318 BELL SYSTEM TECHNICAL JOURNAL 



momentarily doubled in intensity. The energy is predominantly magnetic, 

 and the type of retlection may be regarded as inductive. When the wave is 

 reflected from the ideal open-end line, a reverse situation prevails. The lines 

 of magnetic force momentarily vanish while lines of electric force, though 

 brought to rest, are doubled in intensity. At this moment the energy is pre- 

 dominantly electrostatic, and the reflection may be considered as being 

 capacitive. 



When a line is terminated in a sheet of metal of good conductivity such 

 as copper or silver, reflection is almost perfect. If the sheet is a poor conductor 

 such as lead or German silver, most of the incident power will still be re- 

 flected; but if a semi-conductor, such as carbon, is used as a reflector, a per- 

 ceptible amount of the incident power will be absorbed. It is interesting also 

 that the penetration into all metals at the time of reflection is very slight, 

 for relatively thin sheets seem to serve almost as well as thick plates. It is 

 therefore possible to use as reflectors extremely simple and inexpensive 

 materials, for example, foils or electrically deposited films fastened to a 

 cheaper material such as wood.^ 



A more general study of reflections on transmission lines shows that the 

 examples cited previously are special cases of a very general subject. Not 

 only may there be reflections from the open and closed ends of a transmission 

 line, but there may be reflections also when the line is terminated in an in- 

 ductance, in a capacitance, or in a resistance. Details concerning the re- 

 flections that may be observed from various combinations of these three 

 impedances are discussed in connection with Fig. 3.6-3. The outstanding 

 results of these discussions may be summarized for the ideal case as follows: 



1. A pure inductance (positive reactance) connected at the end of a 

 transmission line always leads to a reflection coefBcient having a magnitude 

 of unity. The standing wave resulting from this reflection will be charac- 

 terized by the following: (a) If the terminating inductance is infinitely large 

 (reactance of positive infinity), the reflection will be identical with that from 

 an ideal open-end line, and the distance to the nearest voltage minimum will 

 be a quarter wave. [See Fig. 3.6-3(a).] (b) If the inductance is finite but very 

 large, the distance to the nearest voltage minimum, as measured toward the 

 generator, will be somewhat greater than a quarter wave. [See Fig. 3.6-3(b).| 

 (c) If the inductance is reduced progressively toward zero (reactance zero), 

 the distance to the same voltage mininnmi will approach one-half wave- 

 length. In this limiting case, another voltage minimum will appear at the 

 end of the line. [See Fig. 3.6-3(c) and 3.6-3(d).] 



2. A j)ure capacitance (negative reactance) connected at the end of a 



~' One convenient and inexpensive form of reflector is a kind of l)uilding paper coated 

 with copper or aluminum foil. Moderately good reflectors can also he made i)y covering 

 wood with a special ])aint containing i'lnely divided silver in susi)ension (I)u I'ont's 4817). 

 Most aluminum paints are unsatisfactory for this purpose. 



