If you’ve ever looked at APRS LoRa logs and seen cryptic numbers like -127 and -12.5, you might’ve asked yourself, “Is that good or bad?” Don’t worry – you’re not alone. In this post, I’ll walk you through what RSSI and SNR actually mean, how they affect your APRS system, and what you can do to improve them.

🔍 What Is APRS with LoRa?

APRS (Automatic Packet Reporting System) is a digital communication system used by amateur radio operators to send real-time data such as location, weather, messages, and more. When combined with LoRa – a long-range, low-power radio technology – it becomes a powerful way to send packets over large distances using very little power.

You’ll typically see these packets logged like this:

03:03:03  9W2BKF-8>APLRT1,WIDE1-1:!/M!GTh1eR(96Q   -127   -12.5

🧠 Breaking Down the Line

📶 What Is RSSI?

RSSI stands for Received Signal Strength Indicator. It measures how strong a signal is when it reaches your receiver, expressed in dBm (decibels relative to 1 milliwatt).

• Closer to 0 = stronger signal
• More negative = weaker signal

So if you’re seeing -127, it means your LoRa module barely managed to pick up that packet – it’s right at the edge of detection.

🌐 What Is SNR?

SNR stands for Signal-to-Noise Ratio, measured in dB. It compares the level of the signal to the level of background noise.

• Positive SNR = Good
• Negative SNR = Bad

For example:

• An SNR of -12.5 dB means the signal was 12.5 decibels below the noise floor. That’s not great.
• A positive SNR, like +5.5 dB, means your signal was clearly above the noise and much easier to decode.

🔧 How to Improve RSSI and SNR

If your signal quality isn’t ideal, don’t panic. Here are some practical tips:

📡 Improve Your Antenna

• Use a higher gain antenna
• Position it as high as possible
• Ensure it’s vertical and unobstructed

🧵 Use Better Cables

• Use low-loss coaxial cables
• Keep cable runs short

🏡 Change Your Setup

• Move your node away from interference (metal walls, routers, etc.)
• Try an outdoor enclosure

🔁 Try Different Spreading Factors (SF)

If you’re building LoRa nodes, changing the spreading factor in your LoRa configuration can affect both range and reliability.

🌀 What Is Spreading Factor (SF) in LoRa?

Now, let’s talk about a hidden hero: Spreading Factor, or SF.

🧭 What Is It?

Spreading Factor controls how long each symbol (bit of data) is spread over time on the air. It directly impacts:

• Range
• Transmission time
• Reliability

The higher the SF:

• The longer the signal stays on the air (better decoding in noise)
• The greater the range (good for distant nodes)
• But also slower data rate (more air time per packet)

🎛️ Do Bandpass Filters Improve Signal?

Yes – in the right situation, a bandpass filter can significantly improve signal quality in LoRa-based APRS systems.

🧠 What Is a Bandpass Filter?

A bandpass filter is an RF component that only lets a specific frequency range pass – like 433 MHz or 915 MHz – and blocks everything else.

This helps reduce interference from:

• Wi-Fi routers (2.4 GHz)
• LTE/4G towers
• Noisy power supplies
• Nearby transmitters on other bands

✅ When It Helps:

• You’re in a noisy urban environment
• Your node is near multiple RF sources
• You notice packet loss, even when RSSI looks okay
• You’re getting false or corrupted APRS frames

❌ When It May Not Help:

• You live in a quiet rural area
• There’s no significant interference
• Your main issue is low signal strength, not noise

📊 Real Benefits:

• 🚫 Reduces noise floor
• ✅ Improves SNR
• 🧠 Prevents receiver overload
• 📈 Improves reliability of weak signals (especially with SF12)

🛠️ Filter + LNA Combo

For advanced users: combine a bandpass filter + LNA (low-noise amplifier):

1. Filter first → clean up the signal
2. Then amplify → boost only the good stuff

This gives the best results for distant or mobile stations.

📈 Real World Example

Here’s a simplified snapshot of real LoRa APRS signals received on 9M2PJU-2:

🎯 Final Thoughts

Understanding RSSI and SNR is crucial for maintaining a healthy APRS LoRa system. By monitoring these values, you’ll know whether your setup is working well or needs a boost.

So the next time you see -127 and wonder “Is that good?”, now you know – it’s not! But with some tweaks, you can push your packets further and clearer than ever.