ximately 292 O and it By definition, flux density in air cored coil (Btesla) is is easily coupled to 300 O balanced transmission lines. given by The antenna is directive and has to be positioned for maximum signal pick-up. free-space permeability (MO) X magnetic field strength (H) 2.1.5.7 Multi-loop antennas B(tesla) = MO (henry/metre) X H(ampere/metre) (2.1.12) At low frequencies, particularly at frequencies in the medium wave band where wavelengths are long, single or suppose that the incident wave has a magnetic field loop 2/2 length antennas are not practical; multi-loop strength antennas (Figure 2.1-17) are used instead. The multi- loop antenna can be reduced even further in size if H = Hmax sin wt (2.1.13) a ferrite rod is inserted within the loop. The open-circuit voltage induced in multiple loop an- where W is the angular frequency of the r.f. signal. Then tennas can be calculated by making use of Faraday's Law of substituting Equations 2.1.12 and 2.1.13 in Equation Electromagnetic Induction which states that the voltage 2.1.11 yields induced in a coil of n turns is proportional to the rate of : BA = change of magnetic flux linkage. For simplicity in deriva- (2.1.14) tion, it will be assumed that the incident radiation is Taking the rate-of-change¹5 in Equation 2.1.14, then the propagating along the axis of the coil (see Figure 2.1-18). induced voltage is Expressing Faraday's Law mathematically, e = n (2.1.15) (2.1.10) = nwu oAHmax cos wt where For a coil with a ferrite core, the flux density is in- e = open-circuit voltage in volts creased by the relative effective permeability (Mr), giving n = number of turns on coil e = neurou.,AHmax cos wt (2.1.16) do/dt = rate of change of magnetic flux linkage ( = webers and t = seconds) You will see that the ferrite core has increased the Some fundamental magnetic relations are also re- effective area of the coil by a factor Mr. Ferrite cores with quired. These include: effective relative permeabilities of 100-300 are readily available but even with these values, t