Max range and performance within limitations

End Devices (Nodes)
1

Hello,
For some time, I have been launching ultralight (4 grams) LoRaWAN enabled payloads on floating balloons. These balloons fly for a couple of days, weeks or even months travelling many thousand km.

For the newest probes, I have heavily optimized not only the hardware, but also software. I’ve added many safety features for more stable power usage and geofencing. I’ve been researching the Limitations and the Fair Access Policy for quite some time now. The thing is, until now I transmitted messages with these parameters:

Payload size: 5-10 bytes (new payload packets will be around 20 bytes though)
Spreading Factor: SF12
Transmit power: 20dBm
Delay between messages: 10 minutes (though the probe is over land for not very long so there can be only 5 or so messages in a day)

I was launching the balloons from central Europe and none of them got further than to eastern Europe. I got a range of around 400km (judging from the distance between the gateways that were the furthest apart and received the message at the same time). The messages were received by more than 20 gateways in central Europe but only by 1 gateway when flying over western Europe.

Only later I found out that hardcoded SF11/SF12 is forbidden. The problem is, when using SF10, I risk that no gateway will actually receive the message because it is out of range. From what I’ve read, this is not enforced on a global (network provider scale) but can be enforced by the owner of the gateway. How can I deal with this? Can I transmit half of the messages on SF9/SF10 and the second half on SF11/SF12? Also, since the probe is travelling at 150km/h and also is rotating with its directional antenna, almost every other message is received by a different gateway. This means that I’m not “polluting” the same local network. Does that change the fairness of the transmission?

Also, even though my previous probes never left the EU868 band, it’s very probable that the new probes will go much further. Because of this, I’m implementing geofencing to switch the frequency band during flight. But, I found out that for example on the US915 band, it’s not possible to use a spreading factor of more than SF10 because of the maximum dwell time. Will the probe not work at all in this area? Should I just switch to SF10 automatically when switching to that frequency band?

One more thing. How does the airtime actually affect the range of the transmitter? Let’s say I have an airtime of 1812ms on SF12 and an airtime of 452ms on SF10. How much smaller should I expect the range to be? 30%? 40%? 50%? I know that’s impossible to predict precisely, I just want an estimate.

I had a lot of questions here, so thanks a lot in advance to anyone who knows the answer.

1 Like
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To answer about the range vs SF: basically every time you increase the SF by 1 you improve the sensitivity by 2.5dB, meaning an increase in range of roughly 77% (supposing line of sight condition).
There is other factor to take into account like the speed of the node which can have negative impact when you increase the SF but it depends a lot in the payload size (longer payload are more sensitive to this kind of issue)

3

Use Semtechs LoRa calculator.

The relative sensitivity difference between SF12 and SF10 is 5dB, so SF12 should go approximatly 78% further.

4

A directional antenna implies gain, so your transmitting more than 20dBm ?

I thought the legal limit in a lot of places is 14dbm.

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Thanks @Clams and @LoRaTracker. Wow, that’s a pretty large difference. I mean, I experimented with both and had some experience with the range but 78% is huge when you’re somewhere over Turkey or Russia.

So that makes the question of how to use SF12 without being hardcoded and without using ADR even more important.

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Alright. I’m no RF engineer. It’s a wire antenna. I still suppose that the signal strength is different when the probe is rotated by 180 degrees.

As for the legal limit, I did’t know that either. The thing is, the probe is hardwired to use the PA_BOOST pin, which means it cannot go lower than 17 dBm. Is the limit same for ground and airborne applications? I know that this is very speculative, but the strength of the signal on earth should be much lower when the probe is airborne. I’m also wondering, is there even a way to check this? Can you even remotely know at what power is transmitting a device tens or hundreds of kilometers away?

7

The strength of the signal when it reaches Earth is not relavent, neither is it relavent if anyone can ‘remotely’ know that you may be breaking the legal ERP limits.

Of course you can check how much power the tracker is transmitting, you use an RF power meter or do a comparative test of some sort. Basic stuff really.

As a first step you maybe need to re-design your tracker to ensure that it complies with legal limits.

8

This and your other statements feel like you are looking for excuses to exceed about every limit you need to adhere to.

If you search the forum you will notice others have flown balloons as well. Adhering to the maximum transmission power and using SF10 they reached over 600km and hundreds of gateways. May-be you need to do more research and try to adhere to (legal and other) limits.
Check this experiment on ttnmapper for an example.
BTW, SF12 on moving objects is a bad idea in itself. Check this post.

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Alright, I will look into it. The thing is that on the chip I use (SX1276), you need to use a second pin for the lower power transmission and you need a more complex circuit for using both of them.

One more question, is there some easily readable list or a map of countries and their legal limits on transmit power?

10

I was just genuinely interested in this.

Okay. I will try to look into making the BPF filter and the antenna better while transmitting at lower power. The problem with this is I have very limited component selection because of the ultra low weight requirements.

This is very interesting. It would be also interesting to see what transmitting power and spreading factor they used. I suppose it’s there’s somewhere, I will look trough the website thoroughly later.

Well this is very, very interesting. Maybe this has been causing the range for my probes to be small before. When you loose range with lower SF but you increase the risk of the gateways not being able to catch the packet at all, the question of where is the sweet spot is coming to my mind. Because I don’t think the probe will ever be going slower than 25kph.

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14dBm and SF10.

12

Most of the HAB community have the wire antenna vertical with a tin foil ground plane for local flights, not particularly directional at all. A guitar string quarter wave antenna with radials would be better.

There is considerable support available in creating a circumnavigating floater, often switching to APRS over the US and then back to normal LoRa - we get heads up on IRC that one is coming our way and tune in to help with tracking.

Given the patchy coverage in parts of the world for LoRaWAN, maybe you want to rethink your comms strategy.

13

Did you measure the loss through the band pass filter you currently have and the level of harmonics ?

The problem with making the antenna ‘better’ is that you need to reduce the power to compensate for the gain. If this is a directional antenna all you will achieve (over a standard vertical) is the exact same coverage\distance in one direction and reduced range in the other directions.

The one advantage of a directional antenna is that you get the same distance\coverage in one direction (but worse in others) using a lower transmit power, thus saving battery power.

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Okay. Thank you. Having more than 700km of range with that is very impressive.

The wire I use is actually 1/4 wave. A guitar string is most times too thick and heavy for me. I’m considering using a ceramic chip antenna, as I do for the GPS receiver. The problem is that the probe is so small that the ground plane is only 30mm. Maybe I will deal with that with some radials.

Yes. I know that many people use APRS for this but for quite some time I’ve been trying to not use APRS. I don’t have a license for it yet. Also, we are thinking of having balloon kits available for the general public. There we would need it to be operating in the “free” band.

We are finishing backlogging functionality right now. This will enable us to have some limited data from parts of the flight where the balloon was not in covered area. This is simple. But we also plan working to develop the hardware and software for hive floater operation. This means that we could communicate trough multiple floaters from uncovered areas to covered areas.

15

Not really. I’m not sure whether I have immediate access to equipment to test this. These new designs were made during quarantine and while I consulted with an RF engineer, I didn’t have to actually test this yet. Can you do some tests like these with an SDR or do you need something more special (expensive).

I see. Because the plan is to work on hive communication between balloons, it would be best to actually have the signals being omnidirectional. The antenna gain is added to the transmitting power right?

The probe uses a small solar panel with weight of around 2 grams. This is enough to give 0.4W on a sunny day which is enough to transmit at full power. But of course, lower power consumption is never a bad thing.

16

Semtech even has a name for that: Blind ADR.

2 Likes
17

Just asking to be sure:

Have you contacted the ukhas group? They have a lot of practical information and a really nice tracker map. Their website is at
http://habhub.org/

They have a google groups mailing list, and an IRC channel with helpful people.

I myself wrote a kind of bridge between TTN and the habhub tracker site, that receives telemetry packets from TTN and forwards them to habhub for display on the map:
https://revspace.nl/TTNHABBridge

18

Alright thanks. This looks really cool.

Yes I know this. We use the Habhub tracker and predictor for our HAB launches.

Oh, I didn’t know that. I found it on the site now. I will try it.

This is really cool. I wanted to connect my floaters to Habhub myself but I gave up because I didn’t find anything like this back then. But now I’m using Tago IO and I’m really happy with it. It offers a very large degree of customization which is very useful. My dashboard looks like this. The thing I don’t like about Habhub is that the map is so dominant and it’s harder to clearly read all of the other data compared to a chart on Tago. We want to develop our own web interface for this purpose. It will be similar to the Tago dashboard but will have some custom stuff like a 3D globe view instead of a map. But instead of viewing one probe, you will be able to switch between different ones on a map, as you can on Habhub.

19

Well yes, basic RF stuff that is.

Appreciate that a tracker at altitude is capable of causing significant disruption over a wide area both to TTN and other comms. At ground level mistakes in design may go unoticed, but at altitude its different.

Consider getting the assistance of an experience RF guy to check out and test that your tracker is legally compliant and not causing interferance.

2 Likes
20

Yes, I understand. I will try to deal with all of this.