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Is a longer Wi-Fi antenna better? Why do Wi-Fi antennas have different lengths?

The question is about the usual (stick-shaped) antennas used in consumer routers and adapters.

I have read that the length of a 2.4GHz antenna is to be 12 cm, the wave length (yes, inside it is a dipole of two pieces of half wave length).

However, I see much longer antennas sold, such as this (left: normal 12 cm antenna). This long antenna is advertized as "high-gain 18dBi". Does this make sense or is this a lie?

I also don't understand how such antenna can have "18dBi", since it looks omnidirectional -- does it make sense? Apparently it means that it transmits in the direction strictly perpendicular the antenna and almost nothing in other directions, but can this be true for such type of antenna?

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  • See log periodic antenna. – STTR Mar 27 '15 at 14:58
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    @STTR He's asking about stick-shaped antennas with omnidirectional coverage patterns, not triangular directional antennas like log periodic. – Spiff Mar 28 '15 at 3:58
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    @DrMoishePippik Your comment is full of misconceptions. Omnidirectional antennas DO have gain over isotropic antennas. Isotropic is the "i" in "dBi". Simple dipoles ARE omnidirectional antennas. Well designed half-wave dipoles can have as much as 8 dBi gain. – Spiff Mar 28 '15 at 4:04
  • As you state, a truly omnidirectional antenna has no gain (a simple dipole has some gain in a toroidal pattern, see en.wikipedia.org/wiki/Dipole_antenna; gain implies increased directionality). There are do-it-yourself directions for a high-gain, directional antennas at youtube.com/watch?v=udF_02S79fE and instructables.com/id/10--WIFI-16dBi-Super-Antenna-Pictorial. [I find that using a USB WiFi adapter on an a USB extension cable allows me to orient it for best reception, and the adapter is <US $10.] – – DrMoishe Pippik Mar 29 '15 at 3:18
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The typical stick-shaped rigid plastic-covered antennas on consumer Wi-Fi routers are typically half-wavelength dipoles (each element is a quarter wavelength for a total antenna that's a half-wavelength long).

They are not technically rubber duck antennas; rubber duck antennas are very flexible because they have a springy helical antenna covered in flexible rubber. A rubber duck antenna for CB frequencies (26-27MHz) happens to be similar in length to a half-wave dipole for Wi-Fi frequencies (2.4GHz), so laymen incorrectly assume based on external appearance that they're the same thing.

Well-designed half-wave dipoles max out at about 8dBi gain. However, you can stack them up to make a collinear dipole array, with each additional element adding a theoretical max of 3dBi gain. In practice, the marginal gain is probably more like 2dBi.

So:

   8 dBi and  6 cm for the 1st dipole element
+  2 dBi  "   "  "  "   "  2nd    "      "
+  2 dBi  "   "  "  "   "  3rd    "      "
+  2 dBi  "   "  "  "   "  4th    "      "
+  2 dBi  "   "  "  "   "  5th    "      "
+  2 dBi  "   "  "  "   "  6th    "      "
= 18 dBi and 36 cm for a 6-element collinear dipole array at 2.4GHz.

There's probably a little spacing between the elements, so it could be a little longer than that in reality.

So, it seems plausible that you could have something like a 6-element collinear dipole array omnidirectional antenna with 18dBi gain. Instead of a donut-shaped coverage pattern, it's got to be more pancake-shaped. :-)

But honestly, the cynic in me thinks it's much more plausible that you have something much less than an 18dBi omni antenna, but it's sold as 18dBi and they get away with it because no one in the wholesale/retail distribution chain has the equipment and expertise and time to properly measure it, so they just pass along the BS that the manufacturer told them.

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