Time to reach out for some advice (TIA). I have a field site that is located 3.75 km from a gateway that is positioned on a multistory city building. TTNMapper shows reception to and beyond the area. The node is located about 4m above the ground. The area is in a park and there are large trees surrounding.
I started with a 5 dBi 915 MHz “duck” antenna but had difficulty connecting from the site. I changed the spreading factor to LMIC_setDrTxpow(DR_SF12, 14); to try and speed up the join. I did get a connection but this soon dropped out. I purchased a 1/2 Wave Fiberglass Outdoor Antenna 915MHz 6dBi and this certainly made an improvement, readily joining and making successful sends every 10 min for about half a day. Then the transmissions stopped and did not restart unless I rebooted. This pattern has been repeated several times.
I have read that lower antenna gain may pick up scattered signals better as they are less directional. I tried a 2dBi “duck” antenna but this only made one successful transmission from an already joined node.
It’s been very frustrating as the node is 30 min travel away and has required multiple site visits. I have considered including some sort of watchdog routine to reboot if the connection drops but this seems inelegant and would mean rejoining possibly every day which I know is against best practice.
Any suggestions?
I’m not sure if this is the correct solution but we put a 900mhz directional yagi on a weather station that was struggling for reception.
It took SNR from -3 to +7 and improved reliability and stability of the connection.
Its location was behind several large clumps of tall trees.
regards
Paul
I did get a connection but this soon dropped out.
Are you sure this is an antenna issue? If it is the node should do transmit regularly and your application should get data intermittently. Or are you using acknowledged uplinks or downlinks?
So take a node that is portable and see if it connects when its halfway between the field site (@3.75km) and the gateway. That should give you a 6dB improvement in the link, and a clue as to wheter its an antenna problem.
Looking at the RSSI and SNR of received packets is useful too.
Adding 10dB of gain to the weather station antenna increases the radiated power to 250mW, the legal limit in most places is 25mW.
The SNR limit for SF7 (the lowest spreading factor used) is -7.5dB, so if you were receiving packets at -3dB, you still had 4.5dB of link margin left.
thanks for these responses. I’m pretty sure it is an antenna/reception problem as I have been able to get good connection in the area surrounding the trees where there is good line of sight.
Since it isn’t my gateway I don’t think I have access to info such as data rate. I’m not sure how to get SNR and RSSI.
Should I go out and get a full wave antenna? Is there anything I can do with transmission power/spreading factor?
Actually if you expand an uplink message in the devices data window there is a wealth of info regarding signal strength, channel, sf and more
If its an ‘antenna problem’ explain this;
Then the transmissions stopped and did not restart unless I rebooted. This pattern has been repeated several times
Why would rebooting the gateway somehow temporarily improve the antenna ?
We have a limit of 1 W (30dBm) in Australia in the 915 band (not sure if @awootton is in Aus or US on 915)
The RN2903 chip on this weather station board has a max of 18.5 dBm, the antenna has an 8db gain and there is about 3 db loss in the cable (its left over from a mobile phone install so didn’t shorten it)
Seems the directional antenna works much better than the little 2db whip antenna we had on it to start with.
I’m in Australia. Thanks for your interest. Can you give details of the 900MHz directional yagi please.
Sorry not sure what it was, found it in a shed covered in dust and was used for 850Mhz Telstra 3G
We were lucky it just happen to have the same SMA connector we needed.
Most of those old 3G antennas have a range from 850-950 or there abouts to cover both Telstra and Optus 3G.
You can usually find them second hand cheap as people upgrade them to 4G antennas.
cheers
Paul
Andrew, the trees are the problem, not the antenna. 915Mhz doesn’t pass through trees very well, and most likely the problem is scatter and multipath. It may also be affected by moisture (dew), wind etc.
A YAGI antenna will be more directional and may help filter scattered signals out since they come from all directions.
Try getting higher to have LOS, or moving location to a spot where you can see through the trees.
Dave
thanks for this advice, appreciated. I’ll try raising the antenna and if this doesn’t work shift to a clearer location.