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Answer Posted / gaurav sharma
Hi, If you feed an antenna with a signal a part of the
signal may reflect from the antenna and will return back to
the sender. This reflection only happens if the impedance of
your antenna does not match with the impedance of your
cable. So if your antenna is 50 Ohm and your coax cable is
75 Ohm, you will have a problem. The VSWR is a ratio that is
telling how well your system is matched and if the energy of
your transmitter is getting into your antenna. 1:1 is the
optimum and it means all energy made by your transmitter is
put into your antenna and nothing is reflected back to your
transmitter. if Your VSWR is 1:3 it still looks OK you may
think but now some 25% will return to your transmitter and
only 75% is put into your antenna.
If your VSWR is low it DOES NOT mean your antenna is good !.
. It only is telling its good matched. Every antenna also
has an efficiency, how much of the power put into the
antenna is effectively radiated by the Antenna. This is
expressed in dB. Mobile phone antenna's have normally an
efficiency of some -2 or -3 dB but their VSWR is 1:1.5 or
below which is very good
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