One interesting problem encountered with mobile devices that are carried on the person is the detuning of the antenna or antennas on those devices when in close proximity to the person’s body.
Antennas on mobile devices are peaked for optimum performance…and then fall short of the hoped-for performance when actually put into use. The trouble is that the human person has an intriguing ability of tuning things simply by being in close proximity.
Detuning by proximity is worse at high frequencies. The higher the frequency, the more and more small changes in capacitance and inductance will alter the tuning.
The Skin Effect
A related phenomenon, christened the “skin effect,” was a problem in the early days of radio. The skin effect was simply a notable change in tuning caused by a person’s hand as he or she tried to adjust the radio receiver’s tuning. The receiver’s tuning consisted of a simple LC tank circuit. As the user reached for the tuning knob, the person’s presence would notably change the resonance of the LC circuit, despite the low frequencies these early receivers operated at — often less than 2 megahertz. The result was that tuning became very difficult. The exact frequency the receiver was set to was dependent on where the listener was, and as soon as the user retracted his or her hand after tuning the receiver, the radio would become detuned again. The solution was to add a grounded metal shield between the control knobs and the tank circuits.
Just like an LC tank circuit, antennas have resonance as well, but unlike a radio’s LC tank circuits, the antenna cannot be shielded, at least if it is expected to radiate well.
The Broadband Solution
One solution to the problem of antenna detuning by proximity to the human person is to use broadband antennas. The broader the bandwidth of the antenna, the less other external influences will affect resonance. This is an excellent catchall solution, except for one detail: broadband antennas, as a rule, are relatively sizable, which means that they may not fit in some small devices. A cheat is to create a small antenna and broaden its bandwidth by loading it down. This, of course, is a lossy solution, and as such is rarely acceptable.
Adapt to the Human Presence
In some cases, like a watch that is always around a person’s wrist, the antenna can be optimized such that it is tuned properly when placed on the person. The obvious downside of this is the fact that, should the device in question be used off of the human person, it will become detuned.
RF Body Blocking
Another relevant problem sometimes encountered is not so much detuning of a device by the human person so much as RF being blocked by a person. Especially in Bluetooth applications very close to the resonant frequency of water, the human body (being composed largely of water) can cause a significant loss of signal. This can cause sporadic performance of certain small devices, which can cause the user significant annoyance. It is best to be aware of these RF-blocking effects, so that the designer can plan for them, determining the best way to design both the device and the antennas in the device to counteract them.
In all cases, RF-simulating software can be used to get an idea of what to expect for various situations. Models of the human body from an RF point of view are available. When coupled with a simulation of the device in question, they can provide valuable information on how a given device will act under various conditions. This can prevent the headaches that result from poor performance, by allowing for modification before the device goes into production.
ARSI 
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