RF Current Sheets
At radio frequencies (RF), current in conductors tends to concentrate near the surface, forming a thin "skin" rather than flowing uniformly throughout the entire cross-section. This phenomenon is primarily caused by two interrelated effects:
1. Skin Effect: As the frequency of the alternating current (AC) increases, the current gets "pushed" towards the surface of the conductor. This happens because the rapidly changing magnetic field associated with the AC induces eddy currents within the conductor. These eddy currents oppose the main current, and their strength increases with depth due to the diminishing influence of the original AC field. Consequently, the main current gets "crowded" towards the surface where the eddy currents are weaker, effectively reducing the usable cross-section for current flow.
2. Proximity Effect: When multiple conductors carrying AC currents are placed close together, the magnetic fields generated by each current interact with each other. These interactions further influence the current distribution. If the currents are in-phase (both flowing in the same direction at the same time), the magnetic fields add up, pushing the currents towards the outer facing surfaces of the conductors. If the currents are out-of-phase (flowing in opposite directions), the magnetic fields tend to cancel each other out in the region between the conductors, further concentrating the current towards the outer edges.
How They Work Together
In shaped conductors, the skin effect and proximity effect work together to determine the overall current distribution. For example:
In rectangular conductors: The current tends to concentrate on the wider faces due to both skin effect and proximity effect (assuming the currents are flowing along the length).
In circular conductors: The current primarily flows on the outer surface due to the skin effect alone. However, if multiple circular conductors are bundled together, proximity effect comes into play and further confines the current towards the outer edges of the bundle.
In complex geometries: The combined effects of skin and proximity can create intricate current distributions, which are important considerations in high-frequency circuit design and analysis.
Understanding these effects is crucial for various applications involving RF systems, such as antennas, transmission lines, and microwave circuits. By carefully considering the shape and arrangement of conductors, engineers can optimize their performance at desired frequencies.
