Understanding RF Noise Sources

RF noise, as received by an antenna system and considered disruptive, arises from multiple sources. It can be categorized into two main types: radiated noise and conducted noise. Radiated noise refers to noise that directly reaches the antenna from atmospheric, natural sources such as solar emissions, or man-made sources. Conducted noise, on the other hand, is transferred via cables and other conductive pathways into the receiver system, originating from both external power sources and internal circuit design flaws. While both types are always present, one may be more dominant depending on the situation.

Radiated noise includes:

• Atmospheric noise, which varies based on environmental conditions.
• Man-made noise, originating from digital equipment, electrical appliances, and other electronic devices.

Conducted noise primarily consists of disturbances near the receiver that enter the weak signal path through cables. It can penetrate the system due to limited shielding effectiveness or through common-mode to differential-mode conversion. Notably, radiated noise can transform into conducted noise and vice versa.

Managing Radiated and Conducted Noise

Reducing radiated noise requires either eliminating the noise source or increasing the distance between the receiver and the interference source, as field strength decreases with the square of the distance. Conducted noise can be mitigated by implementing chokes in unwanted cable paths.

Conducted noise is problematic both during reception, where it raises the noise floor, and during transmission, where it can cause RF feedback, disrupting radio communication. A common example of conducted interference during transmission is RF feedback into the transceiver.

Addressing Conducted Interference

One of the primary causes of conducted interference is the imperfect transition between symmetrical and asymmetrical media, typically at the antenna feed point. At this juncture:

• Differential-mode signals (intended RF transmission) can be converted into unwanted common-mode signals, causing sheath currents on the coaxial cable’s outer surface.
• Common-mode noise currents present on the outside of coaxial cables during reception can be converted into differential-mode signals, adding unwanted noise to the received signal.

To mitigate this:

1. Use High-Quality BALUNs – A BALUN with strong common-mode rejection should be employed to minimize noise conversion.
2. Implement Proper Grounding – A low-impedance RF ground at the antenna side helps eliminate noise. However, the electrical safety ground in a house wiring system is insufficient for RF grounding due to its high impedance at RF frequencies.

Establishing a Clean RF Ground

A proper RF grounding system includes:

• Several interconnected ground rods placed at least 10 meters away from safety ground connections to prevent RF coupling.
• Isolation of the clean RF ground from the electrical safety ground using common-mode chokes on coaxial lines and control cables.
• Bonding station equipment (transceiver, amplifier, tuner) together using short, heavy-gauge interconnections.
• Avoiding the use of ferrite cores or sleeves on grounding interconnections, while applying them appropriately to interface cables.

Though the station’s equipment must remain connected to the electrical safety ground for protection, ensuring proper shielding integrity prevents noise ingress. Care must be taken to avoid the conversion of common-mode noise into differential-mode noise, which can impact weak signal reception.

Shielding and Choking Techniques

• High-Quality Coaxial Cables – Using coaxial cables with superior shielding effectiveness, such as double-shielded or foil-braided coaxial cables, minimizes noise penetration.
• Proper Connector Shielding – Ensuring strong shielding connections at coaxial connectors prevents unwanted interference.
• Line Isolators and RF Chokes – Installing line isolators on coaxial cables and RF chokes on external cables exiting the shack further reduces noise ingress.
• Supplementary Ground Rods – Additional ground rods placed near the station can help reduce RF impedance in the safety ground mesh but should not be too close to the clean RF ground.
• Common-Mode Chokes for Isolation – Isolating the clean RF ground from the safety ground using common-mode chokes often yields the best results.

Considerations for Mains Filtering

Electrical safety ground has a high RF impedance, making it ineffective for noise suppression. Attempts to lower RF impedance in electrical safety ground should be made cautiously.

• Mains Filters – Installing additional AC mains filters at the shack entrance can complement built-in equipment filters. It is important to select an RF-rated mains filter to ensure effective suppression of high-frequency noise.
• Filtering Individual Devices – Equipment known to generate RF interference, such as those with switched-mode power supplies, may require dedicated mains filtering.
• Avoiding Redundant Choking – If a mains filter is deployed, adding extra ferrite chokes is usually unnecessary, as these filters already incorporate sufficient common-mode suppression.

Conclusion

Reducing RF noise in a radio environment requires a comprehensive approach that includes proper antenna feedpoint treatment, effective grounding, superior shielding, and appropriate filtering techniques. By addressing both radiated and conducted noise sources, radio operators can significantly enhance their reception and transmission quality, ensuring a more stable and interference-free communication experience.

Article written by Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru. With extensive experience in active and passive antenna systems, high-power RF transformers, and custom RF solutions, Joeri shares insights into cutting-edge radio communication technologies.