A Fresnel zone, named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitting antenna and a receiving antenna system.
Okay, that is a mouthful, but what does it actually mean? Let us backup a step and understand first the background and it’s most common applications.
Augustin-Jean Fresnel (10 May 1788 – 14 July 1827) was a French civil engineer and physicist whose research in optics led to the almost unanimous acceptance of the wave theory of light, excluding any remnant of Newton’s corpuscular theory, from the late 1830s until the end of the 19th century.
But he is perhaps better known for inventing the catadioptric (reflective/refractive) Fresnel lens and for pioneering the use of “stepped” lenses to extend the visibility of lighthouses, saving countless lives at sea. The simpler dioptric (purely refractive) stepped lens, first proposed by Count Buffon and independently reinvented by Fresnel, is used in screen magnifiers and in condenser lenses for overhead projectors.
While often discussed in radio communications, the Fresnel zones are seen in many other fields of the physical sciences, including: optics, radio communications, electrodynamics, seismology, acoustics, gravitational radiation, and other situations involving the radiation of waves and multipath propagation.
See my other recent article of how Phased Fresnel optics are used in recent state of the art camera optics for DSLR cameras. Focus of Light
In simple terms, the Fresnel effect is a bubble of energy that extends all around a main beam of source energy, often discussed in microwave and radio communications. Since microwave signals are magnetic fields, they can be polarized and accept physical properties of polarization such as vertical, horizontal, and circular patterns. So, the Fresnel zone is often depicted as being above and below the main beam.
When building microwave links, it is often thought that just line of sight is all that is needed for the link to function, but this is only partially correct. We must have sufficient Fresnel zone clearance for the link to work as well. Any significant blockage of the lower Fresnel zone will obstruct the link even if the main beam has a clear line of sight. I cannot tell you how many times I have been requested to troubleshoot a microwave link that is in a failed condition only to arrive and discover an obstructed Fresnel zone rendering the link inoperative.
Having direct line of sight will not always give you a perfect connection. Even when radio waves are aimed straight from transmitter to the receiver, the problem arises with radio waves that emanates from the transmitter that spread out at an angle from the transmitter. If these signals do not encounter any obstacles, then they will just keep going until they dissipate. However, more often the case is that they will hit a solid surface, which can deflect the signal and upon reaching the receiving end via this indirect pathway, they will be “out of Phase”. An out of phase signal can reduce the power of the main beam arriving signal, also called “Phase Cancelling”. So, the main beam gets negated by the out of sync multipath signals that are delayed in arriving coming from the reflected paths.
So in order to maximize the signal strength at the receiver, you want to minimize any out of phase signals from reaching the receiving end by making sure the strongest signals do not bump into any obstacles. The general rule of thumb is that the 1st Fresnel zone must be 60% clear of obstruction from the centre line of sight to the outer boundary of the 1st Fresnel zone to maintain a good connection.
For longer distance links, let us say 10 kilometres and above, the curvature of the earth comes into play and may become an obstruction into the Fresnel zone and cause signal loss. This is because the longer the distance between the transmitter and receiver, the greater the radius of the Fresnel Zones. Therefore, the height of the transmitter and the receiver antennas becomes an important consideration for long distance links to ensure clearance of the ground level to maximize signal strength.
Toronto’s CN Tower was built primarily as a communications tower. It has numerous antennas on multiple levels within this structure. Most tourists see it as an observation platform and restaurant. And, it is those things too. But its main purpose is to raise antennas high above the ground to permit great distances to be covered radiating out from the city’s downtown core. By using this height, it can overcome the Fresnel zone impacts.
There are many tools, both online, offline, and even for smartphones, that can be used to simply calculate the Fresnel zone for you. The bubble effect of the Fresnel zone varies greatly over distance and is dependent upon the frequency of the signal.
The formula to calculate the Fresnel zone is shown above.
For most folks, the concept of the Fresnel zone is difficult to comprehend since it cannot be experienced with any of the human’s five senses. Therefore, it must be imagined. And, for most people, this is an abstract concept that can be mentally challenging to comprehend.
A physical world example that can be leveraged to imagine the Fresnel zone is the meniscus (plural: menisci, from the Greek for “crescent”) is the curve in the upper surface of a liquid close to the surface of the container or another object. This effect is caused by surface tension and is not exactly the same as the Fresnel effect, but visually it can look similar, so it can be an effective representative example so people can visualize the Fresnel zone. The meniscus can be either concave or convex, depending on the liquid and the surface.
A convex meniscus occurs when the particles in the liquid have a stronger attraction to each other than to the material of the container. Convex menisci occur, for example, between mercury and glass in barometers and thermometers.
Visually, an overflowing glass of beer is often seen with a convex bubble at the top and demonstrates a similar effect to the bubble of the Fresnel zone effect. (see the photo at the top of this article.
In radio communications we do not worry about the Fresnel zone above the radio line of sight as its energy is sent towards the heavens, with nothing to block it or reflex it back towards the receiver. However, the Fresnel zone below the main line of sight is the troublesome energy as it can hit objects like the earth, trees, buildings and other structures and reflect back at the receiver arriving along a much longer path compared to the main bean and thereby generating competing signals that the receiver cannot often adequately reconcile, thus confusing the receiver and causing the radio link to fail.
References:
Digitalair. (2018). Fresnel Zones – What are they and why are they so important? Retrieved on December 24, 2018 from, https://www.digitalairwireless.com/articles/blog/fresnel-zones-what-are-they-and-why-are-they-so-important
Wikipedia. (2018). Augustin-Jean Fresnel. Retrieved on December 24, 2018 from, https://en.wikipedia.org/wiki/Augustin-Jean_Fresnel
Wikipedia. (2018). Meniscus. Retrieved on December 24, 2018 from, https://en.wikipedia.org/wiki/Meniscus_(liquid)
Wikipedia. (2018). The Fresnel Zone. Retrieved on December 24, 2018 from, https://en.wikipedia.org/wiki/Fresnel_zone
About the Author:
Michael Martin has more than 35 years of experience in systems design for broadband networks, optical fibre, wireless and digital communications technologies.
He is a Senior Executive with IBM Canada’s Office of the CTO, Global Services. Over the past 14 years with IBM, he has worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He was previously a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).
Martin currently serves on the Board of Directors for TeraGo Inc (TGO: TSX) and previously served on the Board of Directors for Avante Logixx Inc. (XX: TSX.V).
He serves as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology.
He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) and on the Board of Advisers of five different Colleges in Ontario. For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section.
He holds three master’s degrees, in business (MBA), communication (MA), and education (MEd). As well, he has diplomas and certifications in business, computer programming, internetworking, project management, media, photography, and communication technology.