Dipole and Loop Antennas: Complete Beginner's Guide

A dipole is two conductors end to end, fed at the centre; the half-wave dipole (length ~ half wavelength) is the world's most used antenna. A loop antenna is a conductor bent into closed turns, fed at a gap. Small loops (circumference far below a wavelength) are magnetic antennas; large loops near one wavelength behave like folded dipoles.

They are the building blocks of countless devices: TV rabbit ears and FM aerials are dipoles; NFC cards, RFID tags and AM ferrite antennas are loops. Their simple theory introduces radiation resistance, resonance and current distribution, the ideas behind every antenna you will meet.

Key Concepts

• Resonance: input reactance zero, so the transmitter sees pure resistance. A half-wave dipole resonates near 0.48 wavelengths due to end effects.
• Radiation resistance: equivalent resistance for power carried away as waves; ~73.2 ohms for a thin half-wave dipole in free space.
• Current distribution: maximum at the feedpoint, falling sinusoidally to zero at the ends.
• Dipole pattern: doughnut shape, strongest broadside, zero along the axis, 2.15 dBi gain.
• Small loop: circumference under ~lambda/10; milliohm radiation resistance, responds to the magnetic field, figure-eight pattern with axial nulls.
• Resonant loop: circumference near one wavelength; ~100-120 ohms radiation resistance, maximum in the loop plane.

Formulas

• Resonant length: L = lambda/2 ideal; L = 0.48 x lambda practical.
• Wavelength: lambda = c / f, c = 3x10^8 m/s.
• Dipole R_r: 73.2 ohms (thin wire, free space).
• Small single-turn loop: R_r = 320 x pi^4 x (A/lambda^2)^2 ohms (A = loop area); tiny, which is why small loops need matching or many turns.
• Assumptions: thin perfect conductor in free space, far from ground and objects.

Worked Example

Half-wave dipole for FM at 100 MHz:

1. lambda = 3x10^8 / 100x10^6 = 3 m.
2. Practical length = 0.48 x 3 = 1.44 m total (two 0.72 m rods).
3. Feedpoint ~73 ohms resistive at resonance: a clean match for 75-ohm FM/TV coax, no tuner needed. This is exactly why FM aerials look the way they do.

Common Misconceptions

• "Longer is better." Past resonance the antenna turns reactive and mismatched; a correctly cut resonant antenna beats a longer mistuned one.
• "Dipoles radiate from the tips." Radiation peaks broadside, perpendicular to the wire, and is zero off the ends where current is zero.
• "Loops receive electric fields." Small loops couple to the magnetic field; rotating a portable AM radio finds a sharp null used for direction finding.

Real-World Applications

• TV/FM reception: rabbit ears and Yagi-Uda arrays built around a dipole element.
• Portable AM radios: ferrite-rod loops, compact thanks to magnetic coupling.
• NFC/RFID: small loop coils coupling magnetically at 13.56 MHz.
• Direction finding: the small loop's sharp null locates transmitters.

The half-wave dipole resonates near 0.48 wavelengths with ~73 ohms radiation resistance and a broadside doughnut pattern: the universal reference antenna. Loops complement it as compact magnetic antennas for NFC and AM radio, or as one-wavelength loops with higher impedance. Cut to resonance, feed at the current maximum, match the impedance.