I was skeptical whether a passive antenna may work, but gave it a try because my office is inside a cinder brick block building that blocks both cellular tower signal and wifi signal from the access point hub that is only 50 ft away.  I was getting zero bar or sometimes barely 1 bar drifting from cell tower once in awhile, and no wifi signal at all.  After I put the passive antenna next to the window close to the ceiling, and my iPhone is 5 ft away from the antenna, it actually got 2-3 bars in cell signal, and 1-2 bars for wifi.  I did not expect it could give such improvements, pleasantly surprised.  I tried it out at home, where I do get a pretty good cell signal, 4 bars, without the antenna.  I thought it would give me 5 bars with this passive antenna, but it didn't change any signal strength.  So I think it works well for very weak cell signal in dead zones, and bring it up to a level where you can actually get some signal, which is better than nothing before.  But don't expect to give you better signal improvement if you are already in a strong signal area.  So it is a good alternative, without having to use powered antenna amplifier, which is very expensive and require wiring and power source, which is not as portable as this.