educing the number of compo- nents and hence the cost of the set. The ferrite rod antenna operates using the high permeability of the ferrite material to 'concentrate' the magnetic component of the radio waves as shown in Figure 4.32. This means that the antenna is directive. It operates best only when the magnetic lines of force fall in line with the antenna. This occurs when it is at right angles to the direction of the transmitter. This means that the antenna has a null position where the signal level is at a minimum when the antenna is in line with the direction of the transmitter. Antenna systems 113 Magnetic field concentrated through the ferrite rod Figure 4.32 Operation of a ferrite rod antenna These antennas are very convenient for portable applications, but their efficiency is much less than that of a larger antenna. The performance of the ferrite also limits their frequency response. Normally they are only effective on the long and medium wave bands, but they are sometimes used for lower frequencies in the short wave bands although their performance is significantly degraded, mainly because of the ferrite. Loop antennas Another form of antenna that can be used to good effect under some circumstances is a loop antenna. These antennas are formed by creating a loop of one or more turns of wire that form a short circuit to direct current. An example of this type of antenna is often seen with hi-fi systems where a small loop is used to provide the reception for long and medium wave stations. Loop antennas fall into two categories, small loops and large loops. As the names imply the category depends on their size. As a rule, small loop antennas are those that have a circumference of less that 0.1 wavelengths. These antennas are generally only used for receiving as their radiation resistance is extremely low and losses when compared to the resistance of the wire can be high. This makes them quite inefficient, although very useful for receiving signals in areas of high signal strength. Loop antennas are direct