I'm thinking of increasing my 3G USB modem's signal reception using an antenna. I know a UMTS(2.1 GHz) antenna is recommended.
I need to know whether a WiFi (2.4 GHz) antenna will do the job (as I can find one easily); will it?
(I found that two frequency ranges won't intersect, if the antenna is built properly !)
A simple whip antenna may work because it will probably have a wider frequency range, but better antennas can't work because narrowing the frequency range is one of the main ways of improving the antenna gain. But I personally don't recommend to do this mess.
Will it work? Likely a little bit depending on whether the WiFi antenna can support a wide bandwidth. In reality, the results are likely to be horrible and the connection non existent. You will then try and troubleshoot problems due to this. I highly recommend that you get the proper antenna.
You can try what I did to improve reception for a 3G cellular wireless modem. Build a reflector for 2100 Mhz. Check out this link. It is for a biquad antenna.
http://buildyourownantenna.blogspot.ca/2014/07/double-biquad-sector-antenna-for-1700-mhz-4g-lte.html
In place of the biquad element you substitute your wireless modem at the specified spacing. The one with the wings is the one I constructed. The results were fantastic. According to what is said on the plans it gives you 13.87 db gain. My modem that used to crank out 100mW in all directions is effectively cranking out 2.5W in one direction. I get full bars from a cell tower about 5-6km distant with decent upload/download speeds.
