8.0 Introduction

In Chapters 2 and 3, I pointed out that an antenna is a transducer, so any antenna is capable of both transmitting and receiving operation. However, in many applications the antenna is not required to transmit. These applications include broadcast reception, direction finding, and spectrum monitoring. Without the need to transmit, the voltage or power-handling capability needed is much lower, which makes the hardware less expensive, and makes it possible to use materials that otherwise wouldn t stand the stress of transmitting. The receive-only application makes signal-to-noise ratio and consistent performance over very wide frequency bandwidth criteria that can be improved at the expense of impedance-matching (maximum power transfer).

The nature and amount of noise at a receiver s detector is a function of the type of amplification used, the noise generated in the receiver, and the electromagnetic noise picked up by the antenna. For AM and SSB receivers, the amplification is linear so that the effect of noise on the amplitude and timing of the voltage at the detector are important. In FM and keyed-signal (digital) receivers, only the noise effects on the timing (zero-crossings) of the voltage are important so long as there is enough signal+noise to drive the limiters (saturating amplifier stages) to full amplitude. In either case, the job of the antenna is to transfer enough of the external electromagnetic noise into the receiver so that it is at least as important as the receiver s own self-generated noise. This implies a necessity for some noise analysis...