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Typical microwave links tend to be 25 miles or less in length, but sometimes they need to be longer – much longer. For example, inter-island hops in the Caribbean and elsewhere need longer links, as do situations like oil pipelines where builders are trying to build a network with as few hops as possible. Oil rigs are another example, where a rig out in the Gulf of Mexico needs to communicate with the shore. So how does a microwave link become an ultra-long link? In this article, we will look at the considerations for building ultra-long links.
Link distance is based on transmit power, modulation, receive sensitivity and path parameters. IP traffic is more bandwidth intensive so it drives designers to use higher modulations (e.g., 16, 64, 256 QAM), which affects path length. The higher the modulation, the shorter the path length and the lower the reliability. IP traffic is a little more forgiving than TDM because it retransmits lost packets, so sometimes designers of IP links can live with lower path availability, or do adaptive coding and modulation, which allows the radios to step between modulation levels.
Now, let’s look at some considerations and techniques for designing ultra-long microwave links.
The first consideration is site selection. A long link has to overcome the curvature of the earth, so this dictates fairly high sites such as mountaintops or tall buildings. Some of the longest links have antennas that are 1,000 feet above sea level.
Frequency selection is another factor. The advantage of a higher frequency is that you can use a smaller antenna and still get the same antenna gain, but the tradeoff is that higher frequencies attenuate more quickly – the signal doesn’t propagate as far. Higher frequencies are good for dense urban areas where you need a lot of bandwidth but you don’t need a lot of distance; the lower frequencies are better for when you need a lot of distance. Most ultra-long links use 6 GHz frequencies in the United States and 7 GHz or 8 GHz frequencies in the rest of the world.
Antenna size is another consideration. The intuitive thought is that bigger is better when it comes to antennas, but this isn’t necessarily true. Up to about 10 feet, the bigger the antenna the more gain and the farther the path will go or the more path reliability you’ll have, but the bigger the antenna the smaller the beam width. With large antennas, it will be hard to align the beam and to keep it aligned. When you get up to a 12-foot antenna, it’s like trying to line up a laser beam. Larger antennas also require more robust tower designs, so instead of a 12-foot antenna that acts as a sail in the wind, designers might choose an 8-foot antenna instead. The tower has to be designed to maintain the antenna on path so you need a robust tower that doesn’t sway itself.
Microwave links are also sensitive to variations in atmospheric conditions that can cause the microwave path to bend. Air temperature and density are the two factors – you get an effect where the lower layers of the atmosphere are denser and the upper layers are thinner, and this tends to make the microwave path sag like a clothesline. This is called the “K factor,” and it can change from daytime to nighttime, because there’s a more pronounced temperature gradient in the daytime and a more homogeneous environment at night when the air cools. And as the K factor changes from day to night the microwave beam can defocus and you can lose the signal. There needs to be a large enough beam width to deal with changing conditions due to the K factor. To overcome the K factor, designers place the antennas higher to get above the lower layers of the atmosphere.
Along with high antenna placement, space diversity is another technique designers use to create links in excess of 25 miles. Space diversity is the idea of using two antennas instead of one to receive a signal. You would have two antennas on the tower, usually separated by about 30 feet vertically for 6 GHz systems, and you would typically transmit on the top antenna and have a receiver on both antennas. When you get signal fade on one antenna, the signals are decoupled enough with the vertical spacing that the other antenna still has a good signal. For example, instead of using a 12-foot antenna on a longer path, you could use two eight-foot antennas to create space diversity.
Another challenge to overcome is multi-path propagation delay. Multi-path propagation delay refers to the microwave signal taking different paths. The main signal goes directly between antennas, but you can have minor signals taking longer paths and they have more delay between antennas. There can be reflective layers in the atmosphere that can actually cause a path to black out. There can also be reflections from the ground – for example, when farmers flood their fields those fields turn into mirrors, causing interference and really long delays. The solution to multi-path propagation delay is to use space diversity and positioning the antenna higher on the tower to get away from the reflection.
Planning software gives you the antenna azimuth and tilt angle, what signal strength to expect, and what the path availability is. It helps designers understand whether they need to use ACM or not to step down modulation levels to preserve link reliability. It also identifies the terrain roughness and any blockages. Smooth terrain has more chances for reflection, so rough terrain increases path reliability. Path loss is one commonly used brand of path planning software.
When deploying the link, antenna alignment is crucial. Technicians start by aiming by compass and level to get a rough alignment based on the original path design, then they tweak the antenna back and forth and up and down until they can maximize the receive signal. The initial path calculations indicate what the receive signal should be, so technicians tweak antenna alignment until they are close to the optimum signal strength.
Designing ultra-long microwave links is a very specialized skill that requires a thorough understanding of the considerations discussed above. It’s a matter of having the right conditions and antenna sites, choosing the right antennas and sizing them properly, and employing space diversity to overcome multi-path propagation. Using these techniques, it’s possible to build links that run for 120 miles or more.