How would you check for antenna resonance in the field?

• A. Use a spectrum analyzer to measure across band and adjust length for minimum SWR
• B. Check battery level
• C. Measure SWR or return loss across the band with a portable SWR bridge and adjust antenna length until resonance is found (minimum SWR).
• D. Inspect color of antenna

Answer: (C)

Resonance means the antenna presents a nearly purely resistive input with little reactive part, so most of the transmitter’s power is radiated rather than reflected. The practical way to confirm this in the field is to measure how much power is reflected back, using SWR or return loss data. A portable SWR bridge lets you sweep the operating band and watch the SWR as you adjust the antenna length. When you reach the point where SWR is at a minimum, the antenna is resonant and the feed sees an impedance closest to the system impedance (often around 50 ohms), allowing efficient power transfer. You then tune the length (or adjust matching) to maintain that low SWR across the desired operating range. Using a spectrum analyzer won’t directly indicate how well the line is matched or reveal resonance in the feed, and checking battery level or simply inspecting the antenna’s color doesn’t provide impedance information.