MAGNETIC LOOP ANTENNA

We often see the Mag Loop shown with the feed loop on top AND with it on the bottom.  Which One is better and why?





WHICH ONE

?





and WHY ?

The capacitor goes at the top (high-voltage point) and the small coupling/feed loop at the bottom (high-current point) mainly for voltage/current distribution, safety, coupling efficiency, and electrical balance.


Voltage and Current Distribution

A magnetic loop (small transmitting loop) is a high-Q parallel resonant circuit: the large conductor forms the inductor, and the variable capacitor resonates it.

Even though the loop is electrically small (circumference typically ≪ λ/4 to ~λ/3), the current is not perfectly uniform. There is a clear voltage maximum across the capacitor and a current maximum at the point diametrically opposite it.

  • Highest voltage → at the capacitor.
  • Highest current (and strongest magnetic field) → opposite the capacitor.

Placing the capacitor at the top and the small coupling loop at the bottom puts each component where it works best.


Why the Capacitor at the Top

  • Safety: RF voltages across the capacitor routinely reach several kilovolts even at modest power (hundreds of volts at QRP levels, multi-kV at 100 W). Keeping this high-voltage point elevated reduces the chance of arcing to ground, nearby objects, or a person.
  • Feedline isolation: The coax and operator stay away from the high-E-field region. Running the feedline near the capacitor can introduce losses, detuning, or common-mode issues.
  • Mechanical and environmental: In fixed or higher-power installations the capacitor (often motor-driven) sits high and clear; the structure is more stable when the heavy feedpoint hardware is lower.


Why the Small Loop at the Bottom

The small coupling loop (usually ~1/5 the diameter of the main loop, often a Faraday/shielded loop) works by inductive coupling to the magnetic field of the main loop. Magnetic field strength is proportional to current, so the coupling is strongest and most efficient at the current maximum—exactly opposite the capacitor.


This placement also:

  • Gives a convenient, low-impedance feedpoint near ground or the support structure.
  • Helps keep the antenna electrically balanced/symmetric, which improves the pattern nulls and reduces common-mode current on the feedline.
  • Avoids routing the coax through the high-voltage region of the loop.


Practical Notes

  • Many portable/QRP designs put the capacitor at the bottom for convenience and lower center of gravity. At low power the voltage is more manageable and the performance penalty is small.
  • For higher power or fixed installations the classic “capacitor top / coupling loop bottom” arrangement is preferred for the reasons above.
  • Alternative feeds (gamma match, transformer, etc.) still work best when located near the current maximum.


In short: the topology follows the natural voltage/current standing-wave pattern on the loop, keeps dangerous voltages out of reach, and maximizes coupling efficiency.