Reddit mentions of TECHTOO Hi-Gain 3G 4G LTE Indoor Outdoor 10dBi Directional Wide Band MIMO Antenna 698-2690MHz Panel Antenna Wall Mounting kit for Cellphone Signal Amplifier/MiFi Jetpack/Mobile Hotspot Device

For best performance from an external antenna on the Nighthawk you are going to need two antennas at a 90 degree offset to each other. You can't split one antenna. Something to do with MIMO not working if you only connect to one antenna port and some other stuff. check out LTE hacks on FB and you can find some pretty good info on antenna and hotspot use.

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Or you can do something like this. It has 2 antenna built into it.

https://www.amazon.com/Proxicast-Cross-Polarized-High-Gain-Fixed-Mount-Antenna/dp/B01NBSLNJ6/ref=sr_1_7?keywords=techtoo+4g+antenna&qid=1565200161&s=wireless&sr=1-7-catcorr

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or this one, I personally have his one connected to my MOFI and have been pleased with it. Unfortunately i is not available at the present time.

https://www.amazon.com/TECHTOO-Directional-698-2690MHz-Cellphone-Amplifier/dp/B0130QUUUU

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there are also other options to go with depending on your needs.