There are many companies that sell 4G hotspots small enough to carry in a pocket and battery powered. It's not quite as convenient as only having to carry the device instead of the device + modem, but is probably the cheapest option if you have 3G or 4G service where you are.
Piece of cake, under $300 take your existing internet signal and create a WIFI hotspot. 2.4GHz 12dBi Outdoor Omni-directional Antenna TL-ANT2412D or equivalent. mast, tower or pole to mount antenna. Wireless Advanced Router with 2.4 ghz wireless G or N radio card and power supply see http://www.mikrotik.com/mfm. Cables cat 5 & coaxial, connectors for same. Board enclosure, pigtail from radio card to antenna. Assemble program and run.
Antenna height of 12 to 15 feet should cover 4 to 9 acres depending on tree cover and buildings, remember LOS line of sight. It is done at airports and camp grounds all the time. We put up 96 foot towers and cover 10 to 12 square miles (rural) with the same antenna but use a dish at the client end. In a town a solid square mile with small CPE (Customer premise equipment) narrow bandwidth modified WIFI cant see it with a laptop. The photo shows a roof top unit and we run 5 clients on it at 2+ miles. The tower is 4 miles away.
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