 Microwave Engineering

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Microwave Engineering
ECE Microwave Engineering Fall 2011 Prof. David R. Jackson Dept. of ECE Notes 6 Waveguides Part 3: Attenuation

Attenuation on Waveguiding Structures
For most practical waveguides and transmission lines the loss associated with dielectric loss and conductor loss is relatively small. To account for these losses we will assume Phase constant for lossless wave guide attenuation constant Attenuation constant due to dielectric loss Attenuation constant due to conductor loss

Attenuation due to Dielectric Loss: d
Lossy dielectric  complex permittivity  complex wavenumber k Note:

Attenuation due to Dielectric Loss (cont.)
Thus This is an exact formula for attenuation due to dielectric loss. It works for both waveguides and TEM transmission lines (kc = 0). Remember: The value kc is always real, regardless of whether the waveguide filling material is lossy or not. Note: The radical sign denotes the principal square root:

Approximate Dielectric Attenuation
Small dielectric loss in medium: Use

Approximate Dielectric Attenuation (cont.)
Small dielectric loss: Use

Attenuation due to Dielectric Loss (cont.)
We assume here that we are above cutoff.

Attenuation due to Dielectric Loss (cont.)
For TEM mode We can simply put kc = 0 in the previous formulas. Or, we can start with the following:

Attenuation due to Conductor Loss
Assuming a small amount of conductor loss: We can assume fields of the lossy guide are approximately the same as those for lossless guide, except with a small amount of attenuation.  We can use a perturbation method to determine c . Note: Dielectric loss does not change the shape of the fields at all, since the boundary conditions remain the same (PEC). Conductor loss does disturb the fields slightly.

Surface Resistance This is a very important concept for calculating loss at a metal surface. C S Plane wave in a good conductor Note: In this figure, z is the direction normal to the metal surface, not the axis of the waveguide. Also, the electric field is assumed to be in the x direction for simplicity.

Surface Resistance (cont.)
Assume The we have Note that Hence

Surface Resistance (cont.)
Denote “skin depth” = “depth of penetration” Then we have At 3 GHz, the skin depth for copper is about 1.2 microns.

Surface Resistance (cont.)
Frequency 1 [Hz] 6.6 [cm] 10 [Hz] 2.1 [cm] 100 [Hz] 6.6 [mm] 1 [kHz] 2.1 [mm] 10 [kHz] 0.66 [mm] 100 [kHz] 21 [mm] 1 [MHz] 66 [m] 10 [MHz] 20.1 [m] 100 [MHz] 6.6 [m] 1 [GHz] 2.1 [m] 10 [GHz] 0.66 [m] 100 [GHz] 0.21 [m] Example: copper

Surface Resistance (cont.)
z x S fields evaluated on this plane = time-average power dissipated / m2 on S

Surface Resistance (cont.)
Note: To be more general: Inside conductor: where “Surface resistance ()” “Surface impedance ()”

Surface Resistance (cont.)
Summary for a Good Conductor

Surface Resistance (cont.)
conductor “Effective surface current” Hence we have For the “effective” surface current density we imagine the actual volume current density to be collapsed into a planar surface current. The surface impedance gives us the ratio of the tangential electric field at the surface to the effective surface current flowing on the object.

Surface Resistance (cont.)
Frequency Rs 1 [Hz] 2.6110-7 [] 10 [Hz] 8.2510-7 [] 100 [Hz] 2.6110-6 [] 1 [kHz] 8.2510-6 [] 10 [kHz] 2.6110-5 [] 100 [kHz] 8.2510-5 [] 1 [MHz] 2.6110-4 [] 10 [MHz] 8.2510-4 [] 100 [MHz] []6.6 1 [GHz] [] 10 [GHz] [] 100 [GHz] [] Example: copper

Surface Resistance (cont.)
We then have In general, For a good conductor, This gives us the power dissipated per square meter of conductor surface, if we know the effective surface current density flowing on the surface. Hence PEC limit:

Perturbation Method for c
Power flow along the guide: z = 0 is calculated from the lossless case. Power loss (dissipated) per unit length:

Perturbation Method: Waveguide Mode
There is a single conducting boundary. C S For these calculations, we neglect dielectric loss. Note : On PEC conductor Surface resistance of metal conductors:

Perturbation Method: TEM Mode
There are two conducting boundaries. For these calculations, we remove dielectric loss. Note : On PEC conductor Surface resistance of metal conductors:

Wheeler Incremental Inductance Rule
The Wheeler incremental inductance rule gives an alternative method for calculating the conductor attenuation on a transmission line (TEM mode): It is useful when Z0 is already known. The formula is applied for each conductor and the conductor attenuation from each of the two conductors is then added. In this formula, dl (for a given conductor) is the distance by which the conducting boundary is receded away from the field region. The top plate of a PPW line is shown being receded.

Example: TEM Mode Parallel-Plate Waveguide
Previously, we showed On the top plate: On the bottom plate:

Example: TEM Mode PPW (cont.)
(equal contributions from both plates) The final result is then

Example: TEM Mode PPW (cont.)
Let’s try the same calculation using the Wheeler incremental inductance rule. From previous calculations: d w

Example: TMz/TEz Modes of PPW
Results for TM/TE Modes (above cutoff): (derivation omitted) TMn modes of parallel-plate TEn modes of parallel-plate