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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

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  • \$\begingroup\$ Have you uncovered anything that might make a decent answer in less than the number of words contained in Deuteronomy? \$\endgroup\$ – Andy aka Sep 24 '15 at 20:36
  • \$\begingroup\$ Hi Andy, I have read about TI launchpad and appropriate modules, Arduino and Raspberry Pi. I am pretty sure the last two are the most expansive in my set of options. Also, I realize that Wifi will not get that distance unless I'll use a directional antenna but I'm not sure about that too. I also prefer not to rely on current GSM network since the coverage is not available in some parts. Thank you for your comment, any help is appreciated \$\endgroup\$ – chegov Sep 24 '15 at 21:00
  • \$\begingroup\$ Have you investigated the allowable ISM frequency bands in your country? \$\endgroup\$ – Andy aka Sep 24 '15 at 21:03
  • \$\begingroup\$ Yes, the followings are available for the public : 2400MHz-2483.5MHz, 315MHz, 433MHz \$\endgroup\$ – chegov Sep 24 '15 at 21:06
  • \$\begingroup\$ Non-LOS propagation is always difficult. There's a chance you'll have enough SNR off a reflection (Rician fading), but the modeling is pretty difficult and requires a good terrain model. The easiest way might be a relay on top of a hill both sites can see. \$\endgroup\$ – Peter Sep 24 '15 at 23:23
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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.

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  • \$\begingroup\$ There is no limitation on the bytes rate here, this is just the minimum threshold I would require for the telemetry. Thank you for a very informative and helpful answer ! Can you also advice on MCU, with\without a launchpad capable of this ? \$\endgroup\$ – chegov Sep 25 '15 at 6:32
  • \$\begingroup\$ More bytes per second means wider bandwidth means more noise means worse receive sensitivity means less distance. I have no idea what a launchpad is and I can't recommend an mcu because SE doesn't allow recommendations due to data becoming out of date. Sorry, but please come back if you find a module that might suit and try to avoid an uplink if possible. \$\endgroup\$ – Andy aka Sep 25 '15 at 7:29

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