What kind of radio transmitter would be used on an data collection unit to transmit telemetry over long distances? For example a weather station in a remote location, or in a buoy just off the coast collecting information on currents.
There is a network of LEO (low earth orbit) satellites that is available for this sort of application. A VHF transmitter of a few watts power can easily communicate with them.
Back in the 'good old days' before satellites you could achieve long distance communication either by using 'longwave' transmission (less than 300kHz) or bouncing the higher frequency band transmissions from ionised layers (skipping). VHF and higher are generally line of site systems (unless you manage to bounce them off something). Thankfully, as Dave points out, all this telemetry is now done by relatively low power transmitters aimed at the sky. Fixed location remote data collecting stations are usually powered by solar panels. The low earth satellites are capable of storing the transmitted information and re-transmitting at a different location.
As Dave Tweed mentioned using an LEO satellite is the usual modern way of communicating small amounts of data over long distances. However I noticed in a comment it sounds like a hobby project so you probably don't want to pay for satellite subscriptions and data traffic:
There are several lakes ~10-100km away from my house, I am hoping to setup something to send the temperature of the lakes to my house.
For the lower end of that range (maybe up to 20km) with a very good antenna system you'd probably find VHF data transceivers with a few watts of power may be suitable. Several commercial units are available that are license-free depending on your location, for example:
For longer distances you start to run into problems because the license-free channels don't allow enough power. If you live in a remote area you might find getting a narrow channel allocation isn't terribly expensive, but once again it would be an ongoing cost and some transceivers like the above aren't all that cheap.
About the only low budget solution that comes to mind is to get a ham radio license, maybe take a look at What is Ham Radio? at the ARRL website. You would need to put considerable effort into obtaining a license but once you passed you'd be able to legally transmit at higher powers and lower frequencies. It would also give you enough background so that you could probably put together something using a form of RTTY / Radioteletype to send the data.
protected by W5VO♦ May 30 '13 at 19:11
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