Loop Antennas

A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line (or possibly a balun). There are two distinct antenna designs: the small loop (or magnetic loop) with a size much smaller than a wavelength, and the much larger resonant loop antenna with a circumference close to the intended wavelength of operation. Small loops have low radiation resistance and thus poor efficiency and are mainly used as receiving antennas at low frequencies. To increase the magnetic field in the loop and thus the efficiency, the coil of wire is often wound around a ferrite rod magnetic core; this is called a ferrite loop antenna. The ferrite loop is the antenna used in many AM broadcast receivers, with the exception of external loops used with AV Amplifier-Receivers and car radios; the antenna is often contained inside the radio's case. These antennas are also used for radio direction finding. In amateur radio, loop antennas are often used for low profile operating where larger antennas would be inconvenient, unsightly.

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1. LOOP ANTENNAS Frank Paul F. Abilay

2. Outline Loop Antenna Electrically small and large loop antennas Small loop antenna Constant and non constant current loop antennas Equivalent Circuit of Loop Antennas Ferrite Loop

3. Loop antenna An antenna constructed in a form of a loop Loops may vary in shape: square, circular, elliptical Can be classified as electrically small or electrically large Based on circumference/ perimeter: Small loop: C < λ/ 10 Large loop: C ∼ λ (free space wavelength) Common band of application HF (3–30 MHz) VHF (30–300 MHz) UHF (300–3,000 MHz ) Microwave bands

4. Loop antenna Are inductive in nature

5. Electrically small and large loops For single loop antennas, the smaller the loop, the smaller the radiation resistance. Based on circumference/ perimeter: Small loop: C < λ/ 10 Large loop: C ∼ λ (free space wavelength) Based on radius of small loop Small loop:  

6. Small Loop Antenna The small loops, regardless of their shape, have a far-field pattern very similar to that of a small electric dipole Has a electrical circumference of C < λ/ 10 Has an equivalent radiation pattern with electric dipoles RADIATION PARAMETERS Power pattern Radiation Resistance for single loop antenna  

7. Small Loop Antenna For loop antennas, as the number of loops (N) increases, the radiation resistance also increases. In equation, However, as the number of loops increases, the losses also increase, thus making the antenna inefficient.  

8. Small Loop Antenna In contrast with the electric dipole, radiation resistance of the small loop decreases much faster than that of the short dipole with decreasing frequency Same with the electric dipole, small loop antennas has a directivity as  

9. Small Loop Antenna

10. Circular Loops of Constant Current For analysis of circular loops with radius a , and with constant current, only far fields will be considered.

11. Circular Loops of Constant Current Equations are only valid for loops of thin wires Far fields are approximated as: Using the Bessel function, far fields can be expressed as:

12. Constant Current Loop For circular loops with constant currents, as the radius of the loop increases, the directivity decreases.

13. Non constant Current loops When wire diameters increase, the constant current approximation does not apply. Variation in the radiation parameters occur. Input impedance changes.

14. Non constant Current loops When wire diameters increase, the constant current approximation does not apply. Variation in the radiation parameters occur. Input impedance changes.

15. Equivalent circuit

16. Loss Resistance Caused by high frequencies and proximity effect High frequency loss: Proximity Loss occurs when two wires are close to each other causing the skin effect, wherein the fields on the nearby wires interferes and cancels each other in some extent

17. Loss resistance Two wire in cross section view, showing the proximity effect

18. Ferrite Loops A ferrite loop is formed when a ferrite core is inserted on the loop of the antenna The radiation resistance and radiation efficiency increases with the insertion of a ferrite core High magnetic permeability in the operating frequency band, which increases magnetic flux, thus increasing induced voltage The increase in the magnetic flux, represented as the effective relative permeability, increases radiation resistance by: d