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Thus, some procedure must be devised for calculating the SWR on a transmission line at a frequency J" if the load has been matched correctly to the line at a frequency f'. A procedure will now be given for a line and load matched by means of a short-circuited stub; the quarter-wave transformer situation is analogous.
One very obvious requirement is for some sort of adjustable stub, which could cope with frequency or load impedance changes and still give adequate matching. Finally, it would be very handy indeed to have a transmission line, for measurement purposes, on which the various quantities such as nodes,- antinodes, or SWR could be measured at any point. All such evennrnlities are covered by special components, or "hardware," which will now be-discussed.
I Since the pattern has two degrees of freedom the SWR and the position of the first maximum , so must the stub matcher. A single stub of adjustable position and length will do the job very well at frequencies below the microwave range. This fre'luency range is not rigidly defined, but it is commonly understood to occupy the spectrum. It is also called broadly the centimeter range, since the two extreme frequencies correspond to wavelengths of30 and 0.
At such high frequencies coaxial lines are employed instead of parallel-wire lines, and difficulties with screened slots are such that stubs of adjustable position are not considered. The two stubs are placed 0. Two variables are provided, and very good matching is possible. Such a matcher is normally C. Qnnected beiween the load and the main transmission line to ensure the shortest possible length of mismatched line.
The method of adjustment for matching is best described as "guesstimation," i. When trial and error is used, the stub nearest to the load is set at a number of points along its range, and the farther stub is racked back and forth over its entire range at each setting point of the first stub until the best possible match has been achieved. The SWR is, of course, monitored constantly while adjustment is taking place.
Unless the load is most unusual, for example, almost entirely reactive, it should be possible to reduce the SWR on the main line to. If almost perfect matching-under all conceivable conditions of SWR and load impedance is required, a triple-stub tuner should be used. This is similar to the doublestub tuner but consists of three stubs, adjustable in length and placed 0. This is often done by a sampling technique, in which a known fraction of the power is measured, so that the total may be calculated.
A number of coupling units are used for such purposes and are known as directional couplers, the two-hole coupler shown in Figure being among the most popular.
This particular one is discussed because it is a good illustration of transmission-line techniques and has a direct waveguide counterpart see Section The probes do not actually touch the inner conductor of the auxiliary line.
They couple sufficient energy into it simply by being near it. If they did touch, most of the energy instead of merely a fraction in the main line would be coupled into the auxiliary line; a fraction is all that is needed. The probes induce energy flow, in the auxiliary line which is mostly in the same direction as in the Forward wave components proceed. The auxiliary line is terminated at one end by a resistive load.
The other end goes to a detector probe for measurement. It now remaios to ensure that only the forward wave of the main line can travel from left to right in the auxiliary line. The outgoing wave entering the auxiliary line at A, and proceeding toward the detector, will meet at B another sample of the forward wave.
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Communication systems refer to the sending and receiving of electrical impulses over a particular distance by radio, telephone, television, cable, telegraph etc. The whole process of communication involves a few essential fundamentals- transmitter, receiver, noise variable and a channel through which the information is transmitted. The book explains concepts in a theoretical and illustrative manner.
In the fifth edition of this book, the form and tone of the book have been retained but various technological updates have been incorporated in order to keep the students abreast of the latest trends regarding communication systems.
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