ESL / nRF5340DK / nRF21540EK

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Fri, 21 Nov 2025 12:29:33 GMT

Hi,

Just to let you know that I swapped out the Molex antenna for this one: Linx ANT-W63WS5-SMA Blade WiFi Antenna with SMA Male Connector, WiFi | RS and I'm now getting decent RSSI levels. I don't know if it is an impedance matching problem or whether the Molex antennae are just of low quality and/or broken in some way. I did try two of them and they both behaved the same. (If I get time, I'll check them with a VNA).

Regardless, that seems to be this problem solved. Thanks for your help.

Scott

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Thu, 20 Nov 2025 14:23:21 GMT

Adding CONFIG_BT_CTLR_TX_PWR_ANTENNA=20 seems to have improved things but the performance with the shield is still worse than without, just not by as much.

The cable is this one: MXHT83QE3000 Murata Electronics | Connectors, Interconnects | DigiKey

I don't have access to a spectrum analyser today but will do tomorrow.

Thanks

Scott

RE: ESL / nRF5340DK / nRF21540EK

Kenneth — Thu, 20 Nov 2025 13:36:35 GMT

Looks like the latest update from you is missing the CONFIG_BT_CTLR_TX_PWR_ANTENNA=20.

Kenneth

RE: ESL / nRF5340DK / nRF21540EK

Kenneth — Thu, 20 Nov 2025 13:18:50 GMT

Hi,

I am really sorry but I have not gotten around to test this myself.

I am wondering if you can send me a picture of each end of the cable you have between the DK and FEM?

Also, if you are able to connect ANT1 or ANT2 to a spectrum analyzer directly for comparison that would be helpful. That is what I plan to do when I get around to it.

Kenneth

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Thu, 20 Nov 2025 12:09:12 GMT

I have now tried running the SDK v3.0.0 peripheral_uart sample app across the following hardware combinations:

- nRF5340DK, nRF52840DK, nRF52833DK, nRF21540DK,
- nRF21540EK (x2)
- Murata cable (x2)

In ALL cases, I see the same behaviour: i.e. significantly lower RSSIs when using the shield than without.

When building for the shield, I'm using the following setup:

-DSHIELD=nrf21540ek

CONFIG_MPSL=y

CONFIG_MPSL_FEM=y

CONFIG_MPSL_FEM_NRF21540_GPIO=y

CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB=20

CONFIG_MPSL_FEM_NRF21540_RX_GAIN_DB=13

CONFIG_MPSL_FEM_NRF21540_GPIO_SPI=y

CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB_POUTA=20

CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB_POUTB=10

The shield has power, and I can see the TX_EN and RX_EN lines toggling. I have Molex antennas connected to both antenna ports on the shield.

If I run the app on the nRF21540DK (built using the CONFIG_ items but not the -DSHIELD option) then the performance is no different to using one of the other DKs without the FEM.

Can someone from Nordic please get back to me? I'm sure the solution has to be something simple like a missing configuration item, but it really shouldn't be this hard to get an evaluation kit working out of the box.

Thanks

Scott

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Wed, 19 Nov 2025 14:42:15 GMT

Using the setup described above:

nRF52833DK only build:

nRF52833DK+nRF21540EK build:

If I run the nRF52833DK+nRF21540EK build but don't plug in the shield (i.e. use the on-board antenna), the results are no different to when the shield is plugged in. That might suggest that it's a configuration problem.

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Wed, 19 Nov 2025 12:34:16 GMT

In an attempt to rule out the hardware as the problem, I have now switched to the following setup:

nRF52833DK + nRF21540EK

SDK: v3.0.0

I'm running the peripheral_uart sample app.

-DSHIELD=nrf21540ek

CONFIG_MPSL=y

CONFIG_MPSL_FEM=y

CONFIG_MPSL_FEM_NRF21540_GPIO=y

CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB=20

CONFIG_MPSL_FEM_NRF21540_RX_GAIN_DB=13

CONFIG_MPSL_FEM_NRF21540_GPIO_SPI=y

CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB_POUTA=20

CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB_POUTB=10

I'm still seeing significantly lower RSSIs when using the shield than without. I've tried two different shields.

All that leaves is the antenna. I'm using this one: https://uk.rs-online.com/web/p/wifi-antennas/2035105.

Is there an antenna that Nordic recommend?

Thanks

Scott

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Fri, 14 Nov 2025 17:18:31 GMT

I've just come across this: RE: nRF21540 FEM TX Power with ESB

I've tried forcing the PDN pin high as per the link but it had no effect in my setup. Does the bug exist in SDK v2.5.0?

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Fri, 14 Nov 2025 16:35:10 GMT

Hi,

I'm still struggling to get acceptable RSSI levels when using the nRF21450EK. I've had a closer look at the GPIO signals on a 'scope and they align with what I would expect to see (e.g. the TX_EN coincides with the ESL subevent advertising interval) so I'm beginning to suspect that the problem is on the RF side.

Can I just confirm that the nRF21540EK is intended to be used with antennas and not just to be connected to lab equipment? (The Product Brief says it is, but the hardware description isn't clear: "The nRF21540 EK is a hardware platform used to monitor the performance of the nRF21540 RF front-end module (FEM) by connecting it to lab equipment or other radios with a SubMiniature Version A (SMA) connector."

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Wed, 12 Nov 2025 15:41:47 GMT

As a test, I tried running the peripheral_uart example on the nRF5340 with and without the FEM enabled (using the extra CMake arguments above). I see the same issue: really low RSSI values with the FEM enabled; this time measured using the nRF Connect app on my phone. With the FEM enabled, I'm seeing activity on the TX_EN and RX_EN GPIO lines, so it looks like the nRF5340 is at least trying to do something with the FEM.

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Wed, 12 Nov 2025 14:43:55 GMT

I've tried plugging in both USB cables and also adding the following to the prj.conf file used by the hci_rpmsg child app.

CONFIG_MPSL_FEM=y

CONFIG_MPSL_FEM_NRF21540_TX_GAIN_DB=20

CONFIG_MPSL_FEM_NRF21540_RX_GAIN_DB=13

CONFIG_BT_CTLR_TX_PWR_ANTENNA=20

These are the sort of RSSIs I'm seeing:

The Rx value is the RSSI of the ESL tag as seen by the Access Point. The Tx value is the RSSI of the Access Point as seen by the ESL tag (and communicated back to the Access Point). The Tx value seems to have improved a bit with the latest changes, but both values are well below what I see when using the nRF5340's onboard antenna:

RE: ESL / nRF5340DK / nRF21540EK

Kenneth — Wed, 12 Nov 2025 14:11:10 GMT

I will take a look, can you plug in both USB cables? I am wondering part of the problem may be caused by that.

Kenneth

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Wed, 12 Nov 2025 13:28:07 GMT

Thanks for the quick reply. I tried setting CONFIG_BT_CTLR_TX_PWR_ANTENNA=20 but that didn't help. The RSSI values are still really low.

The files requested are in the attached .zip file. Note: these were taken from the build\hci_rpmsg\zephyr folder rather than from the build\zephyr folder.

Thanks

Scott

devzone.nordicsemi.com/.../nRF21540EK.zip

RE: ESL / nRF5340DK / nRF21540EK

Kenneth — Wed, 12 Nov 2025 12:24:54 GMT

Hello,

What have you set CONFIG_BT_CTLR_TX_PWR_ANTENNA to? Try 20.

If that doesn't help, maybe you can share the following files from your build folder:
\build\<name>\zephyr\.config
\build\<name>\zephyr\zephyr.dts

Maybe also upload a picture of your hardware setup.

Kenneth

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Wed, 12 Nov 2025 11:45:59 GMT

A further update. I can now see activity on the GPIO lines, so I think the FEM is 'working'. However, the RSSI values that I am seeing at either end of the connection are still much lower than I see when using the nRF5340DK's built-in antenna.

This is the antenna I'm using:
Molex 212860-0011 Whip WiFi Antenna with SMA RP Connector, Bluetooth (BLE), WiFi | RS

(I have one attached to both of the ANT connections on the FEM.)

Scott

RE: ESL / nRF5340DK / nRF21540EK

Scott Grubb — Tue, 11 Nov 2025 17:43:05 GMT

I'm not sure what I did differently, but the following CMake arguments are now generating the .overlay and the .conf settings for the network MCU:

However... the shield still doesn't seem to be working based on the RSSI readings at either end of the connection, which have dropped by ~30dBm compared to when using the onboard antenna on the nRF5340DK. I'm using the Murata cable specified by Nordic and the physical connections seem solid. Is there something else I need to do in the source code to enable the FEM? The TX_EN, ANT_SEL, RX_EN and MODE pins are all low.