Diffraction and general attenuation

Even in urban areas which usually have strong cellular signals D3797D

  throughout, there are often dead zones caused by destructive interference of waves which have taken different paths (caused by the signal bouncing off buildings ma2831

  etc.) These usually have an area of a few blocks and will usually only affect one of the two frequency ranges used by cell phones. This is because the different wavelengths of at49f040

  the different frequencies interfere destructively at different points. Directional antennas are very helpful at overcoming this since they can be placed at points of constructive interference and aligned DS2016

  so as not to receive the destructive signal. See Multipath interference for more.The longer wavelengths have the advantage of being able to diffract to a greater degree so are less reliant on line of sight to obtain a good TX-2B

  signal, but still attenuate significantly. Because the frequencies which cell phones use are too high to reflect off the ionosphere as shortwave radio waves do, cell phone waves cannot travel via the ionosphere.