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Fig. 2L The effect of noise and amplification at various steps in the transmitter output to receiver output chain.

ling process requires some kind of current flow and any kind of current flow is going to produce more noise. The noise produced is directly related to the losses in the system. These losses (attenuation) reduce the amplitude of the coupled signal and noise, but also introduce new noise. The fact that the amplitude of the signal and noise take on a new ratio is what makes the receiving system suffer — not the simple attenuation of the desired signal alone. For the moment, let us assume that the signal-to-noise amplitude ratio does not suffer during the antenna coupling process.

The next noise source encountered is that provided by a preamplifier, if such a device is used. As shown in Fig. 2, this noise source is shown to have a level less than that of the noise portion of the signal and other noise coupled into it The amplifying device amplifies both the signal and the noise coupled into it. The output, then, is an amplified signal, amplified noise, and noise generated by the amplifying device. If the amplifying device is good, the signal-to-noise level ratio at its output will almost be the same as that at its input. But it can never be better.

If a signal-and-noise input is coupled directly to a receiver where the noise input is less than that of the receiver noise, the latter will "mask" part of the signal and a poorer signal-to-noise level ratio' will occur at the receiver output than was coupled into it.

On the other hand, if the receiver-generated noise level is so low that it is always exceeded by the noise portion of the signal and noise input to it, a preamplifer has no value. The use of a preamplifer in such a case will make things sound louder, but it cannot improve the signal-to-noise level at the output of the receiver over what that ratio would have been if the input signal has been directly processed by the receiver. In fact, if a poor preamplifer is used which has an internal noise level greater than that of the receiver itself, the use of such a preamplifier will actually degrade receiver performance.

It should be noted that in all the signal transfer, processing, etc. which takes place, the objective is to preserve as much as possible of the original signal-to-noise level of the input until it can be amplified to the point where it is capable of being used to drive an audio transducer, If one remembers this simple idea, it will be obvious why losses have to be avoided as soon as possible in the receiving process, why preamplifiers should be placed as early as possible in the receiving process, and why a preamplifier, to be useful, has to be a very low-noise device.

The Receiving Antenna

From the foregoing, it would seem that the minimum requirement for a receiving antenna is that it just be large enough to pick up enough external noise to exceed the noise generated internally in the receiving system. Any signal could be received?then, since any signal to be useful anyway must have a level which exceeds the external noise level, This idea is basically correct. What, then, is the value of beams and other very directive antennas for receiving purposes?

A beam may have a transmitting gain of several decibels or more but it is still a passive device and, as such, cannot amplify any signal. It cannot produce a gain for receiving purposes in the sense that it amplifies a received signal. What a beam or other directive antenna can do is illustrated in Fig, 3. The omnidirectional antenna picks up both noise and signals from all directions. The directive antenna picks up signals from one direction but, more importantly, also only noise from one direction. The signal-to-noise ratio at the antenna terminals is enhanced in the sense that the total noise pickup of the antennas has been reduced.

It should be noted that the terms "gain" and "directivity" are related but have dif-

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Fig. 3, The value of antenna directivity (B) for receiving purposes over an -omnidirectional response (A) is that noise coming from directions other than that where a desired signal originates is discriminated against,

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ferent definitions. Both can be expressed in terms of decibels and for some antennas, their value is the same. A large beam has gain for transmitting purposes and also exhibits usually an equal amount of directivity for receiving purposes. A very sitiall antenna can still have directivity (a ¡loop, for instance) but absolutely no gain.

If a very small antenna can exhibit the same directivity for receiving purposes as a large antenna, the question naturally arises

as to why a small antenna should not be used instead for receiving purposes. Then, if one could only erect a modest type of transmitting antenna, it still would be possible to have a very effective receiving antenna. The situation becomes even more intriguing when one considers that, theoretically at least, a very small physical antenna provides the same amount of signal pickup as a large antenna. The amount of signal pickup of an antenna itself is not a direct function of its physical size. The advantage of a full-size antenna is that its terminal impedance is such that it matches to a transmission line or some sort of simple coupling circuit and, therefore, an efficient transfer of power from the antenna can take place.

The recent work which has been done on miniature or ultraminiature antennas has been directed toward finding an efficient method to transfer power frorfi the physically small antenna. Transistor stages have been used as a coupling device instead of tuned circuits to match the highly reactive terminal impedance of an antenna veiy short in terms of wavelengths. Another approach has simply been to use an antenna length just long enough which would theoretically pick up enough atmospheric noise to be greater than the internal noise level in a receiving system. Such an antenna need only be a few inches long, for instance, at almost any frequency above the medium-wave band. The antenna is fed into a preamplifier stage with an extremely high input impedance so that the antenna is not "loaded down." Whatever approach is used, however, the factor of noise still enters into the picture because of the transistor stage used as part of the antenna (whether the stage is called a matching stage or a preamplifier).

Active antennas do work, but the question of whether it is worthwhile to very carefully construct the active antenna using very iow noise components, or simply to try to make a conventional antenna form electrically larger (perhaps by loading techniques) is a moot one. After all, there are very few situations where one cannot put up some sort of antenna that is more than a few inches long. '

Noise Tests

Even with just some understanding of the role that noise plays in a receiving system, one can make some subjective analysis of what effect various components have upon a system. For instance, if the terminals of a receiver are resistor-load terminated, the antenna peaking control should produce a distinct increase in noise level at some point. With a terminated transmission line added to the receiver, the same peaking action should occur and the noise level output should be the same or slightly greater than before. If a preamplifier is used and its input terminated, the noise level should not increase if the receiver gain is adjusted to compensate for the gain of the preamplifier.

The best way, of course, to check the effectiveness of a preamplifier, or any similar device added ahead of a receiver which is supposed to improve its performance, is an actual reception test. If the preamplifer is switched in and out and a very weak signal can be heard with the preamplifer that could not be heard without it, you can be sure that the preamplifier has value. Note, however, that if a signal that can be received without the preamplifier simply sounds louder with the preamplifier being used, this does not signify anything about the preamplifier except that it produces some gain. Antennas can be checked against each other by being careful to keep the gains in a receiving system constant and noting which antenna, or what modifications to one antenna produce the best signal-to-noise level at the output of a receiver regardless of how "loud" a signal may sound. If the signal-to-noise ratio is the best possible, "loudness" can be provided at any point in the receiving system by means of additional gain.

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