nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Sigurd — Wed, 30 Jul 2025 12:29:05 GMT

[quote user="H. Dekker"]I’m running into another issue.[/quote]

Please create a new case, for this new issue.

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Wed, 30 Jul 2025 07:35:21 GMT

Hi Kazi,

I still need to test the fix you’ve made, but the wired connection is working great. It looks like SDK 3.1 will be available very soon as well.

I’m running into another issue. I’m trying to connect the nRF7002-DK with the nRF21540-EK shield, but as soon as I combine the two, I get a build error (note: the nRF5340-DK + nRF21540-EK setup works fine). Something seems to go wrong in the devicetree. Could you help me resolve this issue as well?

If this setup works, I’ll be able to build a gateway with excellent range using ESB or Bluetooth, and forward the data over Wi-Fi to a central computer.

Thanks in advance,

Herke

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Sat, 26 Jul 2025 09:08:23 GMT

Hi Kazi,

Thanks for the update. I’ll go ahead and test the fix.

I also think it’s very important that you update the documentation — it doesn’t reflect the latest SDK versions at all. As far as I can see, there haven’t been any improvements yet. For example, the documentation around configuring TX power or the required physical connection on the board using the MXHS83QE3000 cable along with an ADP-SMAM-SMAM-G RF adapter is still unclear. Nordic is really making it unnecessarily difficult to work with the nRF21540-EK this way.

I’ll follow up shortly to confirm whether the fix you provided resolves the issue. In any case, directly connecting the PDN to VDD worked well, and we were able to carry out our tests successfully.

Best regards,
Herke

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Kazi Afroza Sultana — Fri, 25 Jul 2025 12:20:07 GMT

Hello Herke,

ESB+FEM issue seems resolved with PR https://github.com/nrfconnect/sdk-nrf/pull/23190, but it seems like maybe it was tested with the 3rd party FEMs, not with nRF21540.

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Thu, 03 Jul 2025 14:38:09 GMT

Hello Kazi,

I’m curious to know if you’ve been able to raise the issue with development yet? I really think something needs to be done about the documentation—it doesn’t match the current situation in several areas and is also very sparsely described.

I’ve been busy testing over the past week. What I found is that the TX output power has to be increased in a different way than what’s documented. I’m currently setting the power using:
esb_set_tx_power(22);
or with
esb_set_tx_power(CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB);
If I don’t do this, the range is less than or equal to the situation without a FEM.

I’m now getting really good range with the nRF21540 FEM. I did an outdoor test today using an nRF21540-DK on the PTX side and was able to reach a distance of over 500 meters with ESB at 1 Mbps and FEM support at 22 dBm. There was no direct line of sight—many trees and even houses were between the transmitter and receiver. The PRX receiver was a basic nRF54L15-DK.

There is still something strange on the PTX side with the nRF21540: the ESB_EVENT_TX gives a failed event at just 5 to 10 meters distance, even though my PRX is receiving the packets correctly. I don’t see this issue when I use, for example, an nRF54L15. So apparently, the nRF21540 is still not properly receiving ACK packets due to insufficient receive gain or configuration.

I also couldn’t get it working on the nRF7002-DK + nRF21540-DK (while it does work on an nRF5340-DK, same SoC). Something goes wrong during the build process—so that’s another bug.

I’ve updated my test example again; see the attachment on the right side (“ESB-RSSI (version 3-7-2025)”).

In short, there are several bugs in the ESB module and also undocumented functionality. This makes it extra complex to work properly with ESB. But if you fix these bugs and update the documentation, it will be a very nice protocol to work with.

Thanks in advance for your response,

Herke Dekker

Sample: devzone.nordicsemi.com/.../3771.ESB_2D00_RSSI-_2800_version-26_2D00_7_2D00_2025_2900_.zip

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Kazi Afroza Sultana — Tue, 01 Jul 2025 16:21:38 GMT

[quote user="H. Dekker"]this confirms that there is indeed a bug in the Nordic Connect SDK (versions 2.8.0, 2.9.0, and newer).[/quote]

Yes you are right. There might be an issue in esb_fem_for_tx_set() function - this requires further debugging, I think. It seems PDN is not set by the driver. We are working to fix this.

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Thu, 26 Jun 2025 16:14:45 GMT

Hi Kazi,

When I manually forced the PDN pin high by connecting it to VDD, I achieved proper TX power on my setup using the nRF52840-DK with the nRF21540-EK. The RSSI value in my ESB example increased by 25 dBm.

I applied the same workaround on the nRF21540-DK, which previously had almost no output. After forcing the PDN pin high on that board as well, it started functioning correctly, and I observed excellent reception on the PRX side.

Conclusion:
The current workaround is to manually force the PDN pin high by connecting it to VDD. However, this is clearly not a proper solution. I believe the hardware is working correctly, and—as I have been pointing out for over six months across multiple support requests—this confirms that there is indeed a bug in the Nordic Connect SDK (versions 2.8.0, 2.9.0, and newer).

Documentation Tip:
I recommend updating the documentation to clearly state that an external cable is required to connect the nRF21540-EK. There is currently no mention or image explaining this. For example, a warning like:

"Warning: A connection is required between your development board (J1) and this port to include the FEM in your RF path."

I also strongly suggest including this cable in the box by default. It is quite difficult to find the correct one online. If you choose not to include it, please at least specify the exact cable type (brand and part number).

Question:
Can Nordic fix this bug in the Nordic Connect SDK as soon as possible?

Thanks in advance,

Herke Dekker

Photo 1: nRF52840-DK + nRF21540-EK (with wire connecting PDN to VDD)

Photo 2: nRF21540-DK (with wire connecting PDN to VDD)

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Kazi Afroza Sultana — Tue, 24 Jun 2025 08:18:24 GMT

Hello,

You don't need to do any changes on the PRX code.

I did some test in the lab by watching the improvement of TX output on the spectrum analyzer. Because -60 to -73 dbm (which we saw in the putty output as RSSI) is not optimal at all. So, the idea was to check the TX output. TX output power did not show any improvement on the spectrum analyzer which proved some pins of the Pin did not work.

I checked the FEM pin (PDN, TX, RX and PDN) on the nRF21540 if those were working properly. When we forcefully made the PDN pin up by connecting the pin to vdd then I got the tx value on spectrum analyzer. The RSSI value

You can try to do the same on your nRF21540 DK.

I also have made little changed in the code. Hvae set CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB=20 in the prj.conf file. Now esb_set_tx_power(20) function is declared as an error return after some activities (clock_start(), dk_kleds_init(), esb_initialize()) and

Got this as output on the spectrum analyzer which proves PDN is working(after this forcefully made it up)

and improved RSSI value

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Thu, 19 Jun 2025 14:44:10 GMT

Hi Kazi,

I did exactly what you did. I upgraded from SDK 3.0.1 to SDK 3.0.2 to eliminate any potential differences there. I also properly took the ESB examples from the v3.0.2 folder.

I'm not entirely sure why you added the RSSI code on the PTX side — I’ve placed it on the PRX side in my case.

I also assume that FEM support (MPSL) needs to be enabled in prj.conf. I've added that as well and tested it both with and without.

I tested thoroughly with esb_set_tx_power(10), esb_set_tx_power(8), and esb_set_tx_power(0). For me, this makes no difference in the RSSI values. In all cases, I can only keep my PTX and PRX boards about 5 cm apart — otherwise, the signal does not come through on the PRX side. I consistently get RSSI values around -83 to -84 dBm.

When I use a standard nRF52, nRF54, or nRF54 board from Nordic, I do get proper reception, and I can place the boards about 10 meters apart without issues. So something is definitely not right — I should be getting better reception, not significantly worse.

I'm really curious what’s different in your setup compared to mine. Could you show me your setup and maybe zip and share your project code?

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Kazi Afroza Sultana — Thu, 19 Jun 2025 13:36:09 GMT

Hello Herke,

''Thanks for your response and for testing. I understand that you're using the nRF21540-DK for the ESB PTX and the nRF52840 (with the nRF21540-EK?!?) on the ESB PRX receiving side.''

without the nRF21540EK.

For exb_prx, this is the default sample from NCS (C:\ncs\v3.0.2\nrf\samples\esb\esb_prx).

For esb_ptx, I have added the code in my previous comment what you need to add in the main.c file of esb_ptx sample.

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Thu, 19 Jun 2025 11:26:50 GMT

Hello Kazi,

Thanks for your response and for testing. I understand that you're using the nRF21540-DK for the ESB PTX and the nRF52840 (with the nRF21540-EK?!?) on the ESB PRX receiving side.

Could you please share your project as a ZIP file for both the PTX and the PRX parts? I have the same hardware here, so I can replicate everything exactly and run the same test.

I’ll share the test results with you afterwards.

Thanks in advance,
Herke

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Kazi Afroza Sultana — Thu, 19 Jun 2025 11:12:16 GMT

Hello Herke,

I have tested with two DK, one is nrf21540Dk for running the sample esb_ptx and nRF52840 Dk to run the sample esb_prx.

The samples are default from NCS, I only have added tx power setting fucntion in the beginning of main() function.

esb_set_tx_power(10)

the Case ESB_EVENT_RX_RECEIVED looks like this

case ESB_EVENT_RX_RECEIVED:
		while (esb_read_rx_payload(&rx_payload) == 0) {
			LOG_DBG("Packet received, len %d : "
        "0x%02x, 0x%02x, 0x%02x, 0x%02x, "
        "0x%02x, 0x%02x, 0x%02x, 0x%02x, rssi %d",
        rx_payload.length, rx_payload.data[0],
        rx_payload.data[1], rx_payload.data[2],
        rx_payload.data[3], rx_payload.data[4],
        rx_payload.data[5], rx_payload.data[6],
        rx_payload.data[7], rx_payload.rssi);
		}
		break;
	}

This is with TX power is set,

This is without 8dbM TX power

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Thu, 12 Jun 2025 21:15:21 GMT

Hi Kazi,

Great that you’re testing this on your end. I'm curious to hear whether you experience the same issue. I've encountered the problem both with the nRF21540-DK and with the nRF21540-EK connected to either an nRF52840-DK or an nRF7002-DK.

Thanks in advance,

Herke Dekker

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Kazi Afroza Sultana — Thu, 12 Jun 2025 13:53:14 GMT

Hello Herke,

Sorry for the late reply. We would try option 1 as first step - verify power levels locally on a setup with FEM. I will let you know the test outcome later.

Thanks.

Kazi

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Tue, 27 May 2025 16:35:36 GMT

Dear Kazi,

Thank you for your quick response and for answering the five questions from my initial message. These points are now clear to me. At this point, I practically know the referenced documentation by heart.

We’ve been working daily with the ESB protocol for several months now, and our project is progressing well. We have full control over the ESB part — with the exception of the FEM module, which remains problematic.

To keep things as simple and isolated as possible, we are using the official ESB example from your SDK as the basis for our tests — specifically the esb_ptx and esb_prx samples from nRF Connect SDK v3.0.1. To evaluate signal strength and the effect of transmit power, we made only minor changes to the sample. We added a loop that cycles through all supported TX power levels for the given board and sends that to the PRX side. This allows us to easily test various Nordic development boards, which we have in large supply.

Here’s the issue:
As soon as we activate the FEM module by enabling the appropriate MPSL options in prj.conf and setting -DSHIELD=nrf21540ek, or by selecting a board with integrated FEM like the nRF21540-DK, the transmit power immediately drops, even when we explicitly set it to the maximum via esb_set_tx_power(). The resulting RSSI on the receiver side is consistently worse.

Option 1:

Could your team try the default esb_ptx and esb_prx samples with FEM enabled on your end, and confirm whether higher TX power levels actually lead to improved RSSI? If so, would you be willing to share the exact example setup that works?

Option 2:

Alternatively, we can share our modified esb_ptx and esb_prx samples with you, so you can test them directly and see if anything in the code or configuration might be preventing the FEM from operating correctly.

Depending on your findings, we’re also happy to review the documentation related to FEM + ESB together to check for possible inaccuracies — or even a potential bug in the SDK.

Kind regards,
Herke Dekker

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

Kazi Afroza Sultana — Tue, 27 May 2025 15:46:22 GMT

Hello,

I have feedback from team

Answering the questions:

Q1: No, the FEM module in MPSL can work without the MPSL scheduler and this is how it is used in the ESB, so no separate initialization is required in the application code.

Q2: This relates to Q1 a bit. Since no MPSL scheduler is started, there will be no specific log from MPSL if only the FEM module is used

Q3: It should be done through the esb_set_tx_power() function: https://docs.nordicsemi.com/bundle/nrf-apis-latest/page/group_esb_ga63a3c3e672521b5d7646f45331395e95.html#ga63a3c3e672521b5d7646f45331395e95

Q4: The FEM driver integrated sets up the LNA automatically. You just need to build the application with the nRF21540 EK as described in https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/app_dev/device_guides/fem/21540ek_dev_guide.html.

Q5: FEM are supported in ESB through the integration with the FEM driver in MPSL however without running the MPSL scheduler. We don't have a dedicated sample to show such setup. In theory the existing samples, when built for a configuration with FEM e.g. on nRF52840 DK + nRF21540 EK should work out of the box.

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9 / 3.0.1)

H. Dekker — Mon, 26 May 2025 11:14:17 GMT

Dear Kazi,

I understand that if I don’t configure additional TX power, I shouldn’t expect significantly better performance.

I’m not new to Nordic Semiconductor products. I’ve been working with them daily for over a year now. We've already built several interesting projects and even developed our own custom boards, so I believe I have a solid understanding of how things work.

The only area I’m really struggling with is FEM support. I’ve tested everything again using the ESB samples with Nordic Connect SDK 3.0.1 (we’ve migrated all our internal projects to the latest SDK). However, I just can’t get it to work properly.

As soon as I enable MPSL in my project (as described in the documentation, including your note about CONFIG_MPSL_FEM_NRF21540_RUNTIME_PA_GAIN_CONTROL=y) and correctly set the board target to something like the nRF52840 + nRF21540-EK (via the CMake flag -DSHIELD=nrf21540ek), the transmission range drops significantly. I’ve also tried adjusting the TX power settings, but the range remains very poor. I tested with the nRF21540-DK as well and encountered exactly the same problem.

When I build the ESB-PTX sample with the default settings for the nRF52840-DK (without FEM), the transmission range is noticeably better.

I should also mention that the documentation for FEM support in the ESB sample isn’t entirely accurate. If you're using a standard nRF21540 or any nRF device with the nRF21540-EK front-end module, there’s no need to manually create an overlay file—this is handled automatically unless you're using a custom board. The current documentation is a bit confusing in that regard. Additionally, there's no clear guidance on what project or code settings are required to actually enable FEM support and benefit from the increased TX and RX range.

I’m seriously starting to doubt whether FEM support is working properly with the ESB protocol. Could you please test this yourself using the standard ESB sample, a nRF21540-DK, and an nRF device with the nRF21540-EK? I’d be very interested to hear whether it works correctly on your side.

It’s possible I’m overlooking something small, but I’ve read nearly every Q&A thread on DevZone related to FEM support and ESB. If you do manage to get it working, could you please share your (modified) ESB sample with me and others on DevZone? Also, please consider updating the documentation to make this process clearer for everyone else. The many changes to the SDK over the past two years haven’t made things easier—especially when trying to follow older DevZone threads.

I really hope this issue will now be taken more seriously and addressed more quickly. It’s been almost two months, and I still haven’t received a solid answer.

Thanks in advance for your feedback and example.

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9)

Kazi Afroza Sultana — Wed, 07 May 2025 13:31:47 GMT

Hello,

Enabling any FEM into the build by itself will not automagically cause that the greater output power is used. This is true for all protocols. If the protocol requested for example 0 dBm then with FEM it will be still 0dBm but achieved in a different way. The protocol driver needs to ask for more power.

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9)

H. Dekker — Thu, 24 Apr 2025 07:57:51 GMT

Hi Kazi,

Thank you for your response, and no worries about the delay – I understand the Easter break may have caused that.

Regarding your suggestions: I’ve already gone through all of the steps you mentioned, and I’ve been testing them extensively over the past few weeks. I have several Nordic boards here, including the nRF52840-DK, nRF5340-DK, and the nRF21540-DK kit. I've been struggling for quite some time to get FEM support working reliably with the ESB sample. At this point, I could probably recite most of the documentation by heart, including the link you provided.

Unfortunately, despite all of that, I still don’t see any improvement in either transmit or receive range when using the nRF21540. To me, the FEM support still feels very opaque. Besides the build-time confirmation (which I already mentioned), there's no clear indication that the FEM is actually doing anything. It would be incredibly helpful if there were some visual confirmation – such as a log message or LED – that shows the FEM is active, but even with full debugging enabled, I get no feedback.

My primary goal is to improve reception range, as I plan to integrate the nRF21540 FEM into a gateway device, while keeping the client devices battery-friendly by using a standard nRF52840 or nRF5340. However, right now it feels like the nRF21540 FEM (either with the nRF21540-DK or the nRF21540-EK) is doing absolutely nothing in ESB mode with NCS 2.9 – unless I’m missing something crucial?

It really shouldn’t require rocket science to get improved signal strength out of the ESB sample using the nRF21540. Once I can get it working there, I plan to integrate it into our own software solution that we’re developing.

Would it be possible for Nordic to run a few internal tests using some typical setups? For example:

PTX: nRF52840, PRX: nRF52840 + nRF21540-EK
PTX: nRF21540-DK, PRX: nRF52840 + nRF21540-EK

If these setups work as expected on your side, maybe you could share your modified sample so I can test the same setup locally. That would be very helpful.

Also, if you are planning to run internal tests anyway, I would strongly recommend using nRF Connect SDK v3.0.0 instead of 2.9. I've been testing on 2.9 myself, but if there’s going to be an updated blog or sample, it makes more sense to use the latest version so it remains future-proof for others.

Thanks again in advance,
H. Dekker

RE: nRF21540 FEM Not Improving Range in ESB Project (NCS 2.9)

Kazi Afroza Sultana — Wed, 23 Apr 2025 12:05:55 GMT

Hello,

I am trying to answer your questions here. I am sorry for the late reply as we had easter vacation on last week.

''Q1: We are only using ESB (not Bluetooth). Do we need to explicitly initialize MPSL in our code to enable FEM functionality?''

If you want to explicitly set MPSL then you need to set CONFIG_MPSL_FEM=y in the prj.conf file.

Q2: Should I see additional log output at startup when MPSL is active? Right now, this is all we see:''

Log does not show any indication of FEM activity.

Q3: How can we properly increase the TX output power? We’ve come across many examples, but are unsure which ones are correct for SDK v2.9.

You may need to enable runtime PA gain control for controlling TX power with the nRF21540 FEM.

CONFIG_MPSL_FEM_NRF21540_RUNTIME_PA_GAIN_CONTROL=y

Q4: How can we improve reception quality? Ideally, we want to use the nRF52840 + nRF21540-EK with an external antenna. Do we need to manually activate the LNA for this?

You can go through this

https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/esb/esb_ptx/README.html#fem_support

Q5: It would be very helpful if the ESB documentation or samples included more detailed instructions about FEM configuration, especially regarding what is required vs optional. A lot has changed in the SDK over the past years, and many online examples no longer work with SDK v2.9.

I do agree with you. I will talk about this to the respective team.

Thanks.

Kazi