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Author: J.B. Hoag
It is necessary to understand that the same guide can operate with several types or modes of vibration of the three-dimensional electric and magnetic field patterns. Fig. 38 O shows various methods of feeding energy into a rectangular waveguide, together with the resultant field patterns of the E11 and the H01 waves inside the guide.
The electric fields are shown by solid lines, the magnetic fields by dotted lines or by small dots and circles. These are the patterns at a given instant; actually, the waves are in rapid motion down the guide at velocities somewhat less than that of light.
In Fig. 38 P, the structures for launching waves down a cylindrical waveguide are shown, together with the corresponding field patterns, in the respective cases.
The same electrode structures shown in Figs. 38 O and 38 P can be used for the reception of their corresponding waves, the oscillator being replaced by a crystal or diode detector.
Figure 38 Q shows a detector mounted in a resonant cavity of a cylindrical guide for the reception of H1 waves. In order that the reception shall be strong, the electrode used must not only be of the same type as that used at the transmitter, but it must also be oriented in the same direction. Different types of waves may thus be transmitted down the same tube and sorted from each other at the receiving end.
It is only possible to transfer energy down a waveguide when the wavelength is sufficiently short. As shorter and shorter wavelengths are applied to the end of a guide, a critical value or cutoff wavelength is found. All waves shorter than this will pass down the tube, all longer waves will not; the system acts like a high-pass filter. Table 38 A gives the equations for the cutoff frequencies. It will be observed that the wavelength must be approximately equal to or less than the opening of the pipe before energy will pass down the guide.
When sufficiently short waves are used that the guide is reasonably small and not too costly, waveguides prove to be unusually satisfactory as paths for electromagnetic energy because their losses can be made smaller (by proper operation) than with other forms of conducting systems. The loss of energy in a waveguide depends on the mode of vibration and on the frequency. A typical attenuation curve is shown in Fig. 38 R.
Also, the loss depends on the metal used in the construction of the guide. In the case of an Eo wave, the minimum attenuation in decibels per mile, is as follows: copper, 1.9; aluminum, 2.5; lead, 6.5; iron, 45. The value of 1.9 just cited may be compared with that of 8.3 for a two-and-one-half inch diameter copper coaxial line, and of 25 for a No. 6-gauge open line.
The same guide may be used for the simultaneous and independent transmission of several waves. At the receiving end, short waves can be separated from longer ones by using a constriction in the waveguide, or a diaphragm with a hole in it. Only waves shorter than that permitted by the cutoff values of Table 38 A will pass beyond the constriction or hole. Also, filtering action may be accomplished so as to pass one mode of vibration and restrict another. For example, if a series of radial wires are mounted in a plane at right angles to the axis of a cylindrical tube, the Eo waves will be reflected whereas the Ho waves will pass through and continue on down the tube. If a series of parallel wires are mounted transversely across the tube, H1 waves will pass through at one position of the wires but will be cut off when the wires have been rotated 90 degrees. An examination of Fig. 38 P will show that this filtering action occurs when the electric vector of the electromagnetic wave is parallel to the conducting wires of the filter.
If the end of a waveguide is left open, radiation will take place into the space beyond the mouth of the guide. The end of the guide may be flared in a simple manner, as in Fig. 38 S, or in more complicated fashions, such as the exponential horn, in order to obtain either broadcast or highly directional radiation of microwave energy.
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