VSWRThis 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.

Any questions or comments about Return Loss and VSWR?

About the Author

Derren Oliver

Derren Oliver is director of business development for the Microwave Systems unit at CommScope. He has nearly 20 years of experience in the telecoms industry, beginning in test and measurement for both wireless and wireline networking products at Hewlett Packard, Agilent Technologies and Ixia before joining CommScope in 2010. He holds a bachelor degree with first class honors in electrical and electronic engineering from the United Kingdom’s Heriot-Watt University and an MBA from Edinburgh Business School.

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6 comments for "Back to Basics in Microwave Systems: Return Loss and VSWR"
ravi kumar Tuesday, June 13, 2017 12:47 AM

All information explained by you is best for gaining knowledge about return loss. Can you please explain .......
What is the physical significance of return loss?

Donald Gardner Wednesday, June 21, 2017 9:26 AM

Hi Ravi, Return loss is an electrical parameter and is a result of mismatches between components which result in small reflections of signal back along the signal path. For more information please see our Microwave Communication Basics eBook, downloadable from commscope.com. Hope this helps. Best regards Donald

imran hussain Sunday, December 17, 2017 8:18 PM

Hi, I hope someone can help me, and I would appreciate an email response even if it is 1 paragraph. I have had some issues with my mobile handset data speeds in my home area, and after 11 months of dispute with my provider in the UK it is been reported that there was a fault with the Voltage Standing Wave Ratio (VSWR) either at the mast or antenna. Once that was fixed and set right my data speeds improved, call dropping stopped. The provider has stated that although this fault was found and fixed it would not have caused the dreadful low speeds i was getting. I dispute this, as my 4G speeds were 3mbps download and 0.2mbps upload. Now after the fault was fixed i am now getting 20mbps download and 8mbps upload. Please can someone tell me how the VSWR could have affected the 4G data speeds please I would appreciate it and would be willing to pay to get the answer. The provider is not willing to tell me if they changed the VSWR frequency or something else. My email is imran.hussain11@outlook.com I would really appreciate some help here.

Public Relations Monday, December 18, 2017 11:18 AM

Hi Imran, we recommend you post this question using the Customer/Technical Support section on our website. http://www.commscope.com/SupportCenter/.

Dibyajyoti Mukherjee Sunday, January 14, 2018 1:57 AM

Hello sir,
I am seeking for some help regarding my PG subject. Can you explain me what is a Waveguide Resonator?

Derren Oliver Thursday, January 18, 2018 5:52 AM

Dear Dibyajyoti Mukherjee,

There is a wealth of information on the internet to describe basic devices such as waveguide resonators. See for example the Wikipedia article here: https://en.wikipedia.org/wiki/Resonator and specifically for waveguide here: https://en.wikipedia.org/wiki/Waveguide

We use waveguide with our antennas principally to direct the RF signal to and from the Outdoor Microwave Radio unit (ODU) and the antenna for transmission.


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