Sensor communicating with a base station 3.5km away for a rate of 100bytes every 10 sec

Question

I have a small project that I want to do, I want to deploy a sensor that is capable of measuring the wind speed and direction from a crop field to my house - distance of about 3.5km. There is no line-of-sight and I wish to send up to 100 bytes of telemetry every 10 seconds. The sensor should also be capable of receiving up to 50 bytes from the base station (a command to change the sampling rate). The sensor will have to be powered by a battery.

I am new to the MCU field, what would be the cheapest most power efficient way to accomplish the above ? Would I need a launchpad with a RF transceiver module for this ? Would an MCU + RF transceiver module + Battery - be sufficient or would I still need the launchpad ? What frequencies should I be looking into ? What about the base station, Can I connect an RF module to a PC ? What kind of an antenna would I require ? - I have no problem putting a large antenna on the base station side, but on the sensor out there on the field - the antenna will need to be as minimal as possible due to the risk of theft.

I would also appreciate recommendation on suitable books or other relevant reading material, this project is a hobby of mine and I am more than willing to catch up and update on my progress :)

Thank you for your future assistance. Cheg

Answer 1

315 MHz is probably your best option given that you'll get longer range compared to higher frequencies. This is because an antenna at lower frequencies has (what is called) a wider "aperture". It works like this....

A transmit antenna (dipole or monopole) generally sprays the RF power out 360 degrees in all directions horizontally and also in a fairly wide arc vertically. Because of this, RF engineers talk about the EM power flowing thru one square metre of area at a certain distance from the TX antenna - the bigger the distance from the TX antenna, the smaller the power per sq metre because the area-coverage is bigger.

Power per sq metre falls as distance squared in simple terms.

At lower frequencies, the receive antenna's effective aperture (also measured in sq metres) is bigger because the antenna dipole length is proportional to wavelength. A longer resonant antenna means a bigger aperture and that means more power received.

The effective antenna area "captures" all the picowatts per square metre flying thru its aperture hence, at a lower frequency a dipole receives more power.

I hope that justifies why 315 MHz is probably the best frequency to go for. However, if there was an ISM frequency at (say) 1MHz this might not be a good frequency but this is based on a terrain/line-of-sight analysis. Whole books can be written on this!

Moving on....

You might have problems staying within legal power limits AND getting 3.5km. It's not a problem for a bespoke radio that is designed for slow speed data comms but, who on earth is going to design an off-the-shelf module that has a data rate limited to 10 bytes per second. Having said that, in the dim and distant past (probably 1980s) I remember one telemetry radio designer say that their output power was 1uW and it could be recieved by a suitable receiver 1kM away - bandwidth was really low - maybe 10 Hz (from memory)

Slow data rates mean the receiver sensitity can be massively improved because it can have a much tighter RF filter that blocks out a lot more "ambient" noise than a faster receiver. With a low bandwidth receiver you can easily get several km on 1mW but finding an off-the-shelf one is going to be a misery.

If you can avoid the uplink sample rate control back to the field transmitter you are more-than halving the problem re battery power and complexity - find a way of tackling this first is my advice.