Antenna Theory and Considerations

Antennas are commonly known for their ability to translate an electrical signal into an electromagnetic wave which propagates itself.  As the wave propagates through an area, its' amplitude is damped by various factors until the wave dies out. 
Antenna design takes into consideration a number of different factors to maximize the transmission power efficiency of antenna for a given application.  A few of the primary factors are antenna impedance,  polarization and bandwidth.

Antenna impedance determines how much of the electrical signal is being reflected back into the circuit.  The Standing Wave Ratio (SWR) is the ratio of maximum transmit power to the attenuated power due to reflection.  Ideally, there would be no impedance, therefore no reflection and the ratio would be 1:1.
In practical design for low power applications, this ratio is typically around 1.5:1.  Impedance matching the circuitry to the antenna at the desired resonant frequency can reduce the SWR.

Polarization denotes the physical orientation of the antenna relative to the Earth plane.  Electromagnetic waves propagate from the antenna in a direction perpendicular to the length vector of the antenna.  So maximum power transfer between a transmitting and receiving antenna is achieved by orienting the two with similar polarities.  Aside from horizontal and vertical configurations, circular polarization is commonly used in mobile transmission.  For example, an airplane could be in any given location relative to a receiver that is picking up its' radio transmissions.  So maximum power transfer due to polarization can never be guaranteed.  Instead, circular polarization propagates electromagnetic waves which travel in every direction.  This way, contact between transmitter and receiver is continually maintained.

 The bandwidth of an antenna denotes the range of frequencies, centered on the antenna resonant frequency, which the antenna will transmit with maximum transmission power.  Varying the thickness of the antenna can effect the bandwidth.  Also, bandwidth increases as the SWR ratio gets closer to 1:1.

 The Sleep Sound monitor will require an antenna to be built into the sensor.  Due to the relatively short expected range of transmission, this antenna will not need to be designed around maximum transmission power constraints.  Rather, it will be designed around the physical size and evasiveness constraints of the infant sleepwear shirt. 

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