nRF21540 FEM TX Power with ESB

RE: nRF21540 FEM TX Power with ESB

Glen M — Tue, 29 Jul 2025 16:02:05 GMT

Thanks very much for the reply, I'll keep after it!

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Sat, 26 Jul 2025 08:53:59 GMT

Dear Glen,

I saw your question come in and had a bit of time today to take a look. The Nordic / Zephyr platform is really impressive, and development is progressing rapidly. But the learning curve for this platform is very steep. I'm normally used to making big strides quickly, but with this solution, you quickly learn that slow and steady wins the race. It can be super frustrating to work with the Nordic setup at times. I also sometimes need to blow off some steam when something turns out to be undocumented or there's a bug in the SDK. Sometimes it feels like you're working with a beta product. I often end up diving into the SDK source code to really understand how things work. But once you finally get something working, it's also incredibly rewarding.

A tip I can give: take very small steps and document your own code and changes thoroughly. After a week, you'll have tried so many things that you might not remember what actually worked. Another tip: always start by building your solution using official Nordic DK hardware. Get everything working there first—it's easier to debug. Once that’s working, you can port the code over to your own PCB or a third-party module like Fanstel or Raytec. Nordic can offer less support when you use custom or third-party boards. Just last week, I found two bugs in the devicetree and config of a Raytec module. Took me a couple of days to figure out—just a really dumb bug. The code simply couldn’t run as-is, which makes me wonder how much third-party vendors actually test their modules.

I personally have nearly every Nordic board except the nRF5340_Audio_DK. But I can test things without hardware too. Without FEM, I was able to make a build immediately. But as soon as I activated the FEM via build config with CMake arguments: -DSHIELD=nrf21540ek, I got a build error. I looked into it and it's a bug. As soon as you specify the nRF21540EK shield, Zephyr tries to include the NPM1100 PMIC in the project, which is missing data in the devicetree: “undefined reference to __device_dts_ord_8”. I worked around it by using no-sysbuild. The issue is in the cpuapp part of the build, which remains empty in my ESB project but suddenly pulls in the PMIC from Nordic. After that change, I could build the project again. This allowed me to use the devicetree visual editor to find the PDN pin on the nRF5340_AUDIO_DK with FEM. It’s P1.13, which you need to connect to VDD.

In the new SDK 3.1, which should be released very soon, the bug related to MPSL FEM combined with the ESB protocol has been fixed. But you can also manually patch the current SDK if needed—see the PR: https://github.com/nrfconnect/sdk-nrf/pull/23190

Personally, I don’t think the nRF5340 is one of Nordic’s better SoCs. The dual-core setup is nice in theory but makes everything a lot more complex. The radio part is also pretty bad—it can’t output more than 0 dBm, whereas the old nRF52840 from 8 years ago does way better with 8 dBm. Thankfully, they now have the nRF54L15, which is a fantastic chip. It’s fast and has an excellent radio. So we're currently transitioning everything to the nRF54 series.

I’ve updated and re-uploaded my example code. See: ESB-RSSI (version 26-7-2025).zip on the right. I’ve also included the nRF5340_Audio_DK in it.

Best of luck with your project!

Best regards,
Herke Dekker

RE: nRF21540 FEM TX Power with ESB

Glen M — Tue, 22 Jul 2025 23:54:48 GMT

Hi Mr. Dekker - I know you are a user like me and not a Nordic employee but it seems like you've figured out a ton on this issue and I'm hoping you can point me in the right direction as I'm stuck.

We started with the nrf5340_audio_dk doing Bluetooth BIS broadcast audio and we spun a custom board with a Fanstel BT40N module (just an nrf5340 with an nrf21540 on a module) with a new codec. I'm using SDK 2.9.1.

I believe I've followed all of the instructions but my receiver barely sees the transmitter a couple feet away and it doesn't work reliably. I get a bunch of sync errors and it disconnects after a few seconds.

Based on what you found here I tried setting my pdn pin in devicetree to an unused pin and then I manually set the real GPIO pin for it to output high in my startup code, but it didn't fix my issue. I don't have physical access to this pin to just solder VDD to it as it's all inside the module.

Could you maybe post a zip file of that nrf5340 project that you got working, or do you have any advice? I think I have my devicetree stuff right and that my issue is in what CONFIG items to put in the app and net cores, though I could be wrong. My support ticket for reference is HERE

Thanks very much,

Glen

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Thu, 03 Jul 2025 14:34:17 GMT

Hello Asbjørn,

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-EK (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

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Wed, 02 Jul 2025 18:59:03 GMT

Hello Jean,

Thanks for your response. Great to hear that you’ve come to the same conclusion. It indeed works differently than described in Nordic’s documentation. The documentation is already quite lacking—especially with all the changes in the SDK over the past year, which are not always reflected properly.

I’ve updated my example a bit. It’s not very clean or well-programmed, but it does provide insight into how the ESB protocol is controlled and what the effects are of certain settings. Nordic’s standard ESB example is really minimal and doesn’t give you much of a feel for its possibilities.

I’m still struggling with reading the current raw TX power using the function nrf_radio_txpower_get(NRF_RADIO); and have implemented a less elegant workaround just to make it clear what power level is being used. At this point, it’s still unclear to me which TX power level the nRF5x uses to transmit and how much is added by the FEM. It seems the base level is 0 dBm and the rest is added by the FEM. I don’t want to spend too much time on this since it’s not critical, but still—another issue that doesn’t sit quite right in Nordic’s SDK.

I’ve also added a repeat function, so I can place the PTX at a fixed spot and walk around with the PRX to measure how far the signal reaches at a given dBm value. This is a nice way to determine real-world range.

I’ve re-uploaded the new ZIP file(see attachments on the right panel) for this case so others can benefit from it too. Let’s hope Nordic comes out with a fix soon for the PDN issue—since the PDN pin is not set to HIGH by default, you currently have to power it manually with VDD to get the nRF21540 working at all.

I also haven’t been able to get the nRF21540-EK working with the nRF7002-DK—this also seems to be a bug or driver issue. It does work on an nRF5340-DK, which has the same SoC. I’ll see if I can get to the bottom of it, or else Nordic needs to fix that soon as well.

Best regards from North Holland in the Netherlands,

Herke Dekker

RE: nRF21540 FEM TX Power with ESB

JPhilippe — Tue, 01 Jul 2025 16:41:16 GMT

Hello Herke,

I have also been working on the ESB + nRF2154 issue. Taking into account all of your previous comments I have come to the following conclusion:

Actually, it's as simple as pie!!! >>> When an nRF21540 FEM is present, the antenna power is determined by the parameter passed to the esb_set_tx_power() function; esb_set_tx_power(22) gives the best result.

Everything specified in prj.conf must be consistent for this to work.

I used your esb_prx application and modified esb_ptx slightly to confirm the result.

Best regards from Brittany in France,

Jean Philippe Glesser

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Mon, 30 Jun 2025 15:00:58 GMT

Hello Asbjørn,

I’ve been doing some further testing with the nRF21540 FEM. As I mentioned earlier, the nRF21540-EK does not work with the nRF7002-DK, but it does work fine with the nRF5340-DK. I’m still investigating what’s going wrong here, but it seems to be a driver or SDK issue, since I get a compile-time error as soon as I switch my board from the nRF5340-DK to the nRF7002-DK in my MPSL integration.

Today I came across another strange behavior, and I’m not sure if this is also an SDK bug. From what I understand from the documentation and DevZone articles, I configure the FEM TX gain using the option CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB=20, and then I set the base TX power on, for example, the nRF52840-DK using esb_set_tx_power(ESB_TX_POWER_8DBM);. These values together should form the total TX output power.

However, in practice, the output power from the nRF52840-DK alone seems about the same as with the nRF21540-EK connected, when I use esb_set_tx_power(ESB_TX_POWER_8DBM);. I can confirm this by checking the RSSI on my ESB-PRX, and I expected a stronger signal with the FEM.

Just out of curiosity, I tried increasing the TX power manually using esb_set_tx_power(10);, and I immediately noticed a better RSSI. I continued increasing it step-by-step, up to esb_set_tx_power(21);, and now I’m seeing about 20 dBm better RSSI, which is more in line with what I expected from the FEM.

I haven’t seen any documentation suggesting that values above ESB_TX_POWER_8DBM are valid for the nRF52840, since those enums don't even exist—but it turns out that manually passing a higher integer does increase the TX power. The RSSI at level 21 seems to reflect the added gain from the FEM as expected.

So my question is: am I using some undocumented functionality here? Is this a bug in how TX power is being applied? Or is Nordic in the process of changing how FEM power control works in the SDK 3.x series, and it's simply not documented yet?

Thanks in advance for your response.

Best regards,

Herke Dekker

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Fri, 27 Jun 2025 13:33:27 GMT

Hello Asbjørn,

Glad the bug has been identified. I can imagine that this issue only occurred when using the ESB protocol, which is why it might not have been immediately noticed—most people probably use standard Bluetooth by default.

FEM support on an nRF5340-DK and nRF7002-DK:
I’m currently testing FEM support for ESB on both the nRF5340-DK and the nRF7002-DK. On the nRF5340-DK it works right away, but I’m not getting any extra range, which is strange.

However, on the nRF7002-DK I can’t get MPSL to work. I’m just using the same example I used previously. It’s odd, because both boards use the same nRF5340 SoC. So I think I might have found another bug.

Thanks in advance,

Herke Dekker

RE: nRF21540 FEM TX Power with ESB

Asbjørn — Fri, 27 Jun 2025 13:07:34 GMT

Hello Herke,

Based on your experiences there will be an update to the nRF21540-EK out shortly with better figure and the part number for the cable that is needed.

Regarding the PDN bug, I will try to recreate this observation here and report it to our development team as a bug. It should certainly be corrected and not sure how it could have gone undetected for a few SDK releases.

Thank you for your feedback and I hope the evaluation goes better from here on out. I'll let you know when the update to SDK would be scheduled if you are interested.

Best regards

Asbjørn

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Thu, 26 Jun 2025 16:02:39 GMT

Hi Asbjørn,

I ordered an MXHS83QE3000 cable from Mouser, along with an ADP-SMAM-SMAM-G RF adapter. I couldn’t find another cable with a male SMA connector. I also ordered an extra nRF21540-DB Kit to rule out the possibility of a defective development board. I received the items from Mouser yesterday.

Today, I tested the setup using the cable between an nRF52840-DK and an nRF21540-EK. Unfortunately, the tests show no improvement compared to last week. I still observe around 10 dBm worse reception (my RSSI value is 10 dBm lower) compared to a standard nRF52840-DK board without a FEM.

As soon as I disable the MPSL driver, the reception immediately improves, and the values return to those seen in the standard nRF52840-DK setup.

As far as I can tell, the cable is connected correctly. See attached photo.

I also tested with a standard nRF21540-DK board, of which I now received a second one. On this board as well, the reception is so poor that I can only transmit ESB packets when the boards are just a few centimeters apart.

Working solution:
I continued testing and checked the pin connections between the nRF52840-DK and the nRF21540-EK. It turned out that the PDN pin was never activated, although it should be, as far as I understand. When I manually forced the PDN pin high by connecting it to VDD, I achieved good TX power on the spectrum analyzer. The RSSI value in my ESB example is now 25 dBm higher.

I did the same on the nRF21540-DK, where I previously had almost no output. After also forcing the PDN pin high on that board, it started working properly and I had very good 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 not a proper solution. I assume the hardware is functioning correctly, and—as I have been pointing out for over six months through multiple support requests—this confirms that there is indeed a bug in the Nordic Connect SDK versions 2.8.0, 2.9.0, and later.

Tip:
I recommend updating the documentation to clearly state that an additional cable is required to connect the nRF21540-EK. I also strongly suggest including this cable in the box by default, as it is quite difficult to find the correct one online. If you choose not to include a cable, please at least specify the exact type needed (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 TX Power with ESB

Asbjørn — Fri, 20 Jun 2025 13:44:21 GMT

Hello Herke,

I can feel your frustration and I'll be honest with you, I thought there would be such a statement in the documentation as well that the connection between J1 and TRX was needed. For some it might be obvious, but for others there's nothing indicating that it is needed. I'm sorry you've been struggling with this. I've made a request here to the documentation team to add the sentence about this connection for future updates.

Let us know how the cable works out, I can't see anything else that could be causing this.

Best regards

Asbjørn

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Fri, 20 Jun 2025 09:48:15 GMT

Hi Asbjørn,

Thank you for your response. I’m not sure whether I should feel relieved or completely frustrated. At the very least, I really appreciate your comment.

I’ve been struggling with the nRF21540 for two months now. I’ve already submitted multiple Q&A posts on this topic. At some point, I really started to lose hope. I work with the Nordic platform every day and feel like I have a pretty good grasp on it, but not being able to get the nRF21540 working has been incredibly frustrating. The cable doesn’t solve the issue I’m also experiencing with the nRF21540-DK — that board has a range of just 5 cm (not a typo).

I went through all the Nordic documentation again, including every relevant webpage I could find. Nowhere is it mentioned that you need to bridge the J1 port. I assumed that was only used for lab equipment, like when testing output power with a spectrum analyzer. It’s honestly bizarre that this isn’t documented anywhere. There’s even a schematic showing which pins to enable and which ports to use — but still, no mention of this. Why isn’t there a simple diagram showing how to connect this cable, or a sentence in the text like: "⚠️ Warning: A connection is required between your development board (J1) and this port to include the FEM in your RF path." Why doesn’t Nordic include the cable in the kit? You always need it, and the type of cable is quite specific. Yet even that isn’t documented anywhere.

I’ve now ordered the cable as well as an additional nRF21540-DB kit so I can also check whether the one I already have(nRF21540-DK) might be defective (max range 5 cm).

I should receive the items next week. I’ll test everything immediately and let you know if this solves the issue.

Thanks again!

Kind regards,
Herke Dekker

RE: nRF21540 FEM TX Power with ESB

Asbjørn — Thu, 19 Jun 2025 14:49:57 GMT

Hello Herke,

I would agree with you that the information related to the nRF21540 EK is scattered, in some aspects I think we could say the same about all Nordic documentation. It's not perfect and there's room for improvement.

However, on the pictures I noticed something, and I might be seeing the incorrectly, but the output from the nRF52840-DK that you have plugged the nRF21540 onto doesn't seem to be connected. There should be a cable between J1 on the nRF52840-DK and the TRX SMA connector on the nRF21540-EK.

Connector on the nRF52840 is a female type of MM8130-2600 connector onto the male SMA on the EK.

Best regards

Asbjørn

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Mon, 16 Jun 2025 09:23:31 GMT

Dear Asbjørn,
Sorry for my late reply — I was abroad for a while. Today I took some time to make a few photos of the test setup.
Here are the answers to your questions:

Q1: The firmware without FEM works fine.
Q2: I've taken several photos of the test setup — see attachments.
Q3: I have about 20 Nordic boards here, so I can try all possible combinations. I flash the firmware specifically for each configuration. So, in the case of an nRF52840-DK + nRF21540-EK, I enable FEM support in my build. I also see during the build that the nRF21540-EK is detected/activated. However, an nRF52840-DK has much better range without the external nRF21540-EK than with it. The same problem occurs when I attach the nRF21540-EK to an nRF5340-DK or an nRF7002-DK.

For the nRF21540-DK, the range is so poor that the nRF21540-DK only works when I physically hold it right next to another board.

Q4: To ensure a fair comparison, I test all boards at exactly the same distance. I’ve created test setups at 2m, 5m, and 10m distance. On the PRX side, I log all values so I can compare them easily afterward. For all tests, I now always use the same board on the PRX side — in this case, an nRF54L15-DK. I also tried with an nRF21540, but that didn’t help either and only made the tests more complicated. I’ve tried to keep the setup as simple as possible in order to present clear cases to Nordic Support.
Eventually, I also want to equip the PRX with an nRF21540 chip, but that’s the next step — first, I just want to get this part working.

I hope the answers and photos give you a better idea of my test environment. I also hope you’ll have some answers for us soon. We’re currently experiencing significant delays in our project, and at this point, that’s really no longer acceptable. I believe my colleague and I are quite experienced in software development and the Nordic ecosystem — we’re not beginners — and everything else in our project is running smoothly.

I genuinely think there’s a bug in the SDK; otherwise, we’re overlooking something massive — and if that’s the case, then it’s also not well documented. Aside from that, I find the documentation around FEM support to be rather poor — you have to gather bits of information from all over, and some things have already changed again in the latest SDK version.

Thanks in advance for your response.

Best regards,

Herke Dekker

devzone.nordicsemi.com/.../IMG_5F00_7867.MOV

devzone.nordicsemi.com/.../serial_2D00_terminal_2D00_04062025_5F00_140101-_2800_prx-nrf54L15_2900_.txt

RE: nRF21540 FEM TX Power with ESB

Asbjørn — Wed, 11 Jun 2025 14:40:01 GMT

Hello Herke,

Vidar asked me to have a look at your questions as the description of "signal so weak that I can only transmit packets if the PTX and PRX are placed directly next to each other". This is the same behaviour I would expect if for some reason the antenna on one (or both) of the boards was missing. The matching network and board itself will radiate enough that it will be transmit a very short distance.

Q1: The FW without the FEM works ok, if I understand you correctly?

Q2: Could you send a picture of the actual PTX and PRX boards and setup you are using?

Q3: Are you manually connecting the FEM between the non-FEM tests and the tests with FEM?

Q4: It sounds like you have several boards available, what RSSI readings are you actually seeing on the PRX when you are not using the FEM and at what distance would that be? How does this change when you add the FEM with same distance and location?

I suspect there's something with the connection that is causing this based on the description. If you are able to get packets through at very short distances with FEM and "normal" 10-20meter range when there's no FEM, with the same FW, I'd check the connections through the FEM. If you have a FEM on both sides, I'd debug with just one side first to see if it's the PRX or PTX to narrow it down.

Best regards

Asbjørn

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Sun, 08 Jun 2025 21:36:23 GMT

I’m taking the radio TX power of the nRF5xxx and the nRF21xxx into account — you can see that I sum these values later on. This is the only way to calculate the total TX power. I’m using a loop to test various radio TX power levels. On the PRX side, you can then see which TX power is needed to send ESB packets reliably over a certain distance. So you can increase the distance and see which nRF configuration allows successful packet transmission. This works well and is stable.

At the top of mail.c in the PRX example, you can also adjust the range within which you want to test. I’ve currently set it from 0 dBm to 10 dBm. I do check whether each TX power level can be used without causing errors during the test. For the FEM, I’m leaving the power at its default value in this demo, but it can be adjusted in fem.conf.

My point is that the range becomes worse when using an nRF21540, even though it’s supposed to improve the range. The actual TX power I calculate isn’t that important — the key issue is which packets actually arrive at the PRX. And that’s a real problem when I activate the nRF21540 FEM — both with the nRF21540-EK and the nRF21540-DK.

With the example I attached, you can clearly see this.

Now the question is: am I doing something wrong in how I control the nRF21540? Is there something off in the Nordic Connect SDK 3.0? Or am I dealing with multiple faulty Nordic development boards?

Thanks in advance for your response,

RE: nRF21540 FEM TX Power with ESB

Vidar Berg — Fri, 06 Jun 2025 12:19:45 GMT

This is the code I'm looking at:

And as far as I can tell, the line esb_set_tx_power() will only be reached when txpower is set to a value supported by the RADIO.

RE: nRF21540 FEM TX Power with ESB

H. Dekker — Fri, 06 Jun 2025 12:11:59 GMT

Hi Vidar,

thanks for your response. This is known to me — as you can see in my code, I add the FEM’s TX power also to the value. So I am taking that into total value. This is purely for reference, and I also include it in the ESB packet so that on the other side I can also see with which TX power value something was sent. But the main issue is that my RSSI value is lower when I activate MPSL.

RE: nRF21540 FEM TX Power with ESB

Vidar Berg — Fri, 06 Jun 2025 08:49:23 GMT

Hello,

Thank you for providing the sample projects. From the PTX project I see that you have implemented a function named tx_power_supported() which checks whether the requested output power is supported by the nRF RADIO. However, this does not apply when using the FEM, as the MPSL library will always attempt to achieve the requested output power by adjusting the RADIO TX gain, see TX power split.

Best regards,

Vidar