Another factor affecting a “perfect” link is line-of-sight. When radio waves encounter an obstacle (such as a tree, a building, or even a hill), some of the signal energy is lost trying to penetrate the object. Some will bounce, scattering seemingly at random. Worse, the effects of bounce may vary depending upon weather (for example, wet leaves absorb more signal than dry leaves). Thus, if it all possible, it is desirable to have a clear line-of-sight between your transmitter and receiver. (Where it isn't possible, using lower frequencies might get you through – and it might not. It's a good idea to do whatever is needed to get line-of-sight, even if it means building a taller pole!). An easy way to judge line-of-sight is to stand at the location of the transmitter, and look towards the receiver. If you can see it – you have line-of-sight.
Line-of-sight by itself is not enough! Radio waves do not leave the transmitter in a perfectly narrow beam, and are not received from a single tiny point. Rather, radio waves are emitted in a cone, and received across the surface of the antenna. The angle of the cone varies by transmitter and antenna. For example, the PowerBeam boasts a 2-4 degree emission cone, as do the large Rocket Dishes (in practice, the Rocket Dishes have the smaller cones; a good rule of thumb is that the larger the dish, the more focused the beam can be). NanoStations cover a 60-degree cone. Sector antennas, by their very nature, can cover a 90-degree or 120-degree cone. Omnidirectional antennas are a special case, in that they are designed to beam in all directions at once: they don't really have a cone at all, more like a three-dimensional doughnut!
Now, consider that the radio on the other end of the link is also a transmitter, and both are attempting to receive the other end's signal with the same antenna. The result is two cones facing one another. We can ignore the portions that will completely miss the other end-point, and instead focus on the areas of useful signal. This results in a radio-signal pattern in the air that strongly resembles an airship:
This airship shape is called the “Fresnel zone” (pronounced “Frey-nell”, it's named after the Frenchman who discovered it). There are in fact multiple Fresnel zones surrounding any given link, referred to as the first, second, third, etc. Fresnel zones. You can calculate the diameter of a Fresnel zone for a given link as follows:
The rule-of-thumb for wireless links is that sixty percent of the first Fresnel zone must be clear of obstructions if a great link is to be established. Therefore, for a ten kilometer link at 5.8 ghz, we can calculate:
Hence, we require that the mid-point be at least 6.8 meters above any obstructions if we are to receive great service on this link.
The Fresnel formula also provides some interesting insight into frequencies, and their usefulness over long distances. If we plot the size of Fresnel zones for each of the major frequency bands, over a distance ranging from one to fifty kilometers, we get the following:
This graph shows an interesting property of the Fresnel-zone: the higher the frequency you choose, the smaller the Fresnel-zone clearance has to be. 900 Mhz at 50 kilometers has a 64.5 meter radius of the first Fresnel-zone (requiring 38.7m of clearance for 60%), while 5.8 Ghz at the same range only has a 25 meter zone (requiring 15.2m of clearance for 60%). This is important to remember: while the lower frequencies carry further, they also require significantly taller towers to pass unobstructed across terrain.
The good news is that very few people calculate Fresnel zones by hand. Like most of the parts of this section, there are calculators online to assist you. Radio Mobile, covered in detail later in this book, also provides excellent assistance in determining if your links have sufficient Fresnel clearance.
It is worth noting that 900 Mhz Fresnel zones are enormous. It is very difficult to raise an antenna sufficiently to provide a clear Fresnel zone for a link in this frequency. The good news is that 900 Mhz is also the most tolerant of Fresnel-incursions; in fact, it is typically used because of this property, where other links won’t function.
Note: The exact extent to which you need Fresnel-zone clearance is a topic of some debate amongst wireless operators. The 60% clearance rule-of-thumb is a solid one to aim for, but sometimes links surprise us all by working with very little clearance – and sometimes the opposite is true. Theory helps, but occasionally Mother Nature still surprises even the smartest of us.« Chapter 4: Trees and Other Obstacles Up To Contents Chapter 4: Noise Floor and Interference »
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