In this post, we will assess this subject from radio signal propagation point of view, not the matters related to installation, site access etc.
Why propagation is important? It is signal propagation in a specific environment that plays the key role in the overall reliability of microwave link, especially if we are talking about those high capacity long-haul links. Of course, there are plenty of factors that affect reliability, but let’s focus on geographic location and environmental impact.
Being in the market for over than 15 years SAF links are deployed virtually everywhere. For many links scattered around the globe, SAF engineers had the opportunity to perform feasibility studies and provide recommendations that were followed by our customers. Over time, we gained experience based on the variety of links and their locations, some of them one may call extreme. We are hereby summarizing the conclusions about best and worst geographical properties for a microwave link deployment.
The key to a reliable link is ensuring the signal will be received undisturbed all year long. As daily and seasonal cycles change, so does temperature, humidity, and pressure. These changes affect refractivity of the atmosphere and how radio signal propagates through it. In case refractivity is such that radio signal is abnormally refracted downwards, then multipath fading and ducting phenomena’s might occur. These effects will cause distortions and potential service outages.
Rough terrain, mountainous regions, high altitude, relatively dry, windy and low annual temperatures – these are the conditions that will serve best for radio signal transmission and minimize multipath probability. Install one site on a mountaintop, the other in a valley and you’re good to extend a single link distance up to 200km while providing a reliability grade that fits service uptime requirements of today’s customer. It is worth noting that elevation difference between sites also provides a considerable improvement. One of the most common areas of interest – cities and other urban areas typically represent favorable conditions for microwave propagation due to decreased ground reflection probability. But there is a major challenge - it is necessary to ensure line of sight between the sites in question. You may have all the positives described above, but obstructions such as vegetation, buildings or Earth curvature can easily become a limiting factor.
Plains, deserts and otherwise flat terrain. Hot, humid and low elevation. These properties are a recipe for high multipath probability and can upset your distance expectations. To achieve the desired uptime, link distance will have to be significantly decreased. Rainfall intensity in the region of interest is a particularly major concern for high-frequency links (above 10GHz). And yes, over water links. Water, being highly reflective, can be the cause for excessive multipath fading and reflections, therefore, decrease the reliability.
Often SAF link planners are faced with geographically challenging links, but even the most unfavorable locations can be used for successful SAF microwave link deployment and reliable operation - it is a matter using the best design and technique for the specific case. Here is how our client E.ON Climate & Renewables deployed a reliable SAF radio link in a harsh environment in the Baltic Sea.
Read more on how antenna heights affect link performance here.