nRF52 DK BLE RSSI is much lower than Tx Power

[StevenW807]

I'm using nRF52 DK BLE. 

In the code, I used the following code to set the Tx Power = +4dBm. 

On my HUAWEI Android phone, I use the nRF Connect app to see the RSSI. My phone is only 15 cm away from nRF52 DK. However, the RSSI value is only about -35 dBm (see image). 

If I change the Tx Power to -8dBm, I could see the RSSI also reduces. It seems like my Tx power configuration code is correct.

Any idea why, please? Also, how I solve the low RSSI issue, please? 

Thank you

[ketiljo]

The RSSI value you get on your phone is the received power. RSSI = received signal strength indicator. Between the transmitter and the receiver there will be some loss that depends on the frequency and antenna gain mostly, called path loss. This is expressed in dB to make calculations easier. You can calculate the loss with online calculators.  

Due to the nature of radio waves, there will be reflections from objects near by. Depending on the placement of the transmitter and the receiver in relation to these objects, the direct signal and the reflected signal will cause multipath interference, both constructive and destructive interference. The RSSI value will then increase or decrease depending on the placement. It's normal to have more than 10 dB fluctuations in the RSSI due to multipath inference. 

Now, using RSSI isn't the greatest way of measuring output power as you will need knowledge of the antenna gain on the RX and TX, exact distance and an environment with no reflection. Especially the latter is hard to archive in practice. 

So the only way to know for sure if the output power is set correctly, is to hock up a spectrum analyzer to the test connector on the DK as described here: https://infocenter.nordicsemi.com/topic/ug_nrf52832_dk/UG/nrf52_DK/hw_rf_meas.html

[StevenW807]

My phone is only 10cm away from the nRF52 DK, I place them on the table. Tx = +4dBm, Rx=-35dBm, is it normal to get such a large BLE signal loss? 

I would like to use nRF Connect --> RSSI Viewer to see the BLE RSSI and channel (just cross-check with my phone).

Does that mean I have to buy another nRF52 DK: 1 DK is used to Tx BLE, another DK is used to Rx BLE?

Thanks Ketiljo 

[ketiljo]

What exactly are you trying to measure? Check if the output power is set correctly? If so, you either need to measure the TX power or you can measure the current draw. The latter may be difficult without a power analyzer.