he effective area of total flux = flux density (B) per unit area X area (A) the aerial is relatively small when compared with a 2/2 aerial length. The ferrite rod aerial is therefore very inefficient when compared to an outdoor aerial but it is popular because of its convenient size and portability. At webers = Btesla Asquare metres (2.1.11) medium wave frequencies, the inherent poor signal If you do not know how to differentiate to get the rate of change of a value, then please refer to a maths book. Basic features of radio communication systems CHAPTER 2. 1 Direction of Example 2.1.4 H-field A coil of 105 turns is wound on a ferrite rod with an ef- Axis of fective cross-sectional area of 8 X 10-5 m ². The relative permeability of the ferrite is 230 and the permeability of air is 4n X 10-7 henry/m. The r.m.s. field strength is Fig. 2.1-18 Multi-looped antenna aligned for maximum flux 10 uA/m. If the magnetic field is incident along the axis of linkage. the coil and the frequency of operation is 1 MHz, what is the r.m.s. open-circuit voltage induced in the coil? pick-up is acceptable because broad-cast stations radiate Given: No. of coil turns = =105, effective cross- large signals. sectional area of ferrite rod = 8 X 10-5 m², relative In the foregoing derivation, it has been assumed that permeability (Mr) = 230, permeability of air (MO) the magnetic field has been cutting the coil along its axis. = 4 x 10-7 henry/m, r.m.s. field strength n=10uA/m, Occasions arise when the incident magnetic field arrives frequency = 1 MHz. at an angle a with respect to the axis of the coil. This is shown in Figure 2.1-19. In this case the effective core Required: r.m.s. open-circuit voltage induced in coil. area is reduced by cos a, and the induced voltage becomes Solution e = nwu oAHmax cos cos a (2.1.17) Using Equation 2.1.19 This expression shows that the induced open-circuit erms = cos a voltage, e, is dependent on the axial direction of the aerial = x 2 x x 1 x 106 coil with respect to the direction of the propagation. It is