Fresnel Zone – Microwave Planning

In radio communications, a Fresnel zone (/freɪˈnɛl/ fray-nel), , is one of a (theoretically infinite) number of concentric ellipsoids which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture. The cross section of the first (innermost) Fresnel zone is circular. Subsequent Fresnel zones are annular (doughnut-shaped) in cross section, and concentric with the first.    The Fresnel Zone is named after the physicist Augustin-Jean Fresnel.

Importance of Fresnel zones

CableFree Microwave and Radio Fresnel Zone
Microwave and Radio Fresnel Zone

If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are reflective surfaces along the path, such as bodies of water or smooth terrain, the radio waves reflecting off those surfaces may arrive either out of phase or in phase with the signals that travel directly to the receiver. Waves that reflect off of surfaces within an even Fresnel zone are out of phase with the direct-path wave and reduce the power of the received signal. Waves that reflect off of surfaces within an odd Fresnel zone are in phase with the direct-path wave and can enhance the power of the received signal. Sometimes this results in the counter-intuitive finding that reducing the height of an antenna increases the signal-to-noise ratio.

Fresnel provided a means to calculate where the zones are–where a given obstacle will cause mostly in phase or mostly out of phase reflections between the transmitter and the receiver. Obstacles in the first Fresnel zone will create signals with a path-length phase shift of 0 to 180 degrees, in the second zone they will be 180 to 360 degrees out of phase, and so on. Even numbered zones have the maximum phase cancelling effect and odd numbered zones may actually add to the signal power.

To maximize receiver strength, one needs to minimize the effect of obstruction loss by removing obstacles from the radio frequency line of sight (RF LOS). The strongest signals are on the direct line between transmitter and receiver and always lie in the first Fresnel zone.

Determining Fresnel zone clearance

Microwave and Radio Fresnel Zone
Microwave and Radio Fresnel Zone

The concept of Fresnel zone clearance may be used to analyse interference by obstacles near the path of a radio beam. The first zone must be kept largely free from obstructions to avoid interfering with the radio reception. However, some obstruction of the Fresnel zones can often be tolerated. As a rule of thumb the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less.

For establishing Fresnel zones, first determine the RF Line of Sight (RF LOS), which in simple terms is a straight line between the transmitting and receiving antennas. Now the zone surrounding the RF Line of Sight is said to be the Fresnel zone.

Unlicensed Wireless Links

What is an Unlicensed Microwave link?

Unlicensed and light licence wireless links is the most cost effective of all links and can be deployed in a matter of days. Currently in most countries there are a few unlicensed ISM-band frequencies that are used for point to point links and a few light licensed frequencies that provide interference free operation.

What is a Light Licensed microwave link?

Regional regulators (typically, in each country) are responsible for Spectrum Management of the Radio Spectrum.  This naturally varies in each country due to different history of usage and allocation.

A Light License is where the licensee pays a small licence fee to register his/her radio link with regional regulators such as OFCOM  (UK).

The regulator (such as OFCOM in the UK) use the licence to inform other potential users of the spectrum that there is already a radio link or links in the area when they register their own link prior to deployment. This information is also used to resolve disputes should interference arise.

Frequencies used

Depending on which country you are in, these can include:

  • Licence free spectrum are the 5Ghz, 24Ghz, and 60GHz frequencies
  • Light licence spectrum operate in the 64-66GHz and 70/80GHz

Why consider unlicensed or light license links?

  • Low density areas not suffering from RF interference
  • Budget constraints
  • Non-critical data transmission

When are licensed links mostly used?

  • Organisations looking to create a LAN across multiple buildings on the same site
  • Organisations looking to reduce the cost of existing leased lines
  • In low density areas where RF interference is low or free

When to consider opting for a licenced over unlicensed?

  • High density areas suffering from RF interference
  • Mission-critical data transmission

Is unlicensed or light licenced microwave right for you?

If you are looking for the simple answer, please contact Wireless Excellence for details. Our very experienced team are happy to discuss your requirements and advise on the best solution whatever your needs.