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by simple you mean small, so you can carry it with you, right...?
this sort of device would either need to be soldered into your phone to take the output signals and boost them, in addition to adding a better antenna for receiving.
or it would need to have a receiver circuit that received your phone signal and re-transmitted it at higher power, while preventing your phone from communicating with the tower.
and either way its going to require working with RF signals in the several gigahertz range, which is not particularly easy.
if your home has poor reception and you would like to boost your signal while in your house, I believe verizon had some units that plug into your wall and act just like a wireless modem... but for your phone. I do not remember what they were called or if they were actually released.
wrap some tape sticky out around modem ,take 10ft of thin insulated wire and wrap around to make a coil ,leave about a foot straight up. wrap tape sticky in to hold it together . slide off mobile then slip modem back about 2/3rds . Gave me about 1 bar extra which got me from 2 bars to 3 . Use the USB extension cable to find the best spot. mine is stuck to my window
An afterthought for this thread.
The UMTS frequencies on which the modem works and phone and others according to material I was reading all transmit on frequencies 1.885-2.025 and 2.110-2.200 GHz
Now 2.4GHz antennas ,double quad or bowtie antennas, have been made for a while and their 1/4 wavelength is said to be about 31 mm . So I think that anybody seeking to boost their USB modem's signal might look at working out the actual 1/4 wavelength for say 2GHz and constructing one with a good length of coax connected then look at coupling the inner end of the coax by an inductive/capacitative loop as mentioned above. Gain of 20dB is claimed by some of these.
Frequency hopping as mentioned in one of the posts would not affect this at all in my opinion as I think the hopping is only when interference is encountered and the antenna would be broad range enough to cover all the frequencies mentioned above . To be sure it does have a wide band usage make the bowtie wire slightly thicker in cross-section than those mentioned .
There was mention of the upright posts used to attach the bowtie to the coax might radiate but as long as the two remain the same distance from each other this would not be the case either. They pickup on an overall length of wire for the bowtie and then fold it in half then half again ,in half again and in half again. At these frequencies I would be interested whether the 1/4 wavelength sections are affected by the right angle bends and if so which way. Do they electrically shorten it or lengthen ? I would guess at shortening it .
Also the ends that get soldered to the ground sheath of the coax would have an effect of lengthening slightly the antenna so maybe one cancels the other. If you build one of these please post your result back here and send me a PM