Microwaves get through the atmosphere, so they carry the link to a satellite

1. Communication with an artificial satellite is mainly by microwaves, sent up from a dish on the ground and sent back down to a receiver somewhere else.
2. Microwaves pass through the atmosphere and the ionosphere with very little absorption, so enough of the signal survives the trip up and the trip back down to be useful.
   1. The ionosphere is a layer of charged particles high in the atmosphere, and it reflects or absorbs radio waves of lower frequency instead of letting them through.
   2. A radio signal aimed at a satellite can therefore be turned back towards the ground before it ever gets out of the atmosphere, which is no use for a satellite link.
3. Microwaves can also be sent as a narrow beam from a curved dish, so the power travels towards the satellite rather than spreading out in every direction.
   1. A narrow beam lets a modest transmitter deliver a usable signal, and it can be pointed at one satellite without disturbing the next one along.

A geostationary satellite keeps pace with the turning Earth

1. The Earth turns once on its axis in $24hours$, and a geostationary satellite goes once round its orbit in the same $24hours$, travelling the same way as the ground below it.
2. Because the two match, the satellite is always above the same point on the ground and appears to hang fixed in the sky.
3. A receiving dish can then be aimed once, bolted to a wall and left, which is exactly what direct broadcast satellite television needs.
4. Some satellite phones also use geostationary satellites, so the handset always has the same patch of sky to talk to.
5. The orbit that gives a period of $24hours$ lies about $36000km$ above the surface, and from that height one satellite can serve a whole continent at once.
   1. The price of that height is a long path for the signal, which puts a noticeable delay into a telephone call.

A low orbit satellite shortens the path but will not stay put

1. A low orbit satellite circles at a height of roughly $400km$, which is around one hundredth of the height of a geostationary orbit.
2. The microwave signal has a much shorter path to cover, so there is less delay between one person speaking and the other hearing, and this is why some satellite phones use low orbit satellites.
3. The trade is that a satellite this low takes only about $90minutes$ to go right round, so it crosses the sky and sinks below the horizon within a few minutes.
4. One low orbit satellite cannot give continuous coverage, so a network of many satellites is used and a call is passed from one to the next as each rises into view.
   1. Because the satellite moves, the handset uses a short aerial that picks up microwaves arriving from any direction rather than a dish aimed at one spot.

Mobile phones and wireless internet use microwaves because a short aerial is enough

1. Mobile phones and wireless internet both send and receive microwaves.
2. Microwaves can penetrate some walls, so a phone still works indoors and a router in one room can serve a laptop in the next.
3. The length of an aerial is related to the wavelength of the wave it handles, working best when it is a simple fraction of that wavelength.
4. The microwaves a phone uses have a wavelength of a few centimetres, so an aerial a few centimetres long does the job and it fits inside the case.
   1. Radio waves of lower frequency have wavelengths of metres or more, and an aerial matched to one of those would never fit in your pocket.

Bluetooth reaches only a few metres because its waves start off weak

1. Bluetooth uses low energy radio waves or microwaves to link nearby devices such as a phone, a speaker and a pair of headphones.
2. These waves pass through walls, doors and furniture, so the two devices do not have to be in sight of each other.
3. Each wall absorbs some of the energy of the wave, so the signal arriving on the far side is weakened.
4. The transmitter is deliberately low energy so that it drains very little battery, which means only a small amount of weakening leaves the receiver unable to make out the signal.
   1. That is why the range is short, typically about $10m$ across an open room and less than that through a wall.

An optical fibre carries far more data than a wireless link and loses little of it

1. Cable television and high-speed broadband send visible light or infrared along optical fibres.
2. Glass is transparent to visible light and to some infrared, so the wave can travel through kilometres of fibre with only a little of its energy absorbed.
3. The light stays inside the fibre because it is totally internally reflected each time it meets the boundary at the edge of the core, so it follows the cable round bends and along its whole length.
4. Visible light has a very high frequency, around $5\times 10^{14}Hz$, which is thousands of times higher than the microwave frequency a wireless link uses.
5. A wave of higher frequency can be varied many more times each second, so one fibre carries a much higher rate of data than a wireless connection or a copper cable of the same thickness.
   1. Cable television needs that rate because it carries many channels at the same time, and broadband needs it to move large files in seconds.
6. The small signal loss over long distances matters just as much, because a fibre may run across a city, under a road or along the sea bed and the light has to arrive at the far end strong enough to be picked up.