RF communication system, very little data but >1km range

[autobot]

LoRa is a good fit for that.

There are also some mbed libraries for it, making development easy.

[Psi]

A $20  433mhz RC airplane transmitter will go that far without problems.

a OpenLRS clone for example. And its open so you can hack it

[Mechatrommer]

loRa

https://www.google.com/search?q=loRa&client=opera&hs=nIe&tbm=isch&tbo=u&source=univ&sa=X&ei=7V90VfPhH8jFmAWczYL4Dg&ved=0CEQQsAQ
sadly even the SX1272 is stated as "long" range, how "long" is anyone's guess.

[Neganur]

You should be able to get +20dBm output on the PA_BOOST output with a stable power supply (i.e. no longer just low power, >2.4V and some 100mA).
With a proper antenna, some claim 10 miles / 16 km line of sight and some 3 miles / 2 km inside buildings.
You can increase sensitivity by using a split path for rx/tx, large spreading factor and narrow bandwidth. Surely 2 km should not be a problem.

[eneuro]

With a proper antenna, some claim 10 miles / 16 km line of sight and some 3 miles / 2 km inside buildings.

You are talking about 100mW transmiters or 1W for those a few km ranges? 
Flite test below, but 2 years ago, however mention also OpenLRS


I was testing those cheapy $2 modules I've linked above with GPS and than calculated distance between transmiter and receiver from GPS log data.
Tomorow 10V transmiters test with dipole vs quater length antenas.

Trying to figureout what range we can expect from 100mW 433MHz transmiter with decent antena and what power needed for a few km ranges ? 

[NiHaoMike]

Is modding (or even just connecting to the headset jack) a pair of cheap walkie talkies an option?

[niekvs]

Trying to figureout what range we can expect from 100mW 433MHz transmiter with decent antena and what power needed for a few km ranges ?

You can see some LoRa range testing here:

https://www.cooking-hacks.com/documentation/tutorials/extreme-range-lora-sx1272-module-shield-arduino-raspberry-pi-intel-galileo

So a line-of-sight distance > 20km, and around 2km a urban areas.

This was using 868MHz, but 433MHz should generally reach further still - and they seem to be done at 15dBm, while most SX1276 modules seem to max out at 20dBm, so they should theoretically reach further as well (though a lot depends on their design, etc.)

[eneuro]

This was using 868MHz, but 433MHz should generally reach further still - and they seem to be done at 15dBm, while most SX1276 modules seem to max out at 20dBm, so they should theoretically reach further as well (though a lot depends on their design, etc.)

Thx, for this link. That is interesting 20km LOS using only 25mW transmiter power? 

It looks like they use quater wavelength antenas with those LoRa modules-but what about using classic half wavelength dipole antenas with this sort of things?
Is it possible and any benefits? 

However, we are talking a low data-rate down to few bytes per second ONLY in those LoRa modules?
It will be interesting if those cheapy $2 433MHz  modules powered at 10V will be capable to deliver a few bytes per second transfer rate at 1km range with good antenas?
I had already 350m NLOS but it was only ~1Hz 50% pwm 
http://www.icstation.com/433mhz-transmitter-receiver-arduino-project-p-1402.html

They says:
Transmitting power: 10mW
Probably they specified there maximum when transmiter powered from 12V 

[Neganur]

You are talking about 100mW transmiters or 1W for those a few km ranges? 

Have you read any of the data sheets?
I can't see your region, but +14dBm (25mW) is for ETSI 868MHz and with a 3dBi antenna you'll easily transmit 8km. The greater power +20dBm can be used for FCC 915 MHz and should give you some 13 km range with the same antenna type.

For example Multitech has a module called mdot, it's meant to work with the mbed stuff.

[RupertGo]

I've seen something similar done with unmodded walkie-talkies and a couple of boxes. One injected one of a selection of tones via the microphone socket and toggled the PTT line, the other listened to the headphone output and toggled lamps according to tones heard.

Or you could just put a walkie-talkie at each station and tell the teams to call in once they get there

[Elf]

In most countries there are laws against people creating their own transmitters and operating them. There are carefully specified licenced exemptions.

Your best bet of avoiding prosecution is to start with existing legal transmitters.

Radio hams are a good source of information on such topics.

Speaking of which, rather than trying to build this in unlicensed bands, why not just pick up a HAM license and build it there? A lot of the regulatory, interference, and power output concerns just go away.

There are a lot of existing cheap circuit designs and products for embedded TNCs, APRS, etc. Although, admittedly not as cheap as the ultra low power unlicensed UHF modules. For using that low power stuff, I would use as much gain as you can reasonably get (e.g. a yagi or, at 900MHz and above, a panel antenna). Antenna gain has much more to do with path length than power levels. You may also try operating at different polarization than other users of the band (e.g. horizontal vs. vertical polarization) if you are concerned about interference from other uses (team voice comms, etc.).

[BrianHG]

I've used this guy: https://www.alibaba.com/product-detail/universal-3km-433-cheap-rf-module_1809987082.html?spm=a2700.7724838.0.0.r8Q13Y

For RX, i used: https://www.alibaba.com/product-detail/315-433MHZ-RF-ASK-Receiver-Transmitter_2005979852.html

It's a little over powered for 1km, but, dirt cheap and functional (I tried it...) with most of the cheap 2-3$ superhert receivers.
In the city, it should give you 1km, outside of the city, about 2.5km.
Going through the concrete walls of my house + through 15 other houses + some help from reflections, you will get around 0.5km.  See attached photo.  The white ring is 0.5km.

Remember, there is a lot of traffic on the 433 Mhz band since it is use by so many gadgets.  You need good MCU code to drive it and decode on the other side to deal with all that random interference in the city, but it is very doable since you got that 3watt burst during transmit.

2 notes, your transmit data pulses shouldn't exceed 50% duty cycle, or it will overheat.  The data in will work with a full clean 3.3v, preferably 5 v signal.  The power supply needs 12v.  It comes with a tuned antenna.  I recommend modulating your transmit data with a carrier like an IR remote control signal so your receiver on the other side can use your carrier frequency to demodulate the data differentiating it from surrounding noise and interference,..


All those red arrows are the houses the signal had to go thru to reach my receive location.  Out in a flat open area, or, having the transmitter high up, you should get at least double the distance I got in my suburb test.  I was transmitting a 4Khz test carrier through the transmitter & testing the receiver at the other end.

I requested a quote for 5 from this guy:  (Even though he says 200pcs minimum, he still sold me 5, + 5 receivers)
TX
https://www.alibaba.com/product-detail/universal-3km-433-cheap-rf-module_1809987082.html?spm=a2700.7724838.0.0.r8Q13Y
RX
https://www.alibaba.com/product-detail/315-433MHZ-RF-ASK-Receiver-Transmitter_2005979852.html?spm=a2700.details.maylikever.10.697a8c3epkWsf4

[mubes]

Deffo LoRa (you don't need the loRaWAN bit). SX1272 or SX1276. Their range claims are not bs. There are plenty of breakout boards with built in antennas at the usual places.

Dave