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nrf24l01+ increasing distance


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I'm still playing around with the nrf24l01+ and was wondering if there were any tricks to increasing the distance? I'm running at 250kHZ mode and have tried the following combinations.

  • Chip antenna Master <-> Chip antenna Slave
  • SMA antenna Master <-> SMA antenna Slave
  • PA+LNA SMA antenna Master <-> SMA antenna Slave

The problem is none of these seem to get any more distance than the others. I was hoping that the PA+LNA master would give me at least another wall + 10 feet, but it seems to stop receiving at the exact same spot as the others.

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The only thing I know is that metal or water tends to kill the signal, think bushes in the way or metal doors...

Otherwise you have the longest-distance configuration for it, particularly with the PA+LNA version.

 

These do not inherently support mesh networking like ZigBee I think, but it's possible to design a custom repeater protocol and have a man-in-the-middle I guess.  More likely though I'd just recommend trying something else.  (That said, the cost factor of the nRF24 + MSP430 make littering your place with repeaters still an attractive idea...)

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I just tried out the PA+LNA as the transmitter (instead of receiver) and that does give me the extra range I need. Didn't try it before because the specs claim it needed 3-3.6V, but seems to be running at under 2.5 currently. Going to try to measure exact values of current usage to see where else the specs are wrong. Maybe I can just use some of these modules instead of any repeater stuff.

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Nice, good to know... I do know the TX power spec is much higher than the RX spec so maybe that has something to do with it.  Guess the LNA receiver amp is just amplifying the typical 2.4GHz noise in addition to the signal anyhow.  Would be interesting to do experiments to see how this changes by channel over distance.

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Replace the antenna with a wire dipole.  This will take some experimentation, but it's something you can do in an hour or so.

 

First, get two copper wires or rods that are 4cm long (each).  

 

Second, remove the antenna from your board.  It is probably a trace that you can scratch-off with a razor blade.  

 

Third, solder one wire to the feed point of the old antenna, and another to the ground plane closest to the feed point.  If you have a dremel with the grinder bit, use it on low-speed to take-off the solder mask.  Otherwise, you can scrape with a razor or use fine-grit sandpaper.

 

Four, build a dipole antenna 1cm away from your ground plane by extending the wires straight-out (in parallel) and then bending them into 180-degrees from each other.  It should look something like this image below, with 1cm feed and 6cm = lambda/2 (thanks, google images).

z1.jpg

 

Five, it's not over yet.  Very likely, there is some capacitive coupling happening between your PCB and this antenna.  It is making the resonant frequency lower than 2450 MHz.  It is also reducing the impedance of the antenna from 74 Ohm to less... although I'm sort-of counting on that because the NRF module is undoubtedly tuned to 50 Ohms.  Start trimming the wires.  Trim both the same each iteration (try 2mm).  Now test the range with a normal NRF board and take notes.  At some point, the range will be longest, and this is where you're getting the best resonance.

 

Final note: position your wire dipole in a vertical orientation.  This will give it toroidal radiation about the Z axis, which will be the best for earthly pursuits.

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  • 2 weeks later...

Tested out the PA+LNA module. Seems to work down to about 1.8-1.9 volts. In power down mode it + the 2452 I'm using consume about 1.6uA vs 0.9uA for the non PA+LNA module. Both of these are actually much better than I had expected.

 

Not really sure how to go about getting a good measurement of peak transmit current, but setting my multimeter to store 1k readings and scrolling through them the max I saw was around 50mA.

 

Still waiting to get a couple more sets so I can try both the RX and TX using one and see if it makes enough of a difference in my application to work.

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  • 4 months later...
  • 2 weeks later...

Has anybody done the antenna modification thing? I replaced the PCB antenna with just a piece of wire and noticed the range increased by 2 or 3 meters indoor. However, I don't have knowledge of tuning the antenna wire length. Any tips on that?

 

For a monopole antenna, the resonant length is 0.25 * wavelength.  The frequency is 2450 MHz, so the wavelength is: c/f, where "c" is speed-of-light and f is Hz.  In other words, 300000000 / 2450000000 = wavelength in meters.

 

A normal monopole has impedance of 37 Ohms, which will match reasonably well with your board.  However, you need to trim the antenna more than wave/4 because there is always capacitance and inductance in non-ideal wires.  It is causing the actual wavelength to be shorter than the free-space wavelength.  The best way to get your tuning optimized in with a vector-network-analyzer (VNA).  If you don't have one (I can't imagine you do), you can do it experimentally as described above.

 

The dipole configuration described above is a balanced antenna, so it is more reliable than a monopole is.  Monopoles depend on the grounding of the device, dipoles do not.  If your board has a small ground plane or bad grounding, the monopole will have poor performance.  So, I usually recommend half-wave (i.e. dipole) antennas if possible.

 

If you want to get really clever, you could try to improve the tuning by constructing a monopole "inverted-F antenna" (IFA), but I would hold-off on that -- maybe a second project.

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