Antenna (1)

A component for transmission or reception of radio waves.  It's function is to convert electrical energy to radio waves during transmission and to convert radio waves to electrical energy upon receiving.

Antennas

 Antennas are used to transmit and receive radio signals.  They can range from a simple single wire in the air or a trace on a circuit board to complex multi-element specialized antennas.  Each type having their specific purpose.  With the intent on transmitting and receiving radio waves the goal is to provide an antenna that does this the best.   That usually means designing the antenna for a specific frequency or frequencies, direction of signal, and polarization of signal.

   Designing an antenna for a specific frequency is making sure that the antenna is resonant on that frequency.  Meaning at that particular frequency the antenna will transmit or receive the power more efficiently than it would at other frequencies.   This is where the relationship of frequency and wevelength are crucial.  Typically an antenna system will be the most efficient when it is a multiple of the wavelength it needs to transmit or receive.   That is it's resonant point.  For instance if you want an antenna that recieves on for instance around 150Mhz which is the 2 meter band then you want an antenna that is either full wave, 1/2 wave, 1/4 wave, etc.  A full wave antenna would be 2 meters in length.  Basically wavelength equals velocity divided by frequency.  The velocity meaning that of light in a vacuum being 300Km/s. 
λ = v/f

So for this 300 / 150 = 2

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