Effect of Changing Length of Transmission Line on VSWR
Posted: Mon Jun 09, 2025 9:25 am
A common question among boaters or others not particularly familiar with transmission lines, antennas, and VSWR is:
Q1: does changing the length of a transmission line affect the VSWR on the transmission line?
The short answer is no, the length of a transmission line does not affect the VSWR.
A more thorough answer will explain further:
The VSWR or voltage standing wave ratio on a transmission line is a metric that determines how well the characteristic impedance of the transmission line and the antenna input impedance are matched. When the length of a transmission line is changed, neither the characteristic impedance of the transmission line nor the antenna input impedance change, so the VSWR does not change.
With that said, however, the measurement of VSWR is seldom taken directly at the antenna input, and is more commonly taken at the transmitter end of the transmission line. All measurements of VSWR taken at the transmitter end of the transmission line are affected by any loss in the transmission line. Loss in a transmission line is directly related to the length of the line.
The effect of transmission line length and thus loss on VSWR can be explained with a simple example. Let us look at the case of a transmission line whose loss as a result of its length causes only half of the input power to be delivered to the antenna. Let us further suppose that the antenna is completely absent (open circuit) or a dead-short (0-Ohms impedance). In those cases all the power reaching the antenna will be reflected back toward the transmitter. That reflected power travels through the lossy transmission line, and again half the power is lost. If the VSWR is measured by a directional wattmeter at the transmitter, and we assume the initial power was 100-Watts, only 50-Watts reaches the antenna due to transmission line loss. Those 50-Watts are reflected back toward the transmitter, and also experience the loss of half their power in the transmission line, so only 25-Watts arrives at the transmitter. The direction wattmeter would then show forward-power = 100, reflected=power = 25, producing a VSWR of 3:1. The actual VSWR at the transmitter is Infinity:1. So in the case of measurement of VSWR at the transmitter end of a transmission line, we could say that line length affects the measured VSWR, but the actual VSWR at the antenna remains unchanged.
Q1: does changing the length of a transmission line affect the VSWR on the transmission line?
The short answer is no, the length of a transmission line does not affect the VSWR.
A more thorough answer will explain further:
The VSWR or voltage standing wave ratio on a transmission line is a metric that determines how well the characteristic impedance of the transmission line and the antenna input impedance are matched. When the length of a transmission line is changed, neither the characteristic impedance of the transmission line nor the antenna input impedance change, so the VSWR does not change.
With that said, however, the measurement of VSWR is seldom taken directly at the antenna input, and is more commonly taken at the transmitter end of the transmission line. All measurements of VSWR taken at the transmitter end of the transmission line are affected by any loss in the transmission line. Loss in a transmission line is directly related to the length of the line.
The effect of transmission line length and thus loss on VSWR can be explained with a simple example. Let us look at the case of a transmission line whose loss as a result of its length causes only half of the input power to be delivered to the antenna. Let us further suppose that the antenna is completely absent (open circuit) or a dead-short (0-Ohms impedance). In those cases all the power reaching the antenna will be reflected back toward the transmitter. That reflected power travels through the lossy transmission line, and again half the power is lost. If the VSWR is measured by a directional wattmeter at the transmitter, and we assume the initial power was 100-Watts, only 50-Watts reaches the antenna due to transmission line loss. Those 50-Watts are reflected back toward the transmitter, and also experience the loss of half their power in the transmission line, so only 25-Watts arrives at the transmitter. The direction wattmeter would then show forward-power = 100, reflected=power = 25, producing a VSWR of 3:1. The actual VSWR at the transmitter is Infinity:1. So in the case of measurement of VSWR at the transmitter end of a transmission line, we could say that line length affects the measured VSWR, but the actual VSWR at the antenna remains unchanged.