Why Does VSWR Exist?
In order to obtain the maximum power from a load requires that the load and generator impedance match. If there is any mismatch or difference, then maximum power transfer does not occur. This same concept also applies to antennas and transmitters. Since the antenna is normally located at distance from the transmitter, the feed-line has not have no loss and match both the antenna input impedance and transmitter output impedance to have a VSWR of 1:1. The resulting voltage and current would then have to be constant over the length of the feed-line. If there is any deviation from this circumstance, then there will be a standing wave of voltage and current created and the VSWR will exceed the 1:1 ratio.
How is VSWR Measured?
VSWR and its effects can be measured in several ways to include return loss, reflected power, reflection coefficient, and transmitted power loss. These all measure the proportion of power that is reflected back to the transmitter by a mismatched antenna just in different ways. The reflection coefficient is a measure of the mismatch at the antenna by the feed-linee and is expressed by the following formula, P =(Z1-Zo)/(Z1+Zo). Z1 refers to the antenna impedance and Z0 refers to the feed-line impedance with all variables being complex numbers. When Z1 = Zo there is no reflected signal in the antenna system. This condition is rarely seen, however, and is used more in antenna theory than in practical antenna applications.
Where are VSWR Measurements Made?
Along an antennas feed-line, the voltages will have a voltage minimum and maximum based on the relative phase difference of the traveling waves along the line. The minimum and maximum points normally occur ¼ wavelength apart from each other. Since the adoption of coaxial cable in antenna systems, VSWR measurements are normally made on the transmitter end of the feed-line.
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