(Image links from Google)
My parents talk about the Over the Air antenna that used to dot roofs or Rabbit Ears on the TV table top. Cell phones and cordless phones had their own visible antennas as did car radios..
Antenna’s are categorized by their size, type and functional design. Passive antennas are the most basic kind and do not amplify the signal in any way they simply radiate the RF energy received from a transmitter in one direction. Active antennas are basically passive antennas with an amplifier built-in.
Size and shape of an antenna depend on the frequency on which the antenna will transmit and receive signals. The direction of the radiated electromagnetic wave and [ower with which the antenna must transmit. Antenna size is inversely proportional to the wavelength it is designed to transmit or receive. Lower frequency signals require larger antennas
There are two major kinds of antennas – directional antennas and omni-directional antennas. The latter are used to transmit and receive signals from all directions with relatively equal intensity whereas directional antennas transmit a signal in one direction only.
Distance between the transmitter and receiver generally determines the strength of the signal. Antenna performance is measure of how efficiently an antenna can radiate an RF signal that ensure maximum signal strength at the receiver.
Antennas emit signals in two dimensions horizontally and vertically. Antenna polarization is the orientation of the wave leaving the antenna. Vertical polarization
Sine waves travel up and down when leaving antenna. Horizontal polarization
Sine waves travel from side to side on a horizontal plane. Most efficient signal transmission and reception is experienced when both antennas are equally polarized
The height and location of the antenna and distance between transmitter and receiver are the critical variables in of “line of sight” versus “non-line of sight”. There are 3 RF waves types defined - Ground waves that follow the curvature of the earth. Sky waves that bounce between the ionosphere and the surface of the earth. Finally, line-of-sight used by RF waves transmitted in frequencies between 30 MHz and 300 GHz. Directional antennas are the most common and reliable method of transmitting RF waves. Antennas for mobile hand held devices have evolved since their introduction
Whips Retractables Stubbies Embedded
Reduction in size and appearance of the antenna (internal or external) has introduced more complexities. Last year, we had an Antenna-gate [ http://www.pcworld.com/article/201297/apples_iphone_4_antennagate_timeline.html ] . The suffix to this news worthy story became clear only when I finally grasped the context of President Nixon’s cover up and connected the dots. Most of the news stories that covered this issue discussed how the bars on the iPhone4 changed based on the grip. Steve Jobs even hosted a complete press conference on this issue.
http://www.anandtech.com/show/3794/the-iphone-4-review/2
The link above from the AnandTech blog discusses how attenuation gets measurably worse depending how you hold the phone. Squeezing it really tightly, you can drop as much as 24 dB. Holding it naturally, I measured an average drop of 20 dB. The article also discusses how the iOS reports the quality metric with a compressed, optimistic dynamic range. On iOS, 4 bars begins at around -99 to -101 dBm. Three bars sits around -103 dBm, 2 bars extends down to -107 dBm, and 1 bar is -113 dBm. To give you perspective, for a "3G" plant, -51 dBm is the best reported signal you can get - it's quite literally standing next to, or under a block away from a tower. At the other extreme, -113 dBm is the worst possible signal you can have before disconnecting entirely. With a few exceptions, signal power as low as -107 dBm is actually perfectly fine for calls and data, and below that is where trouble usually starts. However, the article notes just how little dynamic range iOS 4 has for reporting signal; over half of the range of possible signal levels in dBm (from -99 dBm to -51 dBm) is reported as 5 bars
The technical explanation for drop in signal written in the article notes is because humans are mostly water and material which happens to be pretty good at attenuating RF - thus increasing path loss between the handset and cellular base station. There's nothing Apple or anyone else can do to get around physics, plain and simple. It's something which demonstrably affects every phone's cellular reception.
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