Cell phone antennas

Multicavity filters. Multicavity Fabry-Perot filters are used to make very narrow transmission filters. A simple Fabry-Perot cavity (see Ch. 2) consists of a halfwave layer surrounded by two reflectors of typically 10 layers each. Figure 4 shows three transmission profiles obtained with one, two or three cavity filters. The three cavity HL) 5HH(LH) 5) " 3 filter has a 1.2 nm bandwidth. It has 60 layers. Note that the three-peak top of the transmittance. Each cavity has to be well adapted to the following one if not the resulting transmittance can be very poor. Such cavities are broadly used in telecoms in between arrays of antennas for cell phones. 

In the dark ages before cell phones, people who really needed mobile communications ability installed radio tdephones in their cars. In the radio telephone system, there was one central antenna tower per dty, and perhaps 25 diannels available on that tower. This central antenna meant that the phone in your car needed a poweihil transmitter—big enough to transmit 40 or 30 miles. It also meant that not many people could use radio telephones—there just were not enoi channels. 

As a consequence, different types of terminal will receive different power values because different terminals have different antenna gains. Another issue found in these systems is related with the attenuation due to obstacles. The PL term in (13.4) is dependent both on the free space losses and the attenuation due to obstacles between the transmitting and the receiving antennas. This means that for the same terminal different RSS values for the same references can be obtained at the same point, e.g. a cell phone will have different RSS values when placed on a table or when it is inside a pocket. 

Cell Phones. The telecommunications tool used the world over is the cell phone. A cell phone system employs a series of cells, about ten square miles in size. Each cell site, or base station, is equipped with a tower and an antenna and serves subscribers only within its bounds. A cell phone moving from one cell to another will change operating frequencies automatically to conform with the frequencies assigned to the acljacent cell. This scheme makes it possible to reuse frequencies over and over again as the user moves across the country. 

A major problem with this approach, of course, was one of mobility. Would a taxicab driver in one cell, talking on one frequency, after moving to a different cell, have to redial Certainly, this was not acceptable. By using very sophisticated electronics, the cell phone in use could transfer the call to the cell site antenna it was approaching and drop it from the... 

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