You cannot and should not use external antennas with the WNDR3700 because the "N" technology it uses is highly specialized and TUNED to the antennas contained within the unit. "N" changes the way antennas are implemented --- so the OLD WORLD way of thinking about antennas [for indoor usage] has dramatically changed.
If your issue is performance and range you can improve both by how and where you locate the wireless router.
The rule for PLACEMENT is as follows:
1. as high as possible
2. within the DWELLING as central as possible
99.9 % of people disregard the rule for placement because they love to look at the LEDs flashing away
and will not bother spending the time and money needed to place the wireless router in the most optimum location in relationship to all wireless clients.--
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Information Technology for Home and Business