Waveguide 3.1.3 3.2.1

Editor's Notes

• #3: A waveguide is essentially a pipe through which an EM wave travels. As it travels along the guide, it reflects from the walls. Most high power microwave energy transmission above about 6GHz is handled by waveguides. It can be used to carry energy between pieces of equipment or over longer distances to carry transmitter power to an antenna or microwave signals from an antenna to a receiver -are made of copper, aluminum, or brass
• #4: Waveguides are used to transfer electromagnetic power efficiently from one point in space to another. Some common guiding structures are shown in the figure. These include the typical coaxial cable, the two-wire and mictrostrip transmission lines, hollow conducting waveguides, and optical fibers.
• #6: The signal exiting the output port of the first transmission line is called the “through” (sometimes called the “direct”) signal since it is directly connected to the input port and the signal exiting the other transmission line is called the “coupled” signal.  Because these coupled signals are related to the direction of the through signals, couplers are called directional couplers. 
• #7: Port 1 and 2 is your primary waveguide. “through” (sometimes called the “direct”) 3 and 4 is the secondary waveguide. “coupled” signal Directional couplers are characterized by their insertion loss, coupling, and directivity. All three are normally specified in decibels. Insertion Loss - is the amount by which a signal in the main guide is attenuated. Coupling - is the specification that gives the amount by which the signal in the main waveguide is greater than that coupled to the secondary waveguide. Directivity-Refers to the ratio between the power coupled to the secondary guide, for signals traveling in the two possible directions along the main guide.
• #8: Waveguide Coupling is generally by means of some sort of flange. The function of such flange is to ensure… A typical piece of waveguide will have a flange at either end. When two pieces are joined, the flanges are bolted together, to ensure perfect mechanical alignment if adjustment is provided. This prevents an unwanted bend or step, either of which would produce undesirable reflections. It follows that the guide ends and flanges must be smoothly finished to avoid discontinuities at the junction.
• #9: With Waveguide Coupling naturally reduced in size when frequencies are raised, a coupling discontinuity becomes larger in proportion to the signal wavelength and the guide dimensions. Thus discontinuities at higher frequencies become more troublesome. To compensate for the discontinuity which would otherwise be present, a small gap circular choke ring of L cross section is used in the choke flange, in order to reflect a short circuit at the junction of the waveguides. This is possible because the total length of the ring cross section, as shown, is λp/2, and the far end is short-circuited. Thus an electrical short circuit is placed at a surface where a mechanical short circuit would be difficult to achieve.
• #11: Reflections in a waveguide system cause impedance mismatches. When this happens, the solution is identical to the one that would be employed for transmission lines: lumped impedance or stubs. In transmission lines there is a need of matching Impedance for maximum power transfer. That is in Microwave, a lumped impedance of required value is placed at a precalculated point in the waveguide to overcome the mismatch, canceling the effects of the reflections. Similarly, If the waveguide impedance is matched to the source or load, then a greater level of power transfer will occur. When waveguides are not accurately matched to their loads, standing waves appear, and not all the power is transferred. To overcome the mismatch it is necessary to use some waveguide impedance matching techniques.
• #12: Like any transmission line, a waveguide has a characteristic impedance. Characteristic impedance of a waveguide is a function of frequency. 377 ohms comes from the intrinsic empedance (eta 𝜼) in free space in which 𝜼 = 𝝻 0 𝞊 𝑜
• #13: There are two velocities in a waveguide, and both change with frequency.
• #14: Uses a probe resembling a quarter-wave monopole antenna. The probe couples to the electric field in the guide, and it should therefore be located at an electric field maximum. Shows another way to couple power to a guide. A loop is used to couple with the magnetic field in the guide. It is placed in a location of maximum magnetic field. Is simply to put a hole in the waveguide, so that electromagnetic energy can propagate into or out of the guide from the region exterior to it.
• #15: It is found that abrupt changes in a waveguide will give rise to a discontinuity that will create standing waves. However gradual changes in impedance do not cause this. This approach is used with horn antennas - these are funnel shaped antennas that provide the waveguide impedance match between the waveguide itself and free space by gradually expanding the waveguide dimensions.
• #16: Matching Devices or Waveguide Irises Inductive Iris: The iris places a shunt inductive reactance across the waveguide that is directly proportional to the size of the opening. Notice that the edges of the inductive iris are perpendicular to the magnetic plane. Capacitive Waveguide Iris: The shunt capacitive reactance, illustrated in view (B), basically acts the same way. Again, the reactance is directly proportional to the size of the opening but the edges of the iris are perpendicular to the electric plane. Inductive and Capacitive Waveguide Iris: The iris, illustrated in view (C), has portions across both the magnetic and electric planes and forms an equivalent parallel-LC circuit across the waveguide. At the resonant frequency, the iris acts as a high shunt resistance. Above or below resonance, the iris acts as a capacitive or inductive reactance.
• #17: In addition to using a waveguide iris, post or screw can also be used to give a similar effect and thereby provide waveguide impedance matching. The waveguide post or screw is made from a conductive material. To make the post or screw inductive, it should extend through the waveguide completely making contact with both top and bottom walls. For a capacitive reactance the post or screw should only extend part of the way through. When a screw is used, the level can be varied to adjust the waveguide to the right conditions
• #18: The attenuators are basically passive devices which control power levels in microwave system by absorpsion of the signal. Attenuator which attenuates the RF signal in a waveguide system is referred as waveguide attenuator. There are two main types fixed and variable. They are achieved by insertion of resistive films.
• #20: Using waveguides require some passive components that are used with feed-lines.
• #21: E-bend: The bend must have a radius greater than two wavelengths.
• #22: H-Bend: It creates a bend around the thinner side of the waveguide. This also must have a radius greater than two times the wavelength. The bend angle is usually 30 degrees, 45 degrees or 90 degrees, however this can be customized based on the requirement. When selecting the waveguide bend the waveguide size and the flange type need to be selected - these usually vary based on the frequency at which you are planning to use the specific waveguide bend.
• #23: Waveguide Twist There are also instances where the waveguide may require twisting. This too, can be accomplished. If a complete inversion is required, e.g. for phasing requirements, the overall inversion or 180° twist should be undertaken over a four wavelength distance. used to change the signal polarization.
• #25: E-Plane Waveguide Tee: Also called series tee. • If a signal is applied to port C, the output will appear at port A and B but half the power and the same phase. • If two input waves at port A and B are in phase, the output at port C is additive and in phase H-Plane Waveguide Tee: Also called shunt tee. • Input at port D has an equal output at port A and B but 1800 out of phase Hybrid tee - Magic tee
• #26: •It can provide an isolation between signal. •If two in-phase and equal signals are fed to port A and B, cancellation will be in port D but reinforcement in port C. Similarly, if energy is fed to port C, energy appears at port A and B but not port D.