E238 Information Systems 12 2007 Data Communications: Attenuation

Attenuation is a decrease in the strength of a signal as the distance increases. The result is that the message received is NOT the same strength as the message that was transmitted (sent).

In the real world, this is often just simply attenuation.
In the electronic world, attenuation is the fading of a signal over distance. This may be because the carrying capacity of the medium is inadequate. It may be because of resistance in wires. It may be that the distance is too great.

Example 1: The Garden Hose

When I’m in the garden, watering the plants, and the hose gets caught on a rock (there are many rocks in my garden), I flick the hose. The size of the flick should be large enough to make the hose jump over the obstructing rock. Unfortunately, the amplitude of the wave attenuates. By the time the flick gets to the rock, the wave is now too small to lift the hose over the rock. The advantage of this non-electronic real world example is that the decrease in the amplitude of the wave as it moves along can actually be seen.

Example 2: Microwave Repeater Towers

Microwave repeater towers are visible when you travel across the Nullarbor [Plain}along the Eyre Highway. They have to be “line of sight”, so the curvature of the Earth means that they also have to be tall enough to pick up the incoming signal. They have to be stationed where the attenuated signal is still strong enough to be read completely and correctly. The signal is relayed at a different frequency to the next repeater station. You can see similar repeater towers along the Brand Highway (north of Perth). You can see other repeater stations south of Perth, such as on the Old Bunbury Road, between Pinjarra and the Old Coast Road.

Example 3: Perth Bells

In an interview on the radio in November 2004, William Haynes was being asked “Why do many people in the city [of Perth] not hear the bells from the bell tower?”

Some of his answer is included in the bullet points here:

• The volume next to the bells is 117 dB.
• The volume just 1 m from the bells is 115 dB.
• Volume decreases as distance increases. (They are inversely proportional.)
• This is a well known feature in physics and is referred to as “decay”.
  (In data communications and computing, this is called “attenuation”.)
• If the ambient noise is higher, then you won’t hear the bells.
  • The King’s Park War Memorial, although further away, only has 45-50 dB of ambient noise.
  • If you’re just the other side of the Freeway, the ambient noise is about 90 dB.
    (The Freeway noise level is greater.)
• There are 18 sound proof doors in the belfry, so if you’re in the shop below the bells when they ring, and if those doors are shut, you will not hear the bells, because no sound emanates.

Example 4: Bureaus of Meteorology

The Bureau of Meteorology uses the term attenuation almost twice in each subsection describing about radar interpretation of rainfall. The term is used 66 times in this page.

“Heavy rain directly over the radar site can cause attenuation of all signals. Path attenuation can also occur when the radar beam passes through intense rainfall, with the returned signals from cells further along that path reduced.”

Example 5: The Slender Banksia (Banksia attenuata)

Although it’s called the “slender” Banksia, it would be better if it were called the “tapering” Banksia. The scientific name refers to the leaf which tapers (becomes narrower, decreases in width) towards the stem end, that is, it attenuates.

Example 6: Skipping

If you are one of the turners on the end of the rope when others are skipping, if more people join in the skip, you have to increase the amplitude (height) that the rope turns through. Of course, you could increase the force and therefore the speed that you turn the rope, but this would just make it pepper for everyone!