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VSWR help

VSWR Myths

VSWR calculator

Overview

This calculator is designed to model the common configuration where a transmitter which is designed for a particular load impedance, is connected to a an antenna of that impedance, over transmission line of the same characteristic impedance.

The calculator allows specification of the transmission line loss 'upstream' and 'downstream' from the measurement point. It is intended to provide a simple analysis of the system at the transmitter and at the antenna where a directional wattmeter is used to take measurements in the system at some convenient intermediate point.

Input values / formats

Limits on range of valid data are shown adjacent to the input field label, ** means a large number.

The input fields may support flexible input format. In general, the formats supported include traditional floating point number (50.00), scientific notation (5.05E1), in combination with qualifiers decibels (-3dB), percentage (50%), per unit (0.5pu).

On the Line Loss field, 3 == 3.0E0 == 50% == 0.5pu. Similarly, you could enter 14dBm (-16dBW or 25 mW) in the Forward Power field as -16dB.

Not all fields support all formats.

Model variables

Power into nominal load

This is the power that the transmitter delivers into a nominal load (ie the same impedance as the characteristic impedance of the transmission line).

Forward Power

Forward Power must be measured with a directional wattmeter, the Power function found in many low end VSWR meters is just an RF voltmeter measuring the main line. You can transfer the directional sensor from an RF voltmeter when the unit is feeding a dummy load. Note though there is scope for accumulating considerable error in this process.

VSWR meters with adjustable sensitivity have a potential problem with scale linearity, they tend to be calibrated (ie the scale shape) for low sensitivity, and when readings are made at high sensitivity, the low readings will be underestimated. Net effect is they tend to indicate too low a VSWR when used in the high sensitivity positions.

Reflected Power

See the comments for Forward Power, they are even more applicable to Reflected Power.

Return Loss

The Return Loss is the ratio of the reflected power of the reflected wave to the power of the forward wave, usually converted to and expressed in dB.

Voltage Reflection Coefficient

The Voltage Reflection Coefficient is the ratio of the voltage of the reflected wave to the voltage of the forward wave, usually expressed as a per unit value (PU) from 0 to 1.

Power reflection coefficient

The Power Reflection Coefficient is the ratio of the power of the reflected wave to the power of the forward wave, usually expressed as a per unit value (PU) from 0 to 1.

Transmission efficiency

Transmission efficiency is the ratio (percentage) of power absorbed by the antenna to the power absorbed by the line from the transmitter.

Effectiveness

Effectiveness is the ratio (percentage) of the available transmitter power (into a nominal load) that is absorbed by the antenna. It takes into account the loss of transmitter output caused by activation of transmitter VSWR protection.

Loss due to VSWR

Loss due to VSWR is the additional transmission loss attributable to the VSWR alone. See Additional loss due to VSWR for discussion on this subject.

Mismatch Loss

The power available in a matched load from a matched source compared to the power available in the mismatched load. See Power in a mismatched transmission line  for discussion on this subject.


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