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This final blog post in our “Back to Basics” series addresses the Return Loss and Voltage Standing Wave Ratio (VSWR) characteristics of antennas. These metrics are important measures relating to the performance of an antenna system. Let me explain:
An antenna’s Return Loss is a figure that indicates the proportion of radio waves arriving at the antenna input that are rejected as a ratio against those that are accepted. It is specified in decibels (dB) relative to a short circuit (100 percent rejection).
Consider the antenna being used in transmit mode. The radio waves from the transmitter are routed via a transmission line to the input flanges of the antenna feed. At all waveguide junctions there is a mechanical mismatch, the size of which will determine the size of the consequential electrical mismatch.
Thus, a proportion of the radio waves will be reflected back down the transmission line from the antenna input flange connection. The remainder will continue into the feed system. However any imperfections within the feed system will again cause small amounts of the incident radio waves to be reflected back again towards the input flange.
Finally, the radio waves will emerge from the feed aperture to be radiated onto the parabolic reflector, prior to being directed into the atmosphere towards their intended target. Again, a small percentage of the radio waves will be reflected back from the reflector into the feed system and back towards the input flange.
The sum of all the reflected components at the input flange represents the total reflected signal. Return Loss is significant to radio system designers for a number of different reasons:
- If a large proportion of the incident radio waves are rejected by the antenna, this represents a loss of signal and the antenna efficiency is therefore reduced.
- In a transmitting system where, for example, the antenna is remote from the radio equipment, rejected radio waves returned from the antenna will travel back down the transmission line to the radio. From there, they will be reflected and returned back up the transmission line to the antenna. A percentage of this returned signal will be radiated and, once again, a proportion routed back to the radio. This secondary radiated component will be a delayed version of the primary signal - the delay being set by the time taken for the signal to travel back down the transmission line to the radio equipment and back again.
- Secondary radiation will look like an echo signal on the main signal, and this can cause errors in the detection of the desired information at the remote receiver. If the reflected radio waves back into the transmitter are of sufficient magnitude, the transmitter performance can be severely degraded.
Voltage Standing Wave Ratio (VSWR) is basically an alternative representation of Return Loss. VSWR is a reference to the actual voltages that are created within a transmission line system when there are forward and reflected radio waves propagating simultaneously. There are equations that relate Return Loss and VSWR, which are presented and explained in the Microwave Radio Antenna Link Fundamentals online course that is available through the CommScope Infrastructure Academy. The Microwave Fundamentals online course content is the source material for this “Back to Basics” blog series.