Maximum RSSI value of -42 dBm

[FormerMember]

Greetings,

During a connection between two nrf51422s running on S120 (2.1.0) and S110 (8.0.0) with a connection interval of 7.5 ms I get only a maximum RSSI of -42 dBm, even when the two antennas as almost touching. When taken further apart the RSSI decreases to a minimum of about -100 dBm.

The RSSI measurement is started by

sd_ble_gap_rssi_start(m_conn_handle, BLE_GAP_RSSI_THRESHOLD_INVALID, 0);

And in a routine in between the two connection events I get the RSSI by the call

sd_ble_gap_rssi_get(m_conn_handle, &rssi_val);

What could be the reason for this behavior of RSSI not increasing beyond -42 dBm? With bare metal coding for advertisements and scanners I've noticed RSSI greater than -30 dBm too.

Thanks and cheers!

[Øyvind Karlsen]

Hi,

Is this on the development kit or using a custom board?

The performance in what is almost guaranteed to be near field will give odd results. If it is a custom board it can be because the matching is poor. Also when you are holding metal object like the development kit close to the antenna you will affect the RF performance.

A minimum of around -100 to -80 dBm is sensible, as the nRF51 has a maximum sensitivity of -90dBm in 1mbps mode. The swing of +/- 10dBm is due to RSSI measuring inaccuracies.

Best regards,

Øyvind

[Luis]

Any update on this matter? I am seeing samer behaviour using nRF51DK and module Core51822, which I believe is very popular (for being so cheap) and I expect it to have a bit tunned/optimized antenna (although I could be wrong). When both antennas are closer than 10cm, I always get RSSI -43 dBm, and that is odd, don't you think? It does not matter if antennas are 10 cm away or touching, I get -43 rssi reading, very stable value inside that range. So I think it is saturating somehow?