Author Topic: RFM96/SX1276 Range Questions  (Read 1702 times)

Badger

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RFM96/SX1276 Range Questions
« on: November 05, 2017, 08:04:17 AM »
Hi,

I've started working with some RFM96 modules and communicating with them using a Microchip Pic for a point to point system around my house. The problem I'm having is that the range doesn't seem to be very good at around 3-4 meters.

I'm using the default values that the module has on start-up ie:
Carrier Frequency 434MHz
Bandwidth 125KHz
Coding Rate 4/5
Spreading Factor 128 chips
Pre-amble 8
PA Power 13dBm
etc.

I am only using a wire (17.2cm long) at the moment for an aerial, but I have seen some videos of people using just a wire and getting quite a good distance. I'm also just sending one packet to be received by another module at the moment. I've tried to turn the power up to 17dBm, but that didn't seem to make a difference.

What do you think the range should be using the standard default settings using a wire as an aerial? Before I go any further I wanted to know if I have missed something fundamental.

Also if I'm ever only wanting to send ~4bytes what would be the ideal settings?

Thanks for you help

Badger

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Re: RFM96/SX1276 Range Questions
« Reply #1 on: November 17, 2017, 12:16:05 PM »
Hi,

So I thought the problem was due to not having a good ground plane, so I've fitted one to the TX and RX side of the tests but this has not made a difference. I've also checked the PSU which I don't think is the problem.
Does anyone know what the issue could be, have I missed something obvious?
I've pretty much read through all of the forum and don't think I've missed anything that could help.

Thanks.

Kilo95

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Re: RFM96/SX1276 Range Questions
« Reply #2 on: November 17, 2017, 09:39:20 PM »
I get 16.46 cm for a 1/4 wave 434 MHz Antenna length.
http://www.csgnetwork.com/antennaevcalc.html

Feel free to double check me. I would just fold it down from the top to shorten it to 16.46cm and then twist that around the part thatís soldered to the board. Try that first. Thatís whatís most obvious to me as an Amateur Radio Operator

Badger

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Re: RFM96/SX1276 Range Questions
« Reply #3 on: November 18, 2017, 07:27:48 AM »
Hi Kilo95,

Thanks for your response, and the link to the calculator.
I've been using the paper from the HopeRF website to do the calculations for the 1/4 wave length:
http://www.hoperf.com/upload/rf/ANTENNAS_MODULE.pdf
Do you think this is incorrect?

I'll try your suggestions, and see if it makes any difference.

Thanks.

Kilo95

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Re: RFM96/SX1276 Range Questions
« Reply #4 on: November 18, 2017, 08:41:31 AM »
Good thing about folding it down instead of cutting it right away is that if Iím wrong, you can just unfold it. I think that may be part of the issue. If the antenna is too long, the SWR will be too high and the radio on the Moteino will cut power to protect itself.

Badger

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Re: RFM96/SX1276 Range Questions
« Reply #5 on: November 18, 2017, 12:09:45 PM »
It didn't make any difference  :(

Kilo95

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Re: RFM96/SX1276 Range Questions
« Reply #6 on: November 18, 2017, 03:05:24 PM »
It was worth a shot

Badger

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Re: RFM96/SX1276 Range Questions
« Reply #7 on: November 21, 2017, 05:35:01 PM »
Hi,

I'm starting to run out of ideas and now am looking at the RFM96 module.
Are there any counterfeit modules out there? If so how do you tell?

Thanks
« Last Edit: November 21, 2017, 05:37:12 PM by Badger »

perky

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Re: RFM96/SX1276 Range Questions
« Reply #8 on: November 21, 2017, 07:54:16 PM »
I'm not sure. What I do know is with the RFM69 you have to explictly turn on one or more of the TX RF amplifiers, if you don't you still get very low transmission distances due to RF leakage. That's only in the order of a metre or so though. Check your code and make sure the RF TX amp or amps are actually enabled if they exist for that module.

Mark.

Badger

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Re: RFM96/SX1276 Range Questions
« Reply #9 on: November 22, 2017, 10:36:49 AM »
Hi perky

Thank you for your reply, that sound exactly like the problem I'm having.

I've had a look at the RFM69 datasheet and it sounds like register 0x11 (RegPaLevel) is the one you are talking about where you have bits 5,6,7 for Pa2, Pa1, Pa0. From the datasheet Pa0 looks to be on by default (at +13db), or is this not the case?

This also seems to be the case for the RMF96 where you have register 0x09 (RegPaConfig), which looks to be on by default (at +13.2db)

Do you think that is correct, or have I miss-interpreted it (which is more than possible!)

Thanks

perky

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Re: RFM96/SX1276 Range Questions
« Reply #10 on: November 23, 2017, 02:48:43 PM »
Those are the registers. I remember getting caught out when I didn't have any RFM69 power amplifiers turned on, but I can't remember why it was in that state to start with.

I'm not sure how the module for the rfm96 is wired up or what restrictions there are, but the datasheet seems to have cut and pasted part of the full chip datasheet and may cover unimplemented functionality. You could try turning on PA_BOOST pin (bit 7 RegPaConfig) just in case the other amplifiers aren't used on the module (?).

Mark.

Badger

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Re: RFM96/SX1276 Range Questions
« Reply #11 on: November 26, 2017, 07:45:53 AM »
Hi Mark,

Thank you for your help, and insight with this and these modules.
After some digging I found some code which set the PA's up:
Code: [Select]
const u16 RFM96PowerTbl[4] = 
{
  0x09FF,                   //20dbm 
  0x09FC,                   //17dbm
  0x09F9,                   //14dbm
  0x09F6,                   //11dbm
};

0x09 being the register value, and then the setting. As you can see they all used the PA_BOOST pin. So I used 0xFC (17dbm) and it sprang into life!
So for anyone who is programming the modules from scratch, the default RegPaConfig will not work as the RFO pin is not connected! They use the PA_BOOST pin.

Thanks Mark again for your help.
« Last Edit: November 27, 2017, 05:34:46 PM by Felix »

perky

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Re: RFM96/SX1276 Range Questions
« Reply #12 on: November 26, 2017, 01:09:51 PM »
You're welcome, I'm glad you got it working. ;)