What is a loop antenna?
A loop antenna is a closed conducting path, circular or polygonal. An electrically small loop (circumference C < about λ/10) acts as a magnetic dipole with radiation resistance Rr = 20π²(C/λ)⁴ Ω per turn (times N² for N turns), the same doughnut pattern as a short electric dipole but with the null on the loop axis; a resonant loop of about one wavelength circumference radiates broadside to its plane with about 3 dBi and an impedance on the order of 100 Ω.[1][2]
Why are small loops inefficient?
Radiation resistance falls as the fourth power of size, so a small loop’s Rr is often milliohms while conductor loss is comparable or larger.[1] Transmitting magnetic loops at HF use large-diameter copper and high-Q vacuum capacitors to resonate the inductance; their bandwidth is very narrow, and the capacitor sees kilovolts.[3]
How does a ferrite rod help?
A ferrite core concentrates magnetic flux, multiplying the effective area by an effective permeability μeff that depends on the rod’s length-to-diameter ratio; radiation resistance rises by μeff². Ferrite-rod loops are standard in AM broadcast receivers, LF time-signal receivers and 125 kHz RFID readers.[1][4]
Where are loops used?
- Receiving at LF/MF/HF, where external noise dominates efficiency concerns; loops are also used for direction finding thanks to their sharp nulls.[5]
- NFC and HF RFID coils, which work by near-field magnetic coupling.[4]
- Full-wave loops and quad beams at HF/VHF.
- Small loops in pagers, hearing aids and wearables, where the magnetic near field is less affected by the body than the electric field.[6]
Frequently asked questions
Is a small loop a magnetic antenna?
Its near field is predominantly magnetic and it is the dual of the short electric dipole.
Where is the null of a small loop?
Along its axis, perpendicular to the plane of the loop.
What is a quad antenna?
A directional array of full-wave loops, analogous to a Yagi.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “Antennas for All Applications, 3rd ed.”. J. D. Kraus and R. J. Marhefka, McGraw-Hill. 2002.
- “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.
- “RFID Handbook, 3rd ed.”. K. Finkenzeller, Wiley. 2010.
- “Recommendation ITU-R P.372: Radio noise”. ITU-R. current edition.
- “Modern Small Antennas”. K. Fujimoto and H. Morishita, Cambridge University Press. 2013.