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CHAPTER 11.1. The R-f 11 R-F COMPONENTS BY A. Transmission E. WHITFORD the Problem.—In block diagram of a basic radar system shown in Fig. 14, the parts shown as heavy double lines transmit the radio-frequency (r-f) energy from the magnetron to the antenna, and receiving book carry the faint apparatus this function considerable around echo is located. is body performed of theory this class of transmission circuit Although elements; space duction terminals lines The function good where k between conductors is here,l A uniform behaves as introduced -... (1) l?llly Micrwave Circuits, the at losses 1 Xfl-....)1",c line if Wavegwid. the in r, of were input the introradar. to the line the known impedance, input as its ZO, is a of the line, and is almost purely inner at material radius resistive. in the of the outer its characteristic annular space and conductor, conductor. any point infinitely long; in there disappears any is into no the discontinuous impedance reflection. Power termination change with along the -.l :_.Ill"1,,,~~""KSUlLILISSSlltX)~h~CL_. L-- L__,.. .L:. --:.-.. -;. 11.. --L.lJ~f... b Uii,, ,UU, ,U vol. Vol. of the inner Transmission Handbook, applied and insulators of is the However, --2.2 .-... l,,edLII, —.-. CALC,,Utl, e,, wavelengths. sufficient line, 'the current which flows dielectrics terminals line. is, size of losses, the terminated line A grown up in microwave characteristic constant dielectric radius practices property The low-loss conductors, outside waveguides. have and to make under- is suddenly of the conductors and the in this from the far end arrive to confuse the line, neglecting the the the by impedance. " For a concentric and be many treatment transmission before reflections of the geometry where treated essential new feature of the line may a limited a voltage is determined "characteristic (3) circuits. of a long and uniform situation, small only Waves.—When in the initial interval, TI is coaxial some of the reasons for current Standing for by will be given to show the general approach, standable the the T-branch of radar is of the same order as the physical the length permits into the types and a new set of techniques of course, that the wavelength the signals For 10; (4) Principles 391 9; (z) of Microwave Microwave ~01. hpk%rs, Circuik?, Vol. 8. 14; RF 392 [SEC.11.1 COMPONENTS line, such as might be introduced by a change in dimension of one of the conductors, or any change in geometry introduced by a sharp bend, or by a dent or an obstacle in the line, will produce reflected energy travels back toward the source. waves in the line. These can be observed a reflection. The This results in standing by sliding a small probe along a slot in the line after the manner shown for a concentric line in Fig. 11.1. Only a negligible fraction of power but, is abstracted fed into by the a suitable the probe, indicator, this M sufficient to regw,ter the voltw~o*~ age;ariati;nsalong~heline. ~~. 11"1.—slot~d VWfi~ ~ne "~ In general there will be voltage maxima for observing utandmgwaves (cutaway view). probe spaced at half-wavelength intervals with minima halfway bet ween them. Only if the line is perfectly matched will the voltage reading be constant the probe moves along. The ratio of the maximum age is called the "voltage criterion of the standing-wave of how well a line is matched. line, circuit, such as would results infinity. in zero at property reflection an open half-wave between PSWR SWR, db and Half-wave Quarter-wave from volt- and is the usual at the end circuit intervals or a short and a VSWR of may also be expressed as a power ratio (PSWR), The relations less transmission long is terminated (VSWR) Complete be expected voltage The mismatch or in decibels. This ratio" as to the minimum these three measures are = (VSWR)Z = 10 log,, (PSWR). Lines.—It can be shown that when a loss- line of characteristic impedance in an impedance 2,, the input is widely wavelength (1) Zi = Z~/Zl. For used. Z, a quarter impedance is example, two transmission lines of differing impedance can be matched to each other by joining them through a quarter-wave line whose characteristic impedance is the geometric mean Or, of that if of the two a quarter-wave impedance is infinite, an open-circuited a short circuit. lines. line i.e., This equivalent quarter-wave to of the has many (Sec. 118). uses by an a matching a short open circuit. line appears at the input a wavelength characteristic also, particularly = impedance in transformer. circuit, For a lossless line half 2, irrespective is called is terminated the input Conversely, terminal to be long 2,, (2) of the line. duplexers (Sec. This 11.5) and principle mixers SEC.11.2] COAXIAL LINES Why a Matched Line?—The fraction to the source from a section of transmission Power reflection coefficient of the incident systems or unpressurized is a possibility, [(VSWR) [(VSWR) = airborne + reflected is 1]' 1]2 (3) 1.5, it is seen that the power surely not serious. In high- systems, where line breakdown the strain is of course higher for a high VSWR. of 1.5 means that for a given breakdown gradient, can be delivered be delivered This power line of given VSWR For the usual upper design limit of VSWR = reflection loss is only 4 per cent, or 0.3 db, power 393 to the load than could A ratio 33 per cent less power in a matched load. can be a limitation. The strongest properties netron load requirement for of the magnetron. exhibits into lower than mum. an output which it a well-matched Like frequency works. 20 at voltage mismatched minimum, or negative. component in Chap. from the the mag- dependent upon represents a resistance line and higher than As explained arises oscillators, and a stability A At other phases it has a reactive positive line all self-excited the ZO at voltage maxi- which maybe either 10, magnetrons are in gen- eral designed to be stable against a VS WR of 1.5 in any phase. the origin of the commonly specified upper limit for mismatch. There is a further long line-for as the frequency and limitation example so also changes the number does the phase seen at the magnetron magnetron of the of wavelengths of the standing the phase happens line with frequency itself, and of opposite netron has no stable netron is This wave. The function known Then line impedance of frequency. to be such that the variation is more In another, is at the end of a in the line changes, sign, a condition frequency. stabilized. occurs from the magnetron. is therefore a rapidly varying If at a certain frequency of reactance if the mismatch 50 to 100 wavelengths This as rapid than that of the results where the magfavorable, the phase "long line the mageffect.' The result is that for long lines either (1) VSWR'S lower than 1.5 are necessary to guarantee stable the effective be included stretcher" in the not unlike or a dielectric outer usually ,I Micrmuaw antenna line. of or a trombone, The are These cables not with Ma#ndrorz.s, aMicrowave Transrntision Circuits, Vol. (2) a method latter may be new. consisting At a solid Vol. 6, 9, Radiation lower dielectric wave done must by a squeeze add an undesirable lines of changing of the standing or in wave guide by Lines.-Coaxial conductors consisted operation, and hence the phase phase shifter.z 1102. Coaxisl and magnetron line length a "line section adjustment. of concentric frequencies and inner they have a braided outer Radiation Laboratory Laboratory Series. Series. is