Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Strange behaviour of Nordic nRF24L01+ 2.4GHz RF module

Status
Not open for further replies.

paulr

New Member
Hi,

I'm using an extended range nRF24L01+ module and antenna, like this one:

**broken link removed**

I've made a small battery-powered transmitter using an Arduino and the module. I'm getting some weird results which are baffling me a little and I wondered if anyone could shed any light. Symptoms are as follows.

My TX circuit can send data a distance of 30m or so, which is all I need, BUT it only works when I hold it in certain ways. After some experimentation, I found the following two positions were most reliable at making it work:

a) Holding the antenna between finger and thumb about halfway to 2/3 from the tip to the elbow.

b) Lightly pressing my thumb over the nRF24L01+ chip, near the module's header.

Without doing this or something similar, data fails to transmit even when the two modules are right next to each other.

I had the circuit wrapped in a ziploc bag while I was trying this, so I know I was making no electrical contact between the circuit and my body, but I must have somehow been affecting the way the radio waves propogated.

I'd be very interested to know what's making it behave like this, and what I can do to maintain the range when it's enclosed in a plastic box and I can't touch it to make it transmit properly!
 
Your transmitter is doing exactly what all transmitters do..

Aerial design is a deep subject.. When you are holding the antenna, you are attenuating / increasing signal strength.

When I mess with these modules, I get the same results...However!! If one of my colleagues holds the antenna, it works fine... Try different antennas... Whip, 1/4 wave, ceramic... I get on well with the simple whip antenna.... When I use a 1/4 wave, I can't be in the same room.... Ultra weird..... Think of you body as a sponge..... An RF sponge...
 
The aerial I'm using is a 1/4 wave, supplied with the module so I assume it's not at fault.

I know RF design is a black art, but because the effect I'm observing is quite specific and dramatic (range goes from zero to 30m with fingers applied to the right place) I had assumed the effect could be replicated with a component or two. But without a better understanding of what my fingers are doing it's hard to know what to try.
 
RF can be funny stuff at times.

My TX circuit can send data a distance of 30m or so,
Does that mean that you have had it working over this distance, or is that info from a marketing hand-out.
Edit, I just read your last post which you made while I was typing.
You have had results at 30 metres.

a) Holding the antenna between finger and thumb about halfway to 2/3 from the tip to the elbow.
b) Lightly pressing my thumb over the nRF24L01+ chip, near the module's header.
These actions could (will) increase or decrease the transmit or received signal in an undefined manner.

Without doing this or something similar, data fails to transmit even when the two modules are right next to each other.
If the modules are adjacent to each other, it could be that the transmitter is overloading the reciever.
Try setting the bits which turn down the RF outout power, try -18dBm when the modules are next to each other on the bench.

Make sure that the transmiter and receiever frequencies are set to be the same.
Try running at the lowest data rate (250kbps) until you get the thing working.


JimB
 
Hi JimB, thanks for your tips, they were really helpful. I think I've solved my problem. Reducing the transmission rate to 250k as opposed to the default of 1M now gives me solid communications up to 30m, at which point I run out of garden to test it in!

I also noticed that removing the aerial altogether allowed transmission at ranges up to 10m or so even at 1MBPS.

Here's my guess at what was happening:

The module is susceptible to overloading if the signal is too strong. This prevents reliable transmission and looks very similar to a weak signal. Operating at a lower transmission speed makes it less susceptible to overloading.

Grasping the transmitter at the points I mentioned wasn't improving the signal like I thought, it was degrading it to the point that it was no longer overloading the receiver. Bringing the data rate down means the module no longer overloading, and range is greater than I was experiencing before because I no longer have to impede the signal to transmit successfully.

[UPDATE: Having done some more tests I'm not sure what I said here is accurate, I'll do some more tests and come back]
 
Last edited:
Paul

I will agree with that, sometimes, depending on the receiever design, it is very easy to overload the receiver with too much signal.
The simpler the receiever, the more susceptible it will be to overload by strong signals. This simple single chip transceiver must be considered as "simple".

JimB
 
After some more testing it seems I wasn't far off in my assessment before, but another factor that's hugely influential on signal strength, more so than data rate, is the orientation of the antenna in relation to the ground plane of the TX circuit. If I find out anything useful I'll post back. If anyone can point me to some good resources so I can educate myself in this area, I'd appreciate it.
 
A 1/4 \lambda monopole doesn't work very well unless it has a ground-plane (counterpoise) underneath it. In other words, to be effective, both the Tx and Rx antenna need to approximate a center-fed 1/2 lambda dipole in size. The ground plane on the PC board is only a tiny fraction of what a proper ground plane should be...

This is especially true at 315MHz or 433MHz; less so at GHz .

Frequently overlooked is the "aperture" of an antenna. A perfect 2.4GHz dipole antenna will radiate (or intercept) only a tiny fraction of what a 433MHz dipole would.
 
Here's a picture of my circuit. I took the plastic cover off the antenna to see what was inside. A similar antenna is shown next to the circuit.

I assume the thin insulated wire is the 1/4 wave part that emits the RF radiation, and the shiny metal shield below it provides some kind of ground plane. If this is the case, should the ground plane of the PCB also be critical to the circuit's operation?

The receiving antenna is the same.

TX circuit.jpg
 
That form of antenna is what is known as a sleeved dipole.
The thin wire is the quarter wave vertical element, and the metal cylinder is effectively a ground plane folded around the feeder coax cable
JimB
 
I'm a little confused, is my antenna a quarter-wave monopole in which the sleeve serves as a ground plane, or a half-wave dipole where the sleeve is one of the poles?

I found some discussion of similar-looking aerials here, in which the sleeve is described as a 'radiator'. It included the following, which could explain my experience with bending the aerial:

"the coax feedline really needs to extend in line with antenna for at least 1.5 to 2 wavelengths, to prevent distortion of the radiation pattern"
 
Last edited:
Looking out the window I can see a very LARGE version of your 'coaxial sleeve dipole' antenna on a neighbor's house. (much lower frequency) It is probably 1 wavelength above the ground.
 
Hi,
paulr asked: I'm a little confused, is my antenna a quarter-wave monopole in which the sleeve serves as a ground plane, or a half-wave dipole where the sleeve is one of the poles?
The quickest way to find this out is to measure the length of the thin wire on top of the antenna. At 2.4 GHz, one wavelength = 12.49 cms. So half-wavelength = 6.245 cms & quarter-wavelength = 3.12 cms. The problem with transmission range is very likely antenna related (I'm inclined to agree with MikeMI in post #8, since increased range when the body is in close proximity to the antenna is strongly suggestive of inadequate ground plane). Rather than belabouring this point to its ultimate conclusion, here are some links on improving the antenna to increase range:
https://www.hobbyking.com/hobbyking/store/uploads/820169487X569826X26.pdf which contains this helpful link: https://achuwilson.wordpress.com/2012/05/18/long-range-rf-link-using-nrf24l01-rf-transceiver/
Here is a link to the product datasheet, which gives no details about the antenna (tsk! tsk!) apart from the fact that its impedance is 15 + 88j (which implies that it isn't a halfwave dipole, whose impedance is ~75 ohms):
**broken link removed**
 
I just looked more closely at the Achu Wilson antenna site, listed in post #13. In his repository in the NEC file which he used to check out the impedance & radiation pattern for his biquad antenna. Although this can be (laborously) translated into antenna dimensions, doing so also requires some knowledge of the NEC acronyms. So I searched online for an alternative source for such an antenna. I believe that this will help:
https://sites.google.com/site/weefiproject/antennas-howto-1
which lists a number of 2.4 GHz antenna related links. The 6th link from the top is for the biquad. The 12th link from the top is for an easily constructed 2.4 GHz helical antenna, and so on.
 
Thanks for the extra info, I'll be taking a good look. In answer to your point ramuna, the insulated wire part of the antenna is 1/4 wavelength; the sleeve as you can see is slightly shorter than that.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top