// electronics beginner here I have been using VirtualWire with 433mhz modules to have an arduino successfully send data to an other one. Now, for telemetry purposes I need this one way communication to be operated 20km away. I do not want sophisticated stuff on that , the ideal way for me would be to amplify the signal and keep using 433mhz. I do not need a high bit rate : 2Kb/s are way enough . I can even go below if needed. Any ideas of an electrical setup that would send binary data that far ? Is it possible to keep using the light and great VirtualWire library or am I doing wrong ?

  • \$\begingroup\$ Can you use a directional antenna? (In other words, are the coordinates of the receiver fixed). \$\endgroup\$ – Peter Smith Dec 26 '16 at 16:44
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    \$\begingroup\$ You would need too much power for 20km range - it would be illegal \$\endgroup\$ – Chu Dec 26 '16 at 16:45
  • \$\begingroup\$ @Chu depends on frequency ... I know people with licenses \$\endgroup\$ – FrProg34 Dec 27 '16 at 9:51
  • \$\begingroup\$ @PeterSmith yes I can , what's more the receiver is on a hill which makes it easier for a directional antenna to point at the transmitter \$\endgroup\$ – FrProg34 Dec 27 '16 at 9:52

Your current set up probably works ok at 100m between units and you want it to work at 200 times this distance (20 km). In a perfect universe (in free space) you will require 40 thousand times more transmit power so, if you are using 1 mW transmit power you will be required to increase this to at least 40 watts.

Here's the Friis transmission equation and this relates to free-space: -

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What it basically says is that for the same antennas and transmitting at the same frequency, the power received relates to the reciprocal of distance squared. It works with ratios too. If 1 mW gets you 100 metres, to achieve the same received power at twice this distance you need to transmit 4 mW. At ten times the distance you need to transmit 100 mW. At 100 times the distance the power required is 10 watts etc etc. so 40 watts is by no means a silly number.

Given that the path between transmitter and receiver will likely be littered with a plethora of other 433 MHz users, to obtain the same performance you are likely to need to transmit over 100 watts.

So, you use an illegal amount of brute force or a legal amount of sophistication with a little bit of brute force.

Of course, some sophistication can be achieved by restricting the data rate sent. This is because the receiver could then be designed to operate with a much smaller bandwidth and not have to fight against the noise. Here's a generally accepted formula for the amount of power needed by a receiver at ambient temperatures: -

Power required in dBm is -154dBm + 10log\$_{10}\$(data rate) dBm

So at 1 bit per second, your receiver could be designed to have a really tight bandwidth (thus excluding noise) and it would need a power of -154 dBm. At 1000 bits per second that receive power rises to -124 dBm and at 10,000 bits per second it will be -112 dBm.

Given that your current receiver might be capable of receiving 10 kbps, redesigning it with a much restricted band width (to say 100 bps) will mean the receive power could be 20 times lower. Thus the 100 watts previously mentioned, with some sophistication (i.e. redesigning the receiver), might now mean a transmit power of 20 dB lower at 5 watts.

Goin back to the Friis transmission equation, if instead of 433 MHz you transmitted using 96.8 MHz (4.5 times the wavelength) the received power would be 20 times higher thus you could reduce transmit power by a further factor of twenty.

Using antennas with gain (directionality) is another way of focussing the power onto a far-distant receiver although at 433 MHz and the curvature of the earth (at 20 km) this is probably not a great option unless you use repeaters.

So, sophistication over brute force usually works and is what I advise.

  • \$\begingroup\$ 100W? You can get around the planet with 100W. 5 or 8W handheld walkie will get that in free space. Especially if you upgrade the stock antenna with a good quality whip and run on VHF. You can get them easily on line. If your conscience bothers you, licences are £75-£100. \$\endgroup\$ – Paul Uszak Dec 26 '16 at 18:16
  • \$\begingroup\$ @paul What is it that you don't understand about power from an antenna falling with the reciprocal of distance squared. And it's worse across land. It's also worse at uhf compared to vhf and it's also worse for crappy receivers of the type implied in the question. I advise you to read the question and my answer again noting particularly that I am not recommending this brute force approach. I will also suggest you research the Friis transmission equation before barking another comment. \$\endgroup\$ – Andy aka Dec 26 '16 at 19:07
  • \$\begingroup\$ @PaulUszak Never mind, I've added it to the answer in case there are more daft comments LOL. \$\endgroup\$ – Andy aka Dec 26 '16 at 19:38
  • \$\begingroup\$ But you've just said that you can do it with 5W! That's what I said. Perhaps we're both confused. I'll butt out and go back to drinking and kicking the cat... \$\endgroup\$ – Paul Uszak Dec 26 '16 at 22:20
  • \$\begingroup\$ You're not grasping it yet. \$\endgroup\$ – Andy aka Dec 26 '16 at 22:31

You probably can't do it with a 433MHz ASK (which virtual wire is predicated on) transceiver, at least not legally. The emerging technology that addresses the design space you are describing, I think, is LoRaWAN, but it's still pretty nascent. Do some Google on it though.

  • \$\begingroup\$ It is true that LoRa is interesting for this type of project, but I have already looked at it and it involves equipment I don't want to deploy at the moment . Do you have any key for building a RF power amplifier regardless of the regulations ? I will then look for the frequencies that allow for some Tx power in my country \$\endgroup\$ – FrProg34 Dec 26 '16 at 16:21
  • \$\begingroup\$ You can use commercially available amateur radio equipment that allows a Morse keying input I think. Then just wire your Arduino to an NPN transistor open collector to the keyer input. \$\endgroup\$ – vicatcu Dec 26 '16 at 16:28

Depend on how long the range you want to cover, there are different solutions.

For the closest ranges, you can use 2.4GHz low power solutions. That is about 10 meters maximum indoors or a few hundred meters outdoors. You can use your nRF24L01+ wireless solution or 802.15.4 Zigbee systems or Bluetooth Low Energy if you are thinking about talking with a smartphone.

Increasing the distance, you will want to look into higher power solutions. 433MHz systems have good propagations without consuming too much power. Full-power Wi-Fi can work if you can get the aim for directional antenna, and it is the computer-friendly solution on this distance.

For even wider coverage, you will want to use someone else's infrastructure - your favorite cellular carrier, and/or the Internet. Two Arduino's, each with a cellular modem, can send text messages to each other. Or you can use the cellular modem to establish a GPRS (or 3G) data connection to talk to a server somewhere.


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