 maximum when cos a = 1, i.e. a = 0°, and minimum 10 x 10 6 x 8 x 10 5 cos when cos a = 0, i.e. a = 90°. This explains why it is = 152.5 V = necessary to position a loop aerial to receive maximum signal from a particular broad-casting station and this is done in a portable radio receiver by orienting its Broadcasting authorities tend to quote electric field direction. strengths rather than magnetic field strengths for their The above reasons apply equally well to ferrite rod radiated signals. This creates no problems because the aerials and for these cases we have an induced voltage two are related by the wave impedance formula given earlier as Equation 1.8. This is repeated below: e = cos wt cos a (2.1.18) electric field strength If the magnetic field strength is given as an r.m.s. value magnetic field strength (Hrms), then the r.m.s. value of the induced voltage is erms = cos a (2.1.19) Example 2.1.5 Finally, ferrite aerials are seldom used at the higher A coil of 100 turns is wound on a ferrite rod with an frequencies because ferrite can be extremely lossy above effective cross-sectional area of 8 X 10-5 m². The rela- 10 MHz. tive permeability of the ferrite is 200 and the perme- ability of air is 4T X 10-7 henry/m. The magnetic field is incident at an angle of 60° to the axis of the coil and the H-field frequency of operation is 1 MHz. If the electric field strength is 100 uV/m, what is the r.m.s. open-circuit Axis of voltage induced in the coil? 000000 Given: No. of coil turns = 105, effective cross- sectional area of ferrite rod = 8 X 105 m², relative permeability (Mr) = 200, permeability of air (MO)=4 X 107 henry/m, incidence of magnetic Direction of propagation field = 60° frequency = 1 MHz, electric field strength = 100 uV/m. Fig. 2.1-19 H field arriving at an angle a. Required: Open-circuit voltage (erms). CHAPTER Basic features of radio communication systems Folded Directors Reflector dipole Table 2.1-1 Typical values for Yagi arrays operating in the TV band. No. of Forward gain Front/back Acceptance e