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Mainly, would it contribute to differences in the range and antenna propagation pattern, affecting my signal at my computer points?

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A lot about how well wireless works in a given area has to do with the specific environment (distance between points, interfering signals, interfering materials, strength of antenna used, even the wireless card at the other end). I have always found routers that sport multiple antenna to be more of a gimmick than anything else. –  MaQleod Aug 19 '11 at 6:08
    
some electrical devices would affect the wireless signal like wireless phone and microwave devices. –  kokbira Aug 19 '11 at 11:49
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It depends on the radio(s) those antennas are connected to.

Let's say you're comparing two 802.11n APs. One has two antennas and the other has three. Well 802.11n's big speed boost over 802.11a and 802.11g is that it can uses MIMO -- ganging together multiple radio chains to boost performance. It takes two radio chains to reach signaling rates up to 300 megabits per second, and with a third radio chain you can reach signaling rates up to 450 megabits per second. So if it turns out that both APs have one antenna per radio chain, it might be and indication that the three antenna unit can go 50% faster than the two antenna unit.

But you can't just count antennas to know how many radio chains they have, you have to confirm it by reading the technical specifications of the device.

If the two or three antennas are all hooked up to a single radio, like in the case of a single-band 802.11g AP, then the third one probably doesn't matter much. With a single radio, it makes sense to have two antennas -- main and auxiliary -- for the sake of antenna diversity. Sometimes the main antenna is in a better place to receive a given signal, and sometimes the aux receives the signal better. A third antenna in this case is just another aux, and it's probably not going to help much given that they're all within a few centimeters of each other. But then again, the antenna designer may have planned out the coverage patterns of those three antennas to complement each other well, so it's possible it can make a difference. But you wouldn't really know unless you had an antenna engineer carefully test it.

Overall, you really can't judge what a wireless router can do in terms of rate-vs-range just by counting antennas.

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is there any way to tell which antennas are doing what on a router? If it is dual band and has 3 antennas.. are some of the antennas dedicated to 5ghz? Or are they all shared? –  Jeff Atwood Nov 15 '12 at 0:53
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@Jeff I can't think of an easy way for a non-expert to determine which antenna does what. And there's a lot of variance from device to device of which antennas are shared dual-band antennas, which are single-band antennas, etc. An 802.11 radio engineer or someone with a related skill set could use lab equipment to figure that out. Or you could try to follow the circuit board traces from the antenna connectors back to the radio(s) and see if you can determine how things are hooked up, but sometimes a trace may be hidden in an internal layer of the circuit board, etc. –  Spiff Nov 15 '12 at 1:35
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@JeffAtwood The only tipoff to look for is if some of the antennas have one shape/appearance, and the others have a different form. This usually suggests that these are single-band antennas, and one set is dedicated to the 2.4GHz radio, and the other set is dedicated to the 5GHz radio. –  Spiff Nov 15 '12 at 1:37
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