cies above about 50 MHz. Receiver noise performance is fashioned to a large degree by the levels of the different types of noise which occur. In view of the high levels of noise entering a receiver from the antenna on frequencies below 30 MHz there is little point in designing very low noise receivers. At frequencies above about 50 MHz to 100 MHz received noise levels fall to a level where the noise generated within the receiver predominates, and accordingly the noise performance of the set becomes far more important. Another form of noise is called thermal noise. This is caused by the movement of the electrons in a conductor. Even when there is no potential difference to cause current to flow noise is present. At any temperature above absolute zero electrons are moving about randomly in the conductor, and this causes random voltages or noise to be generated. It is not surprising that the level of noise is proportional to the temperature. As the temperature rises SO the movement of the electrons increases. It is also found that the level of noise is proportional to the bandwidth being received. The wider the bandwidth, the greater number of noise frequencies which can be received, and hence the greater the level of noise. Finally the noise is proportional to the resistance of the conductor. In fact the level of noise can be calculated from the formula given below: E = V4kTBR where E = EMF in volts k = Boltzmann's constant which is 1.37 x 10-23 joules per degree kelvin T = absolute temperature in degrees kelvin B = bandwidth in hertz R = resistance in ohms Unfortunately there is no way of reducing the noise generated by a resistor, except by reducing the resistance, the bandwidth or its temperature. It is for this reason that some very specialized very low noise amplifiers are cooled. Receivers 171 Signal to noise ratio There are a variety of methods used to measure and define the sensitivity of a radio receiver. As the noise performance is the limiting factor, the sensitivity is specified in terms of its noise pe