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Loop Antenna 1 I EC2009001 2 Introduction Metallic conductor bent into the shape of a closed curve, such as a circle or a square, with a gap in the conductor to form the terminals Simple to construct, inexpensive & very versatile 3 Properties Equivalent to infinitesimal magnetic dipole Loop Antennas’ with small perimeters have small Radiation Resistances(smaller than their loss resistances) Not used in transmitting mode Responds to the magnetic field rather than the electric field. Hence eliminates Noise 4 Principle In receiving applications it works on the principle of the "differences" in voltages induced by the current flowing in the sides of the antenna. These difference voltages can be extremely minute in amplitude and any loop antenna usually requires an associated amplifier capable of at least 25 dB power gain following it. 5 Why Loop Antenna Radio Direction Finding No available space for a longwire antenna: Loop Antennas are small compared to wire antennas To eliminate noise: Loop primarily responds to the magnetic component of the radio wave Noise resides primarily in the electrical component A vertical antenna responds mainly to the electrical component. 6 Uses Used as receivers in portable radios, pagers where Signal-to-Noise ratio is important than antenna efficiency. As probes for field measurements As directional antennas for radiowave navigation 7 Classification Electrically small Circumference ≤ λ/10 Electrically large Circumference ≈ λ 8 Small Loop and Infinitesimal Magnetic Dipole A magnetic dipole of magnetic moment iml is equivalent to a small electric loop of radius a and constant electric current io 9 Ferrite Loop Radiation Resistance of loop antenna is very low Raised by increasing circumference Insert a Ferrite Core Increases Flux, Magnetic Field, Open Circuit Voltage