mobile phones

[Carol Anne]

In as untechnical form as possible, could someone please explain how they work . ie: when I turn my handset on, exactly what happens for the phone to become connected? Does it send a powerful signal to the nearest mast or does it hook onto a signal that is already present in the room? How can a tiny phone with a small battery be powerful enough to transmit to a mast that could be several miles away? ( my little grandson is interested)

[Buenchico]

(2-part post):

If a transmitter is efficiently designed (and uses an appropriate frequency to pass signals over the required distance), you don't actually need much power to transmit over quite large distances. When Marconi first transmitted radio signals across the Atlantic he was just making a big spark (not tuned to any precise frequency). Very shortly afterwards, radio amateurs were routinely transmitting signals from one side of the world to the other with a typical output power of around 100 watts (i.e. the same power as many light bulbs). Other radio amateurs, who liked to show just how efficient their transmitter and aerial systems were, could get a signal across the Atlantic using less than 1 watt.

Mobile phones can transmit at up to around 5 watts (although, to save battery life, they only use this power when they really have to).

The mobile phone system depends upon the network knowing where your phone is. (When someone calls you, the call is only routed to transmitters in your area. If it was sent to every transmitter in the UK, or across the world, the system would rapidly become overloaded).

[Buenchico]

When you first turn your phone on, and periodically thereafter, it sends out a lower power signal which says "I'm here". (If you've ever left your phone near to a radio, you'll have heard this signal). This is usually picked up by one or more local transmitters, which then send a message back saying "Understood. Thanks". If your phone doesn't get an acknowledgement, it re-sends the signal, with a slightly higher power. It keeps doing this until reaching the maximum (around 5 watts) power. If it still doesn't get a signal, it will keep periodically sending a high-power signal to try to connect.

Actually, that last paragraph doesn't tell the whole story. Each transmitter also sends out a continuous 'reference' signal. That's the one which is picked up by your phone and provides the signal strength indication on your phone's display. If the signal strength is zero, the phone won't bother to try a connection. (At least it shouldn't. If you're in a jet, 7 miles up over the Atlantic, your phone should recognise that there's no reception and not try to transmit. The risk is that, if it does, it will send out 5 watt signal which can affect the aircraft's electronics. That's why it's important to keep your phone switched off when onboard an aircraft).

Once your phone has made contact with a local transmitter, it's simply a matter of signals being passed along a chain of transmitters (sometimes linked via satellites or landlines), to connect the caller to the recipient.

Chris

[Carol Anne]

Thanks Chris , that was quite interesting actually. Do transmitters work by sending massive voltages through cables up to the aerials to make the signals radiate further?
(lad is only 7 and facinated by anything electrical)

[Buenchico]

Thanks for your reply. I've spent most of the day trying to work out how to explain radio transmissions to a 7-year-old and, despite the fact that I've taught physics and general science (to 11-year-olds and upwards), I've decided that it just isn't possible :-(

To answer the young lads question about 'massive voltages' means trying to explain that 'voltage' isn't the same as power. A car battery only delivers about 12V but there's enough power to turn over a hefty motor (and it's definitely not a good idea to connect yourself to a car battery - it hurts!).

However, the sparks, that some people feel when they take off a nylon shirt, have a' voltage' of many thousands of volts but they carry very little power.

Mobile phone transmitters are fed by high voltage cables but the actual signals are of quite low power. To give your grandson an idea of how little power is needed to transmit radio signals, you might want to tell him that satellites (like the ones which Sky uses) only have as much power as that used by a small fan heater. Despite this, they can still send a signal to millions of satellite dishes. Then ask him how high up he thinks those satellites are. He'll probably guess at something like "1 mile?" or "10 miles?". Most 7-year-olds will be suitably impressed when grandma tells them that the satellite which provides a signal to millions of homes, using no more power than a fan heater, is 22,240 miles above the earth ;-)

Chris

[Carol Anne]

Hello chris thanks i liked your answers and i want to work on transmitters later on. bye from David

[Carol Anne]

He's been in bed all day with a bad tummy and now can't sleep! You made his day.

Carol Anne

[Buenchico]

Thanks for the replies, Carol.

For David: Thank you for your message. I hope that your tummy is better. I asked a little bird to look in the post box for a Get Well card for you but he couldn't find one. Luckily, a Golden Labrador managed to find it:
http://www.jacquielawson.com/viewcard.asp?code =1230467151952&source=jl999

Best wishes from Chris