At resonance, a center-fed half-wave dipole has an input impedance of about 73 Ω. How does strong coupling to nearby conductors affect it?

• A. About 10 Ω; strong coupling reduces impedance
• B. About 73 Ω; proximity to ground or nearby objects can shift impedance away
• C. About 3000 Ω; metal nearby decreases impedance
• D. About 300 Ω; proximity to ground has no effect

Answer: (B)

The key idea is that antenna input impedance isn’t fixed once you take it out of an ideal free-space model. A center-fed half-wave dipole resonates around 73 ohms in free space because its current distribution is balanced and the radiation pattern is symmetric. But when you bring strong conductors nearby—ground, buildings, other metal objects—the electromagnetic fields couple into those conductors. This mutual coupling changes the current distribution on the dipole and introduces additional reactive loading, so the impedance seen at the feed no longer sits at 73 ohms and can include an added reactance. The exact amount and sign of the shift depend on the geometry, distance to objects, and their conductivities, but the important point is that proximity to conductors can move the impedance away from the free-space value. That’s why the option noting that proximity to ground or nearby objects can shift impedance away is the correct one. The other choices imply a fixed value or no effect, which isn’t generally true in real-world environments.