NRF24L01+ range of only few meters

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I'm currently in the process of trying to figure out why one of my battery powered nodes isn't working very well with a newly purchased NRF24L01+PA+LNA module. I've shielded it as described in earlier posts. There is plenty of pure power in the batteries.

Just as in the case for @pkjjneal it works well only if I'm touching the antenna with a finger.

I decided to look inside the antenna:

As you see above, it starts with an antenna cable that leads to a half wave dipole (where the antenna element is of about 1/4 wave length) and there is a "sleeve balun" going back over the cable . I can find no ground plane though. The problem that I can see is that the antenna length is 27 mm where it should optimally be 28.8 mm.

Now, if I remove the antenna completely my node works quite well. At least better than if I use the original antenna unmodified.

If I insert a wire (DIY antenna made by a paper clip) extending 121 mm out of the antenna sockets forming a full wave length antenna things work great.

I also modified the original antenna so that the antenna element becomes 28.8 mm. It works a bit better but it's far from perfect and communication errors are shown in the log.

As the final test, I added a nice ground plane to the original antenna and now it works really perfect!

So what can I learn from this?

My test rig is an EasyPCB powered with batteries. I'm quite sure that my NRF24L01+PA+LNA module used in another scenario, for example connected to a Arduino Nano fed by a FTDI connector could actually work. It's because it would create a different environment. The antenna will work in relation to what it's connected to, shielding the grounding, and surrounding objects. Capacitive and inductive reactances are involved in mysterious ways.

Anyway. My conclusion is that the two original antennas I have are not working very good. They obviously have the wrong length just a little bit, and I'd say that the the way the antenna is constructed with the sleeve balun and no ground plane, doesn't seem to work well. At least not here.

I'll do some more experimenting with making my own antennas. Starting with taking apart an original antenna is not so bad, I can solder anything (solderable) onto the cable.

Cheers!

From wikipedia:

Sleeve balun
At VHF frequencies, a sleeve balun can also be built to remove feeder radiation.

Another narrow-band design is to use a λ/4 length of metal pipe. The coaxial cable is placed inside the pipe; at one end the braid is wired to the pipe while at the other end no connection is made to the pipe. The balanced end of this balun is at the end where no connection is made to the pipe. The λ/4 conductor acts as a transformer, converting the zero impedance at the short to the braid into an infinite impedance at the open end. This infinite impedance at the open end of the pipe prevents current flowing into the outer coax formed by the outside of the inner coax shield and the pipe, forcing the current to remain in the inside coax.