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What is Antenna VSWR and Why Does it Matter?

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  • What is Antenna VSWR and Why Does it Matter?

    A common antenna spec parameter that is often quoted is VSWR but few understand why they should care about this. A comprehensive description of what the Voltage Standing Wave Ratio is (or simply the SWR if in terms of power) is beyond the scope of this basic forum. The important points however are:
    • The VSWR is related to the reflection coefficient.
    • The reflection coefficient determines describes how much power is reflected back to the transmitter and how much of the incident power actually makes it to the antenna. Of course we would want that no power gets reflected back since this is wastage.
    • How much power gets reflected also depends on impedance matching between the antenna and the cable connector and cable to transmitter. Typical impedances are 50 Ohms for most stuff except cable tv (he're is where picking the right cable comes in handy e.g can't use our wilson electronic antennas with coaxial cable for tv that you might have lying around your house!)
    • A perfect VSWR is 1. The closer to 1 the VSWR spec is the better. This question came up when comparing a 1.8VSWR antenna and a 1.5 VSWR which means the 1.5 was better.
    So what happens when VSWR is large? You lose power or efficiency. Not all the power from the transmitter will make it to the antenna. Some power will be reflected back. This is bad. For one you are wasting your battery. More importantly it leads to overheating which could damage the transmitter. Since RFIC designers (Radio Frequency Integrated Circuit) designers know about this they often design circuits to limit the output power when the reflected power is large. Of course when you limit your output power you limit your range as well which is not good!

    However even if you get an antenna with low VSWR don?t mess up by not matching the impedance between the antenna and the cable connector. Also most people using our cellular amplifers have a reliable abundant power source e.g car battery or a/c outlet so they are more interested in total system gain and less on power efficiency.
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