup$ Gain higher? Not if the cone is shorter than the radius of the dish. But they can still have the same volume, even if the cone height and the dish radius are the same. $\endgroup$
    – hotpaw2
    Aug 14 at 2:06

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