LoRa Transceiver Power Amplifier

Thread Starter

JCL24

Joined Dec 12, 2019
9
Hi Everyone

I have designed an IOT device that makes use of a RFM95 LoRa transceiver module to transmit data from point to point. It works fine but I would like to increase the range. I was wondering if it is possible to "boost" the transmission signal to increase the range? As I understand, Low Noise Amplifiers are used to amplify received signals and High Power Amplifiers are used to amplify transmitted signals. But is it possible to design a high power amplifier for a module like the RFM95 where the same terminal is used for transmitting and receiving?

Any advice will be much appreciated. If you have any suggestions for a more powerful module or IC that I can use to transmit data over longer distances, that will also be very helpful.
 

Ya’akov

Joined Jan 27, 2019
6,597
It is not legal to amplify a LoRa signal beyond what the device is already doing. There are strict limits on power output and even on effective radiated power which means that connecting a gain antenna may require turning the on-board amp down.

The module you are using already has a 14dB PA and pouts out 100mW, you can’t do better.

Almost certainly the area to investigate is the antenna. If you are using a supplied antenna it may not even be resonant on the frequency you are using, this is often the case.

Here are some things you can do to help:

Use a resonant antenna. This can be enough all alone.

Use high quality coaxial cable. Find a datasheet on what you are using or planning to use and look at the loss per enter at the frequency you are using. You might be surprised. Remember that dB is not linear, 3dB is twice as much power.

Use the shortest connection between the module and antenna you can manage. You can easily lose half your power (or more) heating up the coax.

If it is point-to-point, use directional antennas. These provide “gain” but being passive devices what that really means is the use the power to radiate in the direction of what you are trying to communicate with. They will be much more efficient and the receive side will benefit dramatically from this.

Make sure that terrain isn’t a problem. If there are obstacles (hills, buildings, etc.) in the way this will dramatically attenuate the signal. If you can maNate to get the antenna above these on a mast it will make a big difference. Because of the issue of loss in the coax, it is far better to mount the module near the antenna and remote the power and communications.

LoRa has an enormous linK budget and if you do everything right it will probably work for you. If it doesn’t you need differnt technology, you can’t increase the RF output power to extend the range.
 

Ya’akov

Joined Jan 27, 2019
6,597
By the way, amplifiers that have to be used for both trans it and receive are common. They have an amp and a preamp and a switch mechanism of some kind, which varies with application.
 

Thread Starter

JCL24

Joined Dec 12, 2019
9
Hi Ya'akov

Thanks for the detailed reply. I will try to implement the suggestions you made.

About the question of legality. I know for example that there are radio telemetry modules used for drones that transmit data over 40km+ range, and I know these fall within legal limits. Even if you look at a DJI mini drone: it transmits video and telemetry over at least 7-10 km with what I believe is 2.4GHz RF.

If this truly is the legal power limit of RF transmitters, how are these examples possible? Do you maybe know of technologies other than LoRa, with greater range that I can use?
 

Papabravo

Joined Feb 24, 2006
19,315
Hi Ya'akov

Thanks for the detailed reply. I will try to implement the suggestions you made.

About the question of legality. I know for example that there are radio telemetry modules used for drones that transmit data over 40km+ range, and I know these fall within legal limits. Even if you look at a DJI mini drone: it transmits video and telemetry over at least 7-10 km with what I believe is 2.4GHz RF.

If this truly is the legal power limit of RF transmitters, how are these examples possible? Do you maybe know of technologies other than LoRa, with greater range that I can use?
When it comes to RF propagation, unobstructed line of sight is everything. A drone at 400 feet AGL (Above Ground Level) has a huge advantage in terms of transmit range and receiver sensitivity. Whatever you can do with the antenna and receiver sensitivity is by far the best bang for the buck. Raising the transmitter power is the very las thing you should be doing because the payoff is minimal for the expense in the frequency range of interest. Some operators actually use rifle scopes for aiming antennas. There must be a reason for this. If possible choose the best weather conditions of cool dry air. Water vapor can scatter a microwave signal.
 

Ya’akov

Joined Jan 27, 2019
6,597
LoRa operates in the ISM bands (Industrial, Scientific, Medical) and bound by the regulations for operation in this unlicensed band. The ITU writes the regulations for Europe, and in the US it is the FCC that regulates the spectrum. The frequencies vary among countries and regions but are the subject of international agreements.

For example in Europe when using the ISM band frequencies (863 MHz - 870 MHz) users must comply to the following rules:

  • For uplink, the maximum transmission power is limited to 25mW (14 dBm).
  • For downlink (for 869.525MHz), the maximum transmission power is limited to 0.5W (27 dBm)
  • There is an 0.1% and 1.0% duty cycle per day depending on the channel.
  • Maximum allowed antenna gain +2.15 dBi.
[source]

It is very restrictive. The link above includes much more information from a LoRa perspective.

Another excellent resource is Andreas Spiess' YouTube Channel. He is an engineer (though not electrical as I understand it) and has been working with LoRa for a long time. I believe he holds the world's record for distance with LoRa. Here is a link to his LoRa playlist.



As far as the range of drones, which also operate in the ISM bands, as @Papabravo points out, drones are their own towers. If the drone is flying low to the ground its range will be just as bad as any other 2.4GHz device with the same limited power.

Telemetry can be operated in licensed bands and in that case it's a matter of the service they are operated in. Instead of the 25mW you are restricted to they may operate ten times that or more. The downside is the licensing which can be very burdensome and is certainly right out for IoT.

The benefit of LoRa is that it's designed for low-power long-distance operation. The engineers who made it traded data rate for distance and by designing the protocol specifically for that it's exceptionally good. LoRa has an enormous link budget of more than 140dBm.

Please read the documentation linked above and learn about the radio part of this. Unlike some wireless technologies, LoRa really demands you know something about the protocol and the propagation characteristics to use it well.
 

Thread Starter

JCL24

Joined Dec 12, 2019
9
Thank you both for taking the time to give me such detailed feedback. It is much appreciated. I will take a look at all the info you provided. I will also have to check out my local laws in South Africa and see what the limits here are in each frequency band. Thanks again.
 

Ya’akov

Joined Jan 27, 2019
6,597
Thank you both for taking the time to give me such detailed feedback. It is much appreciated. I will take a look at all the info you provided. I will also have to check out my local laws in South Africa and see what the limits here are in each frequency band. Thanks again.
Yes, while some things will be a matter of international agreement and convention, the exact details will vary. Whatever your max power output I am sure your module can do it so if it can be turned up for higher output than the default, go for it. But remember it does no good to be able to shout louder if you can't hear the answer so the real focus is the antenna and tuning the spreading of the protocol.

Please let us know how it turns out.
 

Janis59

Joined Aug 21, 2017
1,488
I love Lora inspite it is 10x expensiver than it`s nconcurent Nordic. Because it may serve from two AAA vells about year whilst Nordic month. And it gives a stable 3 km distance with PCB built in antenna, whilst Nordic with difficulty 1 km. BUT the price is high, the drastic loss of data rate. Lora 0.3...50 kBps depending on signal strengths and Nordic 24L up to 2 MBps.
 
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