Long-range Data Transmission

Hey all.

I've done some searching for the last few months and am looking for advice. I have a little personal project I'm trying to throw together, but i'm entering unknown territory. I'm looking to transmit (+4km) 15-20 charactors to a transceiver and return similarly sized data back. Each character is 8 bits. What would be the "best" way to do this? I've looked into everything from VHF to 9GHz freq, AM and DTMF coding. I'm at a loss. Its going to be on a person and must not require a license. Heck, I've even played with Citizen Band (CB) radios with DTMF. I need long range, low power use, and moderately fast data rates (1-10Kbps). I'm using PIC processor for data management.

Any advice?
Damon

The Piccolo 433MHz radio modem is capable of interfacing with virtually any standard RS232 device, such as PLC, data loggers or desktop computers.
With flow control, the Piccolo transparently moves data from end to end at speeds of up to 38.4kbit/s. Supporting time division duplex data applications, the Piccolo 433MHz radio modem is suited for applications ranging from small point-to-point links through to large broadcast point-tomultipoint data communication networks.
Depending upon the geography and terrain, the Piccolo 433MHz radio is capable of
communicating reliably up to 5km line of sight*.

Click to expand...

Take a look at this licence-free 433MHz radio modem ..
http://www.rfinnovations.com.au/Uploads/Images/rfi433.pdf
With two small Yagi antennas you should be able to cover the 4km distance (line of sight)..

Regards,
IanP

If you can't do it with a 4watt CB radio then no other license exempt equipment will work.

The local geography and the height above ground of your antenna is the main thing. If your base station is at the bottom of a valley or there is a hill in the way then it's not going to work well. Inner citys with lots of tall buildings are also an RF problem.

27MHz CB is probably a good choice for this because the signals go a bit further than line of sight by groundwave propagation. Unless your local geography is against you, a halfwave antenna on top of your house and a CB will easily cover 4KM. an earth wire straight down to an earth spike in the ground may help as well.

The main problems of 27MHz are interference from other CB users and that sometimes the ionosphere is highly charged and every channel has strong signals from thousands of miles away. We are at a low part of the sunspot cycle now so it may be ok but in a few years russian taxi drivers and people in Italy using kilowatt amplifers will again be heard halfway around the world.

Here in the UK it's legal to use CB for telemetry but you still need a license and only ax25 packet radio format data is allowed.

A license exempt UHF frequency would work if you have access to a high enough tower. In the UK we are allowed 0.5watt at 868MHz with 10% duty cycle.