Effect of Transmission Line Loss on VSWR Measurement

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jimh
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Effect of Transmission Line Loss on VSWR Measurement

Postby jimh » Sat Apr 17, 2021 2:16 pm

This article is a brief primer on how loss in a transmission line between transmitter and antenna can affect VSWR measurements taken at the transmitter end of the transmission line loss.

First, to understand VSWR you need to know that if an antenna impedance is a mismatch for the transmission line impedance feeding power to the antenna, some of the power intended to go to the antenna is reflected back toward the transmitter. The reflected power creates standing waves on the transmission line. The ratio of the peak voltage in the incident wave to the peak voltage of the reflected wave is the voltage standing wave ratio or VSWR.

To understand how transmission line loss affects VSWR measurement, assume the VSWR measurement occurs at the transmitter end of the transmission. Assume the antenna is not perfectly matched, and some power is reflected, and for example, consider 25-percent of the power will be reflected. Assume the transmitter outputs 100-Watts

For a perfect transmission line with no loss, the outcome will be as follows:
  • the transmitter sends 100-Watts into the transmission line
  • since there is no loss in the transmission, the antenna receives 100-Watts
  • because there is a mismatch in the antenna impedance, 25-watts is reflected
  • the 25-watts travels back down the transmission line to the transmitter end with no loss
  • a VSWR measuring device (a directional wattmeter) reads incident power 100-Watts and reflected power 50-Watts, giving a VSWR of 3:1

For a non-perfect transmission line with loss of 3-dB (or the power loss is 50-percent), the outcome will be as follows:
  • the transmitter sends 100-Watts into the transmission line
  • since there 3 dB loss in the transmission, the antenna receives 50-Watts
  • because there is a mismatch in the antenna impedance, 12.5-watts is reflected
  • the 12.5-watts travels back down the transmission line to the transmitter end with a loss of 3 dB and only 6.25-Watts reaches the transmitter
  • a VSWR measuring device (a directional wattmeter) reads incident power 100-Watts and reflected power 6.25-Watts, giving a VSWR of 1.67:1

The outcome when the transmission line had 3 dB loss was a true VSWR at the antenna of 3:1 became a much lower VSWR when measured at the transmitter end of only 1.67:1.

As seen above, any measurement of the real antenna VSWR that is made at the transmitter end of a transmission line will be affected by loss in the transmission line. The greater the loss in the transmission line, the greater the masking of the true VSWR of the antenna as measured at the transmitter end, that is, what is meant by masking the results is the measured VSWR will be lower than the actual VSWR at the antenna end.

The example above using 3 dB loss is perhaps a severe case. Generally in a well designed radio system the loss in the transmission line will be limited to no more than 1 dB at most. The masking effect on VSWR of a 1 dB loss is much smaller.