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When I look at networking routers on the market, those that provide external antenna connections offer at least 2 connections.

What is the reason for having 2 antenna? Most if not all, have their antenna virtually at the same locality since they are attached to the device itself, hence it does not seem to provide "greater coverage". Or, does it boost the total gain e.g. 2 x 18 dBi anntena gives 36 dBi, if so, why not just have a single 36 dBi antenna?

Long Story: I recently had to buy antenna to try boost the signal strength going into my 4G LTE router. The router has 2 SMA female connectors on the back. I bought a single antenna and connected to one of the connectors and tested the setup. There was no increase in signal strength and speed a according to the tests. I am thinking of buying another antenna to connect to the router, but not sure if it will help at all.

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Many WiFi devices use (or can use) "diversity antennae". Because the two antennae are physically separated the signals received at each antenna are out of phase, which can be used to help with noise reduction, particularly multipath interference.

If you have a problem with weak signal strength then diversity may help, but a properly positioned directional antenna (a patch or yagi, usually) will be the most beneficial. Diversity is most useful when you have a large number of reflections (multipath).

Also, multiple antennas can be used for beamforming.

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  • \$\begingroup\$ Is the separation between the antennae an important built-in parameter for "diversity antennae" to properly interpret the phase difference? If so, does that mean antennae with lead wires that allows random positioning would not be suitable? The 4G router I have has its SMA connector just 1 inch apart, doesn't make sense to me. \$\endgroup\$
    – Jake
    Sep 9, 2013 at 4:01
  • \$\begingroup\$ The separation (obviously) has an effect on the phase, but the point isn't to make the two antennae x degrees out of phase, it's to maximize the chances of the two being sufficiently out of phase to get a substantial advantage from differential processing. \$\endgroup\$
    – markt
    Sep 9, 2013 at 7:13

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