Why are Cell Towers Placed on Hilltops?

tower on the hill

I’m sure we all notice it, on top of a hill, one would often see a lone solitary telecommunication tower stand amidst the surrounding vegetation and trees. Although not a must, a telecommunication tower is often built on higher elevations. This is due to the mode in which microwave propagate, which is called Line-of-Sight (LOS) propagation. So what is LOS propagation exactly?

Line-of-Sight Propagation

Firstly, we must understand how wave travels. In air or vacuum, wave propagation is direct and travel in a straight line from the source to the receiver. When met with obstructions, lower frequencies are able to pass through them just fine. However, as the frequency of the wave increases, it will become more sensitive to obstructions. These can come in the form of buildings, trees, rain water or any forms of liquid or solid. The electromagnetic (EM) wave spectrum sits just below visible light spectrum. We all know that light cannot pass through obstacles. Similarly, when met with obstructions, EM wave would either be stopped, diffracted or even reflected.

Since the behavior of EM waves is similar to how visible light behaves when met with obstacles, it can be described similarly to how our eyes perceive light. This kind of propagation is known as LOS propagation. Just like how our naked eyes cannot see pass an obstacle, EM waves, especially the higher frequency ones have difficulties getting pass obstacles. Usually this frequency sits around 30MHz and above.

Because of the high signal loss when faced with obstructions, telco towers are usually built on higher elevation to avoid obstacles that comes with the urbanization of our neighborhood and cities. Logically, the closer the towers are to sea level, the higher the chances its signal propagation will be blocked by some tall buildings in its vicinity.

Earth is Flat

Besides, the earth is not exactly flat. Due to the curvature of the earth, the possibility of obstructions on the direct path of wave propagation is also increased. check out the figure below, we can see that due to the earth being round, the height above sea level is relatively speaking, different throughout the whole earth when coming to a LOS propagation.

Source: tutorialspoint.com

Building cell towers on higher elevations would then help to place the path of the LOS propagation well above any possible structures that may block it, increasing coverage area as a result.

Interference. Interference everywhere

Two magnets reacting to one another

We live in the 21st century, where electronic devices have become something of a necessity to our daily life. Everything that is electronic will, to some extend, emit some form of EM radiation. They are mostly harmless to our human body. However as we all know from the simple two polar opposite magnets experiment, EM waves can react to its surrounding, causing distortions and losses. This is what we called as interference, where a signal is affected.

Usually, interference can be reduced by boosting the signal with high power. Think of it like a big muscular guy is less likely to get beaten up by a smaller guy. Similarly, when a signal has high power, it travels further, become less susceptible to losses and more resistant towards interference.

However, another way to address the issue of interference is to build the towers in remote areas where population are less dense. When towers are built on hilltops, they are usually further away from the population center, and thus away from the interference that comes with it. If you think of it, its a sensible option. Instead of spending the extra cost to mitigate interference, might as well just avoid it where you ca.

So there you have it, when you look up to the hill and see some telco towers sitting there, these are the reasons why they were there.

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