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Somebody explain dBi

Can somebody explain to me what dBi exactly is, and whats better? higher dBi or lower? why does higher dBi antennas cost more money? What is different between a 6 dBi omni antenna and a 10.5 dBi omni antenna? Im going to extend the range of my wireless network and im thinking of getting any antenna from »www.hyperlinktech.com/web/an...0···omni.php and probably a amplifier. I already changed the firmware on my router to DD-WRT and set the antenna output to 190mW. It gives me longer distance and stronger strength, but im looking to send my wireless netowork further.

»Wireless Networking Forum FAQ »When comparing add-on antennas, what does dBi mean? How do I choose?

so to get more distance with a omni directional antenna.. i would want higher dBi?

Sort of.

An antenna produces a radiation pattern, specific to the type. The pattern is a three-dimensional area that is effectively covered by the emitted radio waves.

An omni produces a pattern that looks like a doughnut. The higher the gain of the omni, the greater the diameter of the effective range, at the expense of the vertical coverage. That is, the doughnut starts to flatten and look more like a disk. Most omnis also are engineered with a certain amount of downtilt so that it can cover the area below for installations such as a roof mount.

Most manufacturers publish a graph of the radiation pattern for each antenna to help you decide what will work best for your particular situation.

let's try a simplier definition of dBi

first: dB - decibel. A log scale where 3dB is twice power.

dBi - dB's of gain (increased transmitted or received signal strength) for an antenna. That antenna may be directional or not. The dBi rating is the gain of the antenna you use relative to an "perfectly spherical radiation pattern" antenna (isotropic).

dBi - the i means isotropic - the ideal antenna with a spherical radiation/reception pattern. The isotropic antenna has 0 dBi of gain. Any antenna with gain gets that gain (rated in dBi) by shaping the pattern to be non-spherical.

Antenna gain is sometimes rated in dBd - dB relative to a dipole antenna. Rule of thumb: most dipole antennas have 2dBi of gain. So an antennna rated in dBd has that 2dBi of bias in it. A 0dBd gain antenna is 2dBi; and so 2dBd is about 4dBi.

The rubber-duck type antennas you see everywhere are not isotropic or omnidirectional. They have a little gain. The pattern looks like a semi-squashed sphere. So the signal directly off the tip of this kind of antenna (as you see on most WiFi routers/APs) is diminished. Same is true for 180 degrees from the tip, "the bottom". Typical for these is 2dBi of gain.

At the other extreme are parabolic dish antennas. These focus most of the power into a beam about 3-10 degrees in wide/tall, near the dish. The higher the gain (like 25dBi) more narrow the beamwidth. Like a flashlight, the beamwidth enlarges (diverges) as you move away from the antenna - out to miles. Simple math shows this.

Those 7dBi WiFi rubber duck antennas get their gain by "squishing" the 0dBi ideal isotropic pattern down to a more doughnut shape. So they are onmidirectional on the plane extending out from the antenna, but at say 60 degrees off-axis the gain falls (e.g., on the next floor of the house). The other interesting fact about these WiFi antennas is that they cost a tiny fraction of what they are selling for. No magic, just rubber and wire. Same rubber and wire as in the 2dBi antennas. That's marketing 101.

So that's a little antenna 101.

»en.wikipedia.org/wiki/Isotropic_antenna