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Choose LoRa module freq and antenna for best range  XML
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d00m



Joined: 19/10/2015 17:21:10
Messages: 32
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Hello,

In my project I'm using LoRa module 915Mhz. There are two devices - remote server, which measure temperature and client device connected to Android phone via FTDI adapter and receive data from server.

I have successfully received data at range 1km.
and last RSSI was about -98 -100 which meant pretty bad connections I suppose.

Now I need put my server device even further - to 3 km.
I need advice - how to improve range. maybe I need use different antenna or maybe I need choose freq - 433Mhz.
Thank you in advance.
DanPea


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Joined: 01/11/2015 16:41:12
Messages: 5
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Using two RFM95 miniwireless modules (868MHz) I have conducted numerous long range LOS tests.

To date the furthest I have had a two way link reliably communicating was over 20km line of sight. The Arduino code I had running indicated both packet RSSi and background noise floor.

At 20km I still had over 20dB of SNR to hand - with a RSSI of -84dBm!!

this was achieved using the following modem settings and +20dBm TX power:-



For long distances you will need to use narrow bandwidths and high spreading factor - this heavily relies on the relative offset of the 32MHz crystals on each modules. There are ok but by the time they are multiplied up to your TX frequency an unacceptable frequency offset might exist between the TX/RX pair which will mean the carrier will fall outside the receive bandwidth - you may need to play around with the frequency setting for each board to make sure both boards are operating with exaclty (or as close as possible) the same carrier frequency

I also added some code to the RH_RF95.cpp library to output my back ground RSSI as well as my RSSI this allow you to debug your environment and determine if that 4G mobile phone mast is leaking into your receiver.



My experiment location was nowhere near another interferers so my background RSSI was about -97dBm ish

I also used properly grounded coaxial cable feeding a bulkhead Female SMA connector to which a fitted a 3dB whip antenna. the shield of the coax was soldered to an area of the board where i had scrapped of the solder resist to expose the copper ground plane.

http://www.rfsolutions.co.uk/acatalog/info_ANT_8WHIP3H_SMA.html

To optimise the antenna gain I also fine trimmed the length of this antenna so the null in the return loss of the antenna was at exactly 868MHz.

A simple bit of wire is fine for shortish range setups but not really for 3km.

Use this to plan your range experiments:-
http://www.solwise.co.uk/link-budget.htm

In a rural setting I have got about 800m but this was using crude untuned wire antennas at 1m elevation and non optimal modem setting.
Hope this helps.

Regards

Dan
Rick



Joined: 30/05/2014 22:08:56
Messages: 93
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Hi Dan,

Thanks for your excellent contribution here.

We are working with the factory now on producing a custom LoRa with this in mind.
For narrow bandwidth and high spreading factor, it is important to consider crystal tolerance, and in particular a TCXO type crystal would be a useful enhancement.

We have recently received a few crystal samples from the factory, modified a couple radios, and we are also looking into alternative crystals as well.

If you are interested in exploring this further Dan, please send me a private message, and we can arrange getting you a few modified LoRa radios with this in mind.
All I ask in return, is that you report your findings and test results here for the benefit of others who also need to maximize long range usage.

If all goes well, we should have a separate part number for offering the special high-range LoRa radios.
We're focused right now on the RF95/96. but the sample principle could also be applied on the 92/98.

This message was edited 1 time. Last update was at 29/05/2016 10:36:42

 
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