Understanding Signal Strength (RSSI): Why 'Full Bars' Is A Lie

Executive Summary

If your smart lock is "Offline" or your battery is dying in 3 weeks, the culprit is almost always RSSI (Received Signal Strength Indicator).

However, your app's "3 Bars" icon is misleading. RSSI is a negative logarithmic scale. A signal of -73dBm is not "slightly worse" than -70dBm—it is half the strength.

This guide explains the difference between Volume (RSSI) and Clarity (LQI) so you can stop buying extenders that make the problem worse.

The dBm Scale: A Logarithmic Cliff

RSSI is measured in dBm (decibels-milliwatts). Closer to 0 is better.

The "3dB Rule"

In radio physics, a change of 3dB represents a doubling or halving of power.

• Moving a hub from behind a TV to a shelf might improve signal from -76dBm to -70dBm.
• That +6dB improvement means 4x stronger signal energy.

RSSI vs. LQI: The "Volume vs Clarity" Trap

You check your Z-Wave dashboard (Hubitat/Home Assistant) and see your lock has -60dBm (Strong). But it still fails to unlock. Why?

Answer: LQI (Link Quality Indicator).

• RSSI (Volme): How loud is the signal?
• LQI (Clarity): How intelligible is the signal?

The "Noisy Room" Analogy

Imagine you are at a rock concert.

• Your friend screams in your ear (High RSSI / Loud).
• But the band is playing loud music (High Noise Floor).
• You still can't understand what he said (Low LQI / Bad Quality).

Smart Lock Reality: If your neighbor has a bad Wi-Fi router broadcasting on Channel 1, and your Zigbee lock is on Channel 11 (which overlaps), your lock hears the hub loud and clear (-60dBm), but the "Noise" destroys the data packets. High signals do not fix interference.

Protocol Thresholds (The Danger Zone)

Different radios have different sensitivities.

Wi-Fi Locks (Schlage Encode, August Wi-Fi)

• Safe Zone: > -65 dBm.
• Why?: Wi-Fi is high-bandwidth. It is complex complex modulation (QAM). It needs a very clean signal. At -70dBm, Wi-Fi radios struggle to maintain the handshake, causing them to wake up constantly and drain the battery.

Zigbee / Z-Wave (Yale Assure, Kwikset)

• Safe Zone: > -75 dBm.
• Why?: These are low-bandwidth, simple pulses (FSK/OQPSK). They can "hear" a command through static much better than Wi-Fi. A Zigbee lock at -75dBm is often more reliable than a Wi-Fi lock at -65dBm.

Troubleshooting Checklist

1. Check LQI, not just RSSI:

   • If RSSI is good (-60) but LQI is low (<100 on a 0-255 scale), you have Interference. Change your Zigbee/Wi-Fi channel.
   • If RSSI is bad (-80) and LQI is low, you have Distance/Obstruction. You need a repeater.

2. The "Human Body" Test:

   • Stand between the hub and the lock. Your body (70% water) blocks RF. If the signal drops to critical levels when you walk by, your margin is too thin.

3. The Repeater Fallacy:

   • Bad: Putting a repeater right next to the lock. (It receives a bad signal and repeats a bad signal).
   • Good: Putting a repeater halfway between the hub and the lock.

Related Tools

• Signal Strength Analyzer: Input your wall materials to predict dBm loss.
• RF Interference Sources: See why your Stucco wall is a Faraday Cage.

Frequently Asked Questions

Why does my battery die so fast?

Weak signal = Re-transmissions. If a lock has to ask "Did you hear me?" 5 times for every 1 time it sends a status, your battery life is cut by 80%. Fixing RSSI is the #1 way to extend battery life.

Is -100dBm better than -50dBm?

No. Remember it's negative. -50 is hot/strong. -100 is cold/weak. Think of it like temperature (in Celsius) below zero. -5 degrees is bearable; -40 degrees is deadly.

Does Z-Wave Long Range help?

Yes. Z-Wave LR uses 900MHz (vs 2.4GHz). Lower frequency waves are physically larger and pass through walls/furniture more easily. A 900MHz signal often has 10dB better penetration through stucco than 2.4GHz.