how does a cell phone get it right to transmit over 7km

[jacquesl]

Can anyone tell me how does a cell phone get it right to transmit over 7km to a tower and you guys know how small a cell phone is.

How can I build a transmitter so small to also transmitter over 7km +

For data link purpose

[XEAGLEKEEPER]

You wouldn't need much power.Cell phones are Line of sight and require repeater stations(towers) and probably only transmit a carrier of 100 mw or less in the UHF freq.band.

[Nigel Goodwin]

UK cellphones are normally about 6W?, they don't use UHF, they use much higher frequencies - the power output though is switched according to the signal strength from the nearest mast, so would rarely approach the full 6W.

As suggested, cellphones are 'line of sight', so could go a considerable way with fairly low power, however the high frequencies used tend to limit it - which is the whole point of cells.

Using a handheld 2M amateur radio transceiver (1W out) I used to easily get 70 or 80 miles - from the top of a hill! - in the bottom of a valley you get coverage of the valley, and that's about it!.

[jacquesl]

I’ve got a 500mW 446Mhz Oregon Two-ways and it only gives out 5km = 3.106856 Mile and it’s heavy on the battery’s

But a cell phone you can talk hours long.

So what’s the secret behind that, and how can I build my own long range line of sight transmitter that uses 100mA to transmit up to Miles and Miles long just like a cell phone does, my main purpose of this device is to build a data link

[Nigel Goodwin]

Originally posted by jacquesl
I’ve got a 500mW 446Mhz Oregon Two-ways and it only gives out 5km = 3.106856 Mile and it’s heavy on the battery’s

But a cell phone you can talk hours long.

So what’s the secret behind that, and how can I build my own long range line of sight transmitter that uses 100mA to transmit up to Miles and Miles long just like a cell phone does, my main purpose of this device is to build a data link

I explained above - a cellphone uses very low power because it's mostly only transmitting to a nearby mast, and the handwset adjusts it's power output to the minimum required (to extend battery life, and to reduce the possible damage to your brain).

Also, the batteries are MUCH, MUCH better than the ones you're using in your Two-ways - which are also pretty poor things, not like a 'proper' transceiver at all.

[jacquesl]

Do you have a circuit diagram on how to build a device like this?

[ingrast]

Cell phones put the burden of performance on the base station side, sensitive receivers and above all, very high gain antenna arrays with optimized beam coverage.

An answer to your requirements must first start with with knowledge of things like if you want unrestricted portability or you are only interested in a fixed point to point link etc.

Rodolfo

PS And don't even dream of building an amplifier without a fair amount of experience and RF test equipment.

[jacquesl]

It really does matter, well let’s start of by point to point, to make things easier

[Nigel Goodwin]

RF design is complicated, and layout is critical - particularly at the high frequencies you would need to use. Presumably you want a decent data rate?, a normal radio channel probably manages 1200 baud or 2400 baud?. If low rates are all you need?, you might try googling for "packet radio" - radio hams use a device called a TNC (Terminal Node Controller) which is essentially a modem between a computer and a transceiver.

Another problem is the legality of it - you will almost certainly require a licence, which won't be available for what you want to do!.

[jacquesl]

Yea, packet radio will probably be the best option for me now, but I want some scary fast speed over long distances I already checked out the packet radio, but it didn’t tickled me so much, but I’m going to do a monster search on Google now.

The only thing is, how the pc manages it to switch PTT button automatically then you want to TX stuff