Reconfigure 915 Mhz Radio to 868 Mhz (SiK, 3DR, HM-TRP)

I noticed that its possible to use the 915 Radios on 868.
Have successfully done this in the past and confirmed a range of 1.1 km with lowest power settings (1,25mW) and two “Inverted Vee” omni antennas from IBCrazy

Why? 915 cannot be used in the EU, 868 can. Lot smaller antennas then with 433.

Problem is that Mission Planner now restricts the range of frequencies that I can select in the mask so I can not do that anymore.
@Michael_Oborne can we change this in MP please?

  • Does one see any problem doing so?
  • Can we have a 868 Firmware to flash?
  • What exact setting should one use?
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Have you tried using the AT commands?
ATI5 shows all the parameters and among them are the MIN and MAX frequencies.
Using the ATSn=x command you can change the radio parameter n to value x. Then use AT&W to write the changes to memory. More detail here

Was wondering how well that would work, I expected the antenna matching circuit to be design for the specific frequency. Maybe 868MHz and 915MHz are close enough and the loss minimal.

868MHz are allowed in the EU but it’s the conditions which goes with it which makes it more tricky. Take a look at ETSI EN 300 220-1.
To be able to legally use it you have to turn the power down a lot (like you have already done) and conform to some polite spectrum access techniques (like LBT or limited duty). On top of that you have to stay within the allowed channel bandwidth which forces you to drop the air data rate a lot.

Yes, I’m unable to enter command mode via Serial monitor for unknown reason. Commands are received by the module and with MP config it is working.

I found the range with 868 is about 25% lower then with 433 under same test conditions and same antenna design principal.
Antennas were finetuned via a VNA.


Thats out of my competency :smiley:
Why do I need to drop the air data rate?
What settings do you recommend?

I think it’s often the case that serial monitor/terminal programs add a carriage return (CR) or line feed (LF) character when you send a command. Since the firmware expects only three plus characters ‘+++’ it often fails to enter command mode. I used a terminal program called “Termite” and first disable any extra characters then send ‘+++’ to enter command mode. After that then I enable CR again and continue issuing commands.

That makes sense, the higher the frequency the higher the propagation loss, so lower frequencies generally travel further. Sounds like you made some interesting tests already!

Here is the ETSI spec I had in mind:

but I have to eat my words. There seems to be a category (on page 22) which allows up to 500mW and does not place a restriction on the channel bandwidth. The only catch is that you either need to have less than 10% duty or use a polite spectrum access technique like LBT (Listen Before Talk, ETSI calls it Clear Channel Assessment) which the SiK radio can do. I guess this could be an option as long as there are not a lot of other 868MHz traffic in the area.

Typically as your air data rate increases so does the channel bandwidth. Using a better modulation scheme can again reduce the channel bandwidth but at the cost of sensitivity which translate to shorter range. So if you were limited to a given channel bandwidth then you would need to reduce the air data rate to comply.

Yes that was it. I’m using the Arduino Serial monitor. At first you have to set everything off and after entering command mode you need to switch it back on so the commands are accepted.

I was just curious how good my self built antennas are. And they are AWESOME :slight_smile:
Recommend Antennas from VAS aka IBCrazy aka Alex Greve

Can’t imagine that. But anyways can you give a recommended setting for the radio for 868? Min max freq + number of CH?
What should I set as LBT rssi? I guess a bit over the noise floor?

So I must use 869,400 MHz to 869,650 MHz with 25 kHz per Channel so 10 Channels and can transmit full power as long as I stay with 10% dutycycle?

So settings are:

 S8: MIN_FREQ=869400
 S9: MAX_FREQ=869650


Tried the settings and it works, but is very slow. Raised the duty to 100 again and now it’s acceptable speed. I only get max 1 Hz update rate.


Looks like things get very technical at this point. Would be nice if somebody from the manufacturers (eg. RF Design in Australia) or sellers of these links could provide more information how to legally use it.

From glancing over the specs it seems that the number of channels depends of the sub-band and bandwidth:

The RSSI level for LBT depends again on the ERP:

Below is a spectrum sweep of one of the RFD 868 radios (min/max freq was 868MHz to 870MHz using only 2 channels and with an air rate of 64kbps which is the default).

So now we can actually see how wide such a channel is!
Looking at the band for which 500mW is allowed, it is only 250kHz wide, meaning we could probably not even use a single channel. So maybe another band should be used at the lower power level then.

So I looked into this a bit more. In the RFD manual they claim to be EU complaint and give the following configuration details.

Frequency range: 869.525MHz - 869.85MHz (not matching the spec but maybe there is a typo or offset…)
Power: 27dBm
Number of channels: 2

They also have some footnotes saying that there might be some additional parameters which are locked. Would be interesting to know which ones and what value they should be. The radio I have was bought long ago and not EU locked. I thought I could try to flash the latest firmware, set it to EU and read back the parameters to find out. Will try that next week.

The above parameters then looks in general okay, I am just not sure about the number of channels. From the regulation it seems that if the channel is greater or equal to 100kHz (which I guess it would be) then we need at minimum 47 channels. This is just my interpretation, I could be completely wrong.
Does anybody else know how to properly interpret this?

In the previously posted spectrum plot the channels look very wide. Could be that it used a larger frequency deviation to improve performance and maybe this is another parameter they changed (lowered) when in EU mode.

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