Systems

1U79 PRINCETON DftlVE CLtAftWATEP FLORIDA J3515 PHONE (S13i 5Ä1 57 1 S 447 &107

This sorl of situation occurs quite often —

for instance, when ¿1 contact has been

established and the usual tlRST? QTH, and name" information is sent. Many QSOs fall apart after such information is sent because the receiving operator no longer knows what to expect (this assumes that the code speed proficiency of the receiving operator is always adequate.) So, there is a basic human characteristic involved in any type of CW or voice reception and the only overall definitive statement that can be made is that the human operator for aural reception requires the received signal to be some level above the noise level to extract intelligence from it.

Noise in a Receiving System

Starting back at the transmitter, and assuming the noise output of the transmitter is insignificant, the signal radiated in the immediate area of the transmitter is many times stronger than the electrical noise level. As the signal is propagated, its intensity decreases. At the reception point, the signal level will have some level as compared to the noise level at the receiving site. The noise level at the receiving site can be a combination of many things — atmospheric, man-made, and cosmic noise. Some of these noises are shown in Fig. 1. along with their variation with frequency. As can be seen, atmospheric noise can vary with time o:; year as well as with frequency. It also varies considerably with location. The type of noise which predominates depends upon a particular location. Man-made noise is a particularly unknown factor in any location. 1 lie other noise sources lend themselves to prediction and calculation, and many charts, maps, etc. can be obtained relating to them. The very simple but still important consideration at this point, however, is that a desired signal (Fig. 2) arrives at a reception point with a definite level as compared to the noise at that point. Aside from the possibility of eliminating man-made noise, there is nothing that the receiving operator can do about this ratio of signal-to-noise level.

The next step in the receiving process (or perhaps the first step, depending upon how the process is defined) is the coupling provided by an antenna. The idea of an antenna as a coupling device is convenient for remembering that it couples both the signal and external noise to the rest of the receiving system. Unfortunately, this coup-

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