SNR

Noise Floor (NF) Measurement

To determine the Noise Floor (NF) of our receiving system, we tune to a channel where no signal is present and record the S-Value (for instance, S3.5 as in my case). 

According to Graph1, S3 corresponds to a power level of -109 dBm, which we call PNF (Noise Floor Power).

So we have:

PNF = -109 dBm (This is the Noise Floor)

2. Signal Measurement

Now, let's receive a station signal. For example, an S9 signal corresponds to a power level of -73 dBm, which we call PS (Signal power).

So:

PS = -73 dBm (This is the received Signal)

3. Signal-to-Noise Ratio (SNR) Calculation (in dB)

To calculate the SNR, we subtract the Noise Floor (PNF) from the Signal Power (PS):

SNR = PS - PNF

SNR =  (-73 dBm) - (-109 dBm)

SNR = 36dB

So, our SNR is 36 dB.

4. Alternative Calculation: Using Voltage Levels

If you prefer to calculate the SNR using voltage levels, we use the following formula to calculate the SNR using voltages (µV):

SNR = 20x LOG10 (SµV / NFµV)

Where: 

S is the received Signal voltage (e.g., 50.1 µV). 

NF is the Noise Floor voltage (e.g., 0.8 µV). 

Using these values:

SNR = 20x LOG10 (50.1µV / 0.8µV) 

SNR = 20x LOG10 (62.625) 

SNR = 20x 1.7967 

SNR ∼ 36dB

So, using voltage values also gives us an SNR of 36 dB.

5. Interpretation of the SNR Table

From the SNR of 36 dB, we can now interpret the quality of the communication:

• Red Area (Low SNR): Not good for voice or CW communication.

• Yellow Area (Moderate SNR): May allow communication, but not ideal for voice. However, it could work for CW (Morse Code) under some conditions.

• Green Area (Good SNR): This is ideal for communication. An SNR of 36 dB is in the green area, meaning the communication should be clear and not impacted by noise.

6. Real-World Considerations

• Keep in mind that all these measurements are relative. The S-Value you see is not the actual signal at the antenna feedpoint, and the feedpoint impedance might not be exactly 50Ω, so values are relative to your system setup.

7. Conclusion

• 36 dB SNR is considered good for communication.

• SNR values above 25 dB are generally sufficient for reliable communications in professional contexts.

• SNR values in the green zone (like 36 dB) indicate that the received signal is clear, with sufficient separation from the noise floor.