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I need to design a device that can transmit an RF signal about 200 meters. The data transferred is very simple. One device simply has a button, that when pressed transmits a two-digit ID to the other unit. There is no other data or voice transmission.

Because the product will be sold globally I'm leaning toward doing 2.4GHz, but I'm concerned about getting the 200 meter range. This range is way outside of most 2.4GHz products. I'm planning on using TI's CC2541 2.4GHz transceiver SoC along with their CC2592 range extender. The CC2592 is supposed to extend the range by up to 7x. BlueGiga sells a Bluetooth Low Energy "Long range" module with a claimed range of 450 meters. So I think reaching 200 meters should be possible.

The other choice is a 868/915/920 MHz system, but that has its own complications, and I would prefer to use a single frequency 2.4 GHz system. Also I'm not sure if 868/915/920 MHz will cover all countries.

So my question is what frequency would you suggest for an international product transmitting a simple ID up to 200 meters?

Thanks!

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  • \$\begingroup\$ I think it has to be 2.45GHz. \$\endgroup\$ – Andy aka Dec 7 '14 at 15:02
  • \$\begingroup\$ Just a note, I've used the BLE121LR module (long range), and if designed correctly - you can get > 200m line of sight. That might just be for advertising though, because when you want to 'connect' to it, you're often limited by your mobile phone in the BT world (unless you have two BL121LRs talking to each other). For sub-GHz, you'll need two or three frequency bands just to cover Europe and North America. \$\endgroup\$ – SJoshi Apr 19 '15 at 3:04
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The 2.4GHz frequency is more solid international standard than 433 MHz. (source: http://en.wikipedia.org/wiki/ISM_band). Any of the carrier frequencies mentioned could transmit to that distance.

Generally, lower frequencies have a better ability to pass through obstacles than the higher frequencies. But, it's almost always a question of the power of the signal. If you don't have the power of the signal, you need to rely of the manufacturers data. Note that generally the transceiver distances are line of sight distances i.e. no obstacles. The obstacles can reduce that distance considerably.

I would recommend a 2.4GHz transceiver with at least double line of sight transecting distance that what you need. TI are good, another option is the Nordic nRF24L01+ chip. Note that it could also be possible with some Bluetooth modules but at that signal strength they are probably illegal!

Good luck, Data source - I'm an EE Engineer.

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I would go with the 433 MHz, it is ISM frequency and it performs much better in terms of FSL and penetrating walls. As far as I know, the 433 MHz is not widely used like the 2.4 GHz signal so I expect u find less attenuation there which will ease the reception of ur signal at 200 meters.

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I think you'll have to break out the math on this one and do a link budget.

For indoor operation, have a look at the ITU model for indoor attenuation. You'll need to make some assumptions about how many walls/floors you want to be able to operate through. For outdoor operation, there's quite a few urban loss models, such as the Young model.

Basically, you'll want to find the worst-case path loss at 2.4 GHz for 200m with your chosen environment assumptions, then apply a reasonable safety factor.

To ensure that your link closes, you need to at least meet the SNR requirement of the receiver under the worst expected environment.

If you can't do this with the modem chip alone, you have three (basically just two) parameters to work with:

  • Transmit/receive antenna gain.
  • Receiver noise figure (by adding a low noise amplifier).
  • Transmit power (by adding a PA).

Antenna gain is probably out in your case, as you probably are going to work with an omnidirectional antenna.

Most of those single-chip RF solutions have a poor noise figure. You can get relatively low-power low noise amplifiers which you can put in front of the receiver to improve the noise figure and thereby the SNR. Some of these include internal bypass switches so you don't need an external one during transmit to prevent damage.

Maximum transmit power varies from region to region, but I think most countries follow US limits. When using frequency hopping, you can use up to 30 dBm EIRP with less than 75 channels or up to 36 dBm when using more than 75 channels or digitally spread, provided that the input at the antenna doesn't exceed 30 dBm. If you're going to add a PA expect to use a fair bit more power, and don't forget to isolate your receiver with appropriate switching so you don't blow it up.

Good luck!

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