What is the typical impedance of standard coax and why is matching important?

• A. 75 ohms; reduces conductor losses
• B. 50 ohms; matching to 50-ohm system maximizes power transfer and minimizes reflections.
• C. 25 ohms; increases bandwidth
• D. 100 ohms; for higher conductor cross-section

Answer: (B)

Standard RF coax is designed around a 50-ohm impedance. This value is chosen because it provides a practical balance between how much current the line can carry and the voltage it must handle, making it well suited for many transmitters and antennas and offering good power handling with manageable losses.

Matching is important because when the source, line, and load all present the same impedance, energy flows into the load with minimal reflections. If there’s a mismatch, some of the power is reflected back toward the transmitter, creating standing waves along the line (high SWR). Reflected power reduces the actual power delivered to the antenna, wastes power as heat, can overheat components, and can cause distortion or damage. In practice, matching networks or antenna tuners are used to make the system appear as 50 ohms to the transmitter across the operating band, ensuring efficient, safe operation.