AM TRANSMITTER

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AM TRANSMITTER In an  AM  ( amplitude modulation )  transmitter  the amplitude (strength) of the carrier wave is varied in proportion to the modulation signal. In an FM (frequency modulation)  transmitter  the frequency of the carrier is varied by the modulation signal. There are two basic configuration for transmitters. High level modulation low level modulation  High level modulation In high power modulation signal,the carrier voltage is modulated at highest power level.the require power level is obtained by class c power amplifier.The block diagram of high level modulation as shown in fig(a). low level modulation In the low level modulation system, the carrier is modulated at low power level and the carrier power is subsequently raised to the desire level in the class B amplifier . transmitters using high power level are widely use at present .The block diagram of high level modulation as shown in fig(b). advantage AM sign
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YAGI ANTENNA

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 YAGI ANTENNA 
The location  and the length of the director and reflector element are critical, and therefore there can be more than one of each. in practice , adding more reflectors has little benefit ,so  nearly all   arrays have only one reflector element. the Yagi Uda array or simply the Yagi antenna has a single    driven element usually a ƛ/2 Dipole  or folded dipole as show in figure. the reflector is 5% longer than the driven element and space between 0.15 to 0.20 ƛ  Behind the driven element. Single director element, 5% shorter than the driven element is space the same distance on the other side to complete the basic Yagi antenna.Due to the relatively large physical dimensions of that elements and their spacing, the Yagi antenna is rarely used for the frequency below the VHF band. 

  fig (1) basic four element YAGI design

fig 2 radiation pattern 

Advantage
-High gain

-High front to back ratio

- high Directivity 

 Disadvantage
-Gain is limited to around 20db.

-for high gain level of the antenna become very long.

Application
-It is use for vhf television.
-it use for skywave communication.

                                                          video on yagi antenna




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