There is one element in these external cellular antenna systems that most people (except for us ubergeek hams) overlook.
Coaxial cable is naturally very lossy stuff when you're looking at the 500-2000MHz frequency range that cellphones function in. When you're talking about the light weight RG-59A/U or even worse RG-174 coax that is typically used with cellular antennas the power loss is staggering.
For instance, at 1800MHz, the typical lower frequency on 3G networks, you can expect a loss of approximately 5dB in a 10' length of RG-174 (the really thin stuff) This means that about 75% of your signal is being wasted heating your cable. The numbers are a little better if heavier cable such as RG-59 or RG-58 are used, but only slightly. 10' of cable will still result in a 50% reduction in output power and received signal strength.
I've found that by far the most effective setup is to actually run a 10' USB cable up to the roof of the vehicle, and enclose a USB modem in a piece of PVC pipe stuck to a mag mount. That typically bought me a 10-15dB SNR improvement over the USB modem in the cab of the truck, and by adding a quality external antenna on a *very* short coax lead (2' or less) something closer to 20dB is not unreasonable. I tried 5 different spots in my area that are completely dead with the modem inside the cab of the truck, and in all but one I had atleast 4 bars with the modem on the roof. The other spot had 2 bars instead of zero. If I were to add an external amp as well, also on the roof and connected with short coax runs, the improvement would be even more dramatic.
My Dad recently suggested I sell the enclosure I built for my USB modem. Perhaps there could actually be a market for it if I included the external antenna and perhaps even an amp.
