Basics of RF Waveguide Design
The design of RF waveguides is based on electromagnetic theory, propagation theory, and several technicalities that are discussed in this section.
Propagation Theory
According to propagation theory, different kinds of waves can be transmitted within a waveguide that correspond to different elements within the wave. Information about these waves is given below:
- TE waves: In transverse electric waves (or H waves), the electric vector (E) is always perpendicular to the direction of propagation.
- TM waves: In transverse magnetic waves (or E waves), the magnetic vector (H) is always perpendicular to the direction of propagation.
- TEM waves: Transverse electromagnetic waves are included for completeness and cannot be propagated in a waveguide. In these waves, the H vector as well as the E vector is perpendicular to the direction of propagation.
TE and TM waves are normally denoted with integers following them- TE_{mn }where the integers can take value from 0 or 1 to infinity to indicate the wave modes of the waveguide.
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Waveguide Propagation Constant
The propagation constant (γ) for waveguides defines the amplitude and phase of every element of the wave when it gets transmitted along the waveguide. The factor for each element can be expressed as:
exp[j ω t – γm,n z]
where ω is the angular (2 π x) frequency and z is the direction of propagation.
For example, if γ_{m,n }is real,_{}then the phase of each element is constant. So, the amplitude decreases exponentially as the value of z increases. In such a case, there is no significant propagation and the frequency is also below the cut-off frequency of the waveguide.
Let us consider the case where γ_{m,n }is imaginary: then the amplitude of each element remains constant whereas the phase changes with z implying that propagation takes place within the waveguide.
γ_{m,n }is purely imaginary if there is a total lossless system. In fact, there are always some losses involved; so γ_{m,n }will contain a real part (α_{m,n}) as well as an imaginary part (β_{m,n}).
Accordingly,
γm,n = αm,n + j βm,n
This theory of waveguides and the related equations are true for all kinds of waveguides, be they circular or rectangular.
Waveguide Dimensions
RF waveguides are available with different designations and sizes like WR or WG waveguides. The dimension of the waveguide determines its properties including the parameters. The sizes for waveguides are standardized so that units from various manufacturers can be used collectively; these standards are specific to countries. The major standards are-
- WG waveguide system: RCSC Designation (Standard for UK)
- WR waveguide system: EIA designation (Standard for US)
The dimensions of the waveguides are highly significant since they decide the operating range for the waveguide. A specific waveguide size should consider the required frequency range and other factors like weight, mechanical size, loss, etc.
Waveguide Construction
In this section, let us see the construction of the simplest waveguide- the flexible waveguide- as these are the most necessary components for many waveguide installations. There are several forms of flexible waveguides:
- Flexible waveguides can be designed by winding flat ribbons on a rectangular mandrel. Then, the edges can be folded in and interlocked. If desired, it can be soldered; however, this might reduce the flexibility to some extent.
- Such waveguides can also be constructed by shaping thin wall rectangular tubing or by bending and soldering corrugated sheet metal. This forms a corrugated flexible waveguide.
- The common form of construction is to wind a brass strip coated with silver to form a uniform rectangular tube; this is a helically wound system.
Irrespective of the different forms, these waveguides are jacketed in neoprene, viton, silicone, or devcon for protection from mechanical damage.
Applications of RF Waveguides
The applications of waveguides are very wide for carrying RF energy from one point to another. When RF waves propagate in open space, they travel in all directions, which results in a decrease in power intensity as the distance increases. Waveguides are thus useful in confining the waves to a specific area, thus nullifying the scattering of the waves.
Normally, a RF waveguide can be viewed as a transmission line including a hollow conducting tube (circular or rectangular in shape) in which the waves are transmitted. The waves propagate within the confinement of the walls of the waveguide, and this is possible due to the total internal reflection from the metallic walls.
Conclusion
Though RF waveguides are more expensive when compared to other kinds of feeders, they have many advantages. More importantly, they are the only possible solution for several applications.
Suggested Reading
Radio-Electronics.com – WG and WR Waveguide Dimensions and Sizes
Bright Hub – Power Flow in Symmetrical Components
Kyushu University, Fukuoka, Japan – Design of a Coplanar Waveguide (PDF)