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Antenna Evaluation: VSWR and Front-to-Back Ratio

Voltage Standing Wave Ratio (VSWR)

For a radio receiver or transmitter to deliver power to an antenna, the impedance of the radio and transmission line should match well with the impedance of an antenna. Voltage Standing Wave Ratio (VSWR) and return loss are two parameters used to determine whether an antenna is effectively impedance-matched to the line of transmission it is connected to. Often, there is a bandwidth range that antennas must satisfy and is defined in terms of VSWR.

The lower the VSWR is, the better the antenna is matched to the transmission line and the higher the power delivered to the antenna. Furthermore, a small VSWR reduces reflections from the antenna. 1.0 is the minimum VSWR where there is no power reflected, and it’s the ideal condition for an antenna.

Return loss, measured in dBi, which is an indicator of how much incident power is reflected to the source of the signal works in the opposite of VSWR. The higher the return loss, the more power to an antenna meaning you are losing less power and signal strength. Low return loss affects the antenna’s capability to radiate which in turn affects transmission efficiency. Overall, for an antenna to have good signal strength and efficient performance, VSWR should be low while dBi should be high

Front to Back Ratio applies to Directional Antennas Only

Front to Back Ratio is the measure of directivity of a directional antenna:  A ratio of the magnitude of directional radiation in an antenna, directed in a single direction, divided by the radiation pattern to the energy wasted in or by the antenna. 

Increasing the gain of a directional antenna, is the same as increasing the front to back ratio. The best front to back ratio of a directional antenna is a large dish antenna such as 34dBi.

Front to Back Ratio, which is expressed in dB, is important in situations where reverse coverage or interference needs to be minimized.  A low Front to Back Ratio means the less directional an antenna is hence poor reception from the front. A low Front to Rear Ratio can only be good when one is trying to pick up the signal from behind or the side of an antenna.

When evaluating an antenna, Front to Back Ratio doesn’t necessarily fit with antenna gain (dBi). This is because an antenna can have poor front-to-rear ratio but have great gain. The overall performance will depend on the antenna you are using. In case of a directional antenna, both Front to Rear Ratio and antenna gain should be high for it to work efficiently and have a strong signal.

The higher the ratio, the more directionally efficient the antenna is because a directional antenna always radiates some signal out of the back of the antenna. For an antenna to be said to be good, the Front to Back Ratio should be a minimum of around 15dBi. Then again, the antenna gain should be high raise the signal above the unavoidable amplifier noise hence increasing signal strength. 

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