Waveguides

Above 2GHz , the wavelength is short enough to allow practical , efficient energy transfer by different means . A waveguide is a conducting tube through which energy is transmitted in the form of electromagnetic waves . The tube acts as a boundary that confines the waves in the enclosed space . The Faraday cage effects prevents electromagnetic effects from being evident outside the guide . The electromagnetic fields are propagated thjrough thje waveguide by means of reflections against its inner walls , which are considered perfect conductors . The intensity of the fiels is greatest at the center along the x dimension , and must fiminish to zero at the end walls because the existence of any field parallel to the walls at the surface would cause an infinite current to flow in a perfect conductor . Waveguides , of course , cannot carry RF in this fashion .
The X , Y , and Z dimensions of a rectangular waveguide can be seen in the following figure :

There are an infinite number of ways in which the electric and magnetic fields can arrage themselves in a waveguide for frequencies above the low cutoff frequency . Each of these field configurations is caled a mode . The modes may be separated into two general groups . One group , designated TM reaction of propagation , but has a component of the electric field in the direction of propagation . THe other type , designated TE ( Transverse Electric ) has the electric field entirely transverse , but has a component of magnetic field in the direction of propagation .


The mode of propagatyion is identified by the group letters followed by two subscripte numerals . For example , TE 10 , TM 11 , etc . The number of possible modes increases with the frequency for a given size of guide , and there is only one possible mode , called he dominant mode , for the lowest frequency that can be transmitted . In a rectangular guide , the critical dimension is X . This dimensions must be more than 0.5 diumlar at the lowest frequency to be transmitted . in practice , the Y dimension usually is made about equal to 0.5 X to avoid the possibility of operation in other than the dominant mode . Cross - sectional shapes other than the rectangle can be used , the most important being the circular pipe . Much the same considerations apply as in the rectangular case . Wavelength dimensions for rectangular and circular guides are given in the following table , where X is the width of a rectangular guide and r is the radius of a circular guide . All figure apply to the dominant mode .