0
\$\begingroup\$

I am trying to create a PCB which will have a small rf device transmitter and another PCB with the receiver. I want it to be able to transmit for roughly 2km in an urban environment. Since I will be transmitting using ISM band I guess it would be at 433MHz.

I have seen modules like the Extreme Range LoRa 868 / 915MHz SX1272 LoRa module for Arduino, Raspberry Pi but I do not need to transmit over such a big distances and its kinda big. I am unfortunately not knowledgeable enough in order to create my own circuit and I do not know how compact and small it can be compared to the market's.

I have found these guys though I have not idea how they kept the size of the device that small.

So my question is whats the 'best' small rf transmitter using ISM band, able to transmit over 2km in urban area to implement at my project?

If you want me to clarify anything please tell me.

\$\endgroup\$
  • \$\begingroup\$ Kelvin-Mega is not a unit of distance. \$\endgroup\$ – Olin Lathrop Jan 24 '17 at 22:34
  • \$\begingroup\$ I think anyone could hardly confuse it with a temperature unit, but I edited anyways, thanks \$\endgroup\$ – We're All Mad Here Jan 24 '17 at 22:37
  • \$\begingroup\$ It's not about being confused, "KM" is just plain wrong if you mean kilo-meters. There are existing world-wide standards for this. You don't get to ignore them or make up your own. I won't -1 since you fixed it, but don't try to make excuses about how the original was OK somehow. \$\endgroup\$ – Olin Lathrop Jan 24 '17 at 22:43
  • \$\begingroup\$ You are right I am sorry, I saw it but I left it because I thought it was okay and I was too lazy to fix it, you should -1 :p \$\endgroup\$ – We're All Mad Here Jan 24 '17 at 22:51
  • \$\begingroup\$ The device you linked to is a GPS tag, not an RF transmitter, did you post the right link? \$\endgroup\$ – Roger Rowland Jan 25 '17 at 5:56
1
\$\begingroup\$

Semtech make a spread spectrum chip, it is up to the system designer to comply with whatever laws apply in their market.

Some countries (Aus IIRC) allow significantly more power in at least one of the UHF ISM bands if you run spread spectrum with certain characteristics for example, but is highly country specific.

I would note that any UHF comms is almost line of sight, it does go thru buildings, and to some extent diffracts over hills and such, but not well, UHF in an urban environment usually means repeaters on tall buildings and lots of power (And even then reliability is 'iffy').

You should also take range estimates from datasheets with a huge pinch of salt, you can easily have a 20dB or more dead spot just due to multipath, and even when you don't the range estimates will be at the very limit of operation, reliability wants 20dB more then that (100 times that power) in an urban area.

To give you a flavor Back when the London police used to be on VHF (Which does better in urban situations) the handhelds usually linked back to a 25W repeater in the boot of the car, that was how much power it took to make it reliable.

The only way to do this on 433MHz is with a ham license (Then you can run lots of power).

Could you not just stick a couple of GSM modems in there and have done with it, no range headaches, no hairy licensing problems, two modules, two sim cards, job done.

\$\endgroup\$
  • \$\begingroup\$ Yeah I could do it with sim cards but I thought of trying rf cause its easier on the user side. Thank you for your answer! \$\endgroup\$ – We're All Mad Here Jan 24 '17 at 23:59
  • \$\begingroup\$ May I ask this, but I haven't searched much about it so I am sorry if I say something silly. Since 2.4Ghz band is also unlicensed and its a higher frequency, so I can broadcast further with less power cant I broadcast there I mean, isnt it better from the 433MHz band? \$\endgroup\$ – We're All Mad Here Jan 25 '17 at 0:15
  • \$\begingroup\$ Nope, see the friis equation, higher frequency is worse for path loss all else being equal, and the microwave bands do not penetrate concrete in a useful way. Also 2.4GHz ISM band is a mess, Wifi, Bluetooth, Microwave ovens, Video senders, all sorts of stuff much of which does not play nice together, you think 433MHz is bad? \$\endgroup\$ – Dan Mills Jan 25 '17 at 2:24
  • 1
    \$\begingroup\$ Be careful in interpreting the Friis equation .Although the lambda term makes it look like the received power gets worse at high frequencies what is not so obvious is that the antenna gain for a given physical size will go up with frequency. The effectiveness of antennas that are very small relative to the wavelength can be very bad. What is true (and not part of the Friis equation) is that building and atmospheric absorption and multi-path effects can change dramatically at higher frequencies. \$\endgroup\$ – Kevin White Jan 25 '17 at 4:04
2
\$\begingroup\$

You can't just go broadcasting the kind of power it takes to go 2 km (I'll assume for now that's what you meant by "KM") in a urban environment at 433 MHz. There are laws about how much power you are allowed to emit at what frequencies, and what licenses might be required.

The ISM band at 434 MHz is very limited in the allowed transmit power. It is also limited to rather low duty cycle and repetition rate, especially for unattended instruments. You get a few 10s of ms every 10 seconds if I remember right. I was envolved with a product that used this frequency, and its effective RF range was about 100 feet.

This answer is to say you're a long way from slapping something on a board. You need to stop and learn what exactly you're allowed to do, with and without a license, before you proceed.

\$\endgroup\$
  • \$\begingroup\$ So since you seem like a really knowledgeable person, how should I go around with that? I mean I know that I am allowed to broadcast at 433-434MHz but as you said I wont be able to achieve distances of 2km. How does that company that I linked above do it, do you have any clue? \$\endgroup\$ – We're All Mad Here Jan 24 '17 at 22:49
  • \$\begingroup\$ @We'r: It looks like those devices are GPS receivers and loggers. The basic versions don't transmit at all. There was mention of a VHF beacon to help finding the device for recovery, but I didn't see specs on it. I seriously doubt the beacon is powerful enough to be detectable 2 km away. \$\endgroup\$ – Olin Lathrop Jan 25 '17 at 11:38
  • \$\begingroup\$ For distance, you need very narrow bandwidth and antenna gain. Hams do DF direction-finding of beacons using Yagi multi-element antennas --- assume gain is 10X, thus power drops 10X. And instead of 100,000Hertz bandwidth, use precise XTAL filters in the receiver/tracker with 10Hertz BW, and the power drops another 10,000x. You now have 10milliWatt beacons being detected, and DF'd, 6 miles away. \$\endgroup\$ – analogsystemsrf Feb 1 '17 at 18:48
2
\$\begingroup\$

[ERROR: corrected Feb 19, 2017 Used dBm, but always called 0dBm to be 1watt] Lets suppose you want a burst of data every so often. Legal in that ISM band. Lets do 100,000 bits per second, but only a 10mS burst. You'll get 1,000 bits. OK. The bandwidth needs to be approximately 100,000Hz. Approximately. Depends upon type of modulation. Use OOK...on_off_Keying, where you simply turn on/off your PowerAmplifier transistor, controlled by the serial bit stream. And take a couple microseconds to ramp up or ramp down, so your transmitted energy remains (mostly) inside the ISM band.

How much power is needed? We'll work with 'dBm', dB referred to 1milliWatt. 0dBm is 0.223volts RMS across 50 Ohms, or 0.632vPeakPeak, using P=V^2/R. First let's compute the necessary energy into the receiver antenna.

-174 dBm/Hertz is the Boltzmann/Nyquist/Johnson noise floor (comes from K*T)

  • 50 dB from 10*log10(100,000 bits/second) in OOK modulation

  • 20 dB to achieve very low bit error rate

  • 5 dB sloppy receiver noise figure, and other flaws

  • 5 dB sloppy matching from Antenna to Receiver electronics (LNA)

    becomes -174dBm + 80dB

-174 + 80 = -94dBm (I know 0dBm/50ohms is 0.632vpp;-100dBm/50ohms is 6.32uVpp) (This is 6dB stronger, so 12.64uVpp)

What is loss between Transmitter and Receiver? Assume the 2 antennas have unity gain, meaning there is no focusing of the radiated energy. Assume 1meter (300MHz)frequency. Assume 10,000 meter separation, or 6 miles. The path loss is 22dB + 10*log10[ (separation/wavelength)^2 ] ==102dB loss. Thus [ERROR:1 watt] 1milliWatt, radiated uniformly, becomes -102dBm, at 6 miles.

As other people mentioned, the urban environment is not line-of-sight, so another 20 dB margin is wise. If not more. You are not line-of-sight.

How healthy is datalink? Compare receiver floor(RF) to TXpower+PathLoss(TXPL). Looks like you need RF:(-94) - TXPL:(-102) = +6dB additional signal, because the received power(TXPL) is 6db weaker than what receiver needs(RF). Our datalink is 6dB short of what a robust link requires.

Summing Multipath+Linkshortage: 20+6 == +20dB atop 1milliWatt, or +26dBm which is 400 milliWatt.

Working at 433MHz requires a few more dB of power, say 1 Watt.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.