Wave Guides Flashcards
Why do we use waveguides?
Lower losses and greater power handling
BUT
Expensive
How must the EM waves be oriented to the waveguide?
E-field - perpendicular to the wall
B-field - parallel to the wall
What are the two modse of EM propagation?
Transverse Electric
Transverse Magnetic
Describe Transverse Electric propagation
The E-field oscillates transverse to the direction of propagation.
Describe TM propagation
The magnetic field oscillates in adirection that is transverse to the direction of propagation.
Why do we use waveguides?
Losses in transmission lines increase with frequency due to conductor heating and dielectric heating.
Waveguides for > 1 GHz
What is an ideal shape for a waveguide and why?
Ideal: Maximum area to perimeter ratio (circular). Reflection off the conductor induce current, therefore a larger perimeter increases loss.
What would happen if an electric field were near to and parallel to the wall?
It would induce a current in the wall, causing losses.
What would happen if a magnetic field were oscillating perpendicular to a conductive wall?
It would induce a current in the wall leading to losses.
Describe the oscillation directions of the E-field and B-field in free space or a transmission line
Transverse to the direction of propagation
Why do TE and TM modes exist instead of being in TEM mode?
If it were TEM, the boundary conditions would lead to current generation in the walls, leading to losses.
What are “modes” wrt to TM and TE?
There are different combinations of wavelengths which can propagate in a given waveguide that satisfy the boundary conditions.
Why can m and n not equal zero for TM modes in a rectangular waveguide?
Magnetic field lines always form closed loops or we would violate Gauss’s Law of Magnetism.
Therefore the lowest possible TM is TM_11.
Why do higher order modes have greater losses?
The distance between a peak and a valley of the electric field decreases -> the resistance between the peak and valley decreases -> Power lost by conductor heating is proprtional to V^2/R -> Lower losses
Are waveguides high pass or low pass structures?
High Pass.
They will not pass EM waves below the cut-off frequency.
What happens to a signal at the cut-off frequency in a waveguide?
It will reflect back and forth at the mouth of the waveguide.
What is the cut-off frequency a function of?
The dimensions of the waveguide and the mode of the wave.
Why do we only want one mode at a time propagating through the waveguide?
Lower order modes will propagate faster, increasing signal dispersion.
What is bandwidth of a wavetguide?
Each mode has a cut-off frequency. The bandwidth is the range of frequencies from the cut-off of one mode to the cut-off the next higher mode.
What is the Group Velocity?
How fast energy travels down the waveguide.
What can we deduce about the speed of waves through a waveguide?
Waves travel slower through a waveguide than through free space.
Higher frequency waves travel faster than lower frequency waves for a given mode.
What is Phase Velocity?
How fast the wave changes phase parallel to the propagation path.
What can we deduce?
The phase velocity can travel faster in a waveguide than through free space. This is not in violation of relativity since no energy is being transmitted.
At higher frequencies, the phase velocity decreases.
What sources of loss in waveguides?
Reflections due to disconinuities
Currents in waveguide walls
Dielectric filling of waveguide
What can we do to reduce the losses in a waveguide?
Plate the inside with a layer of highly conductive metal such as gold or platinum.
What determines the maximum power rating of a waveguide?
The physical size and the dielectric.
Why do we pressurize waveguides?
Increasing the air pressure increases the dielectric strength, increasing the power rating.
Why do we use air driers in waveguides?
Reducing moisture will increase the dielectric strength, increasing the power rating.
How is dielectric strength related to the performance of the waveguide?
Higher dielectric strength - increased losses but higher power rating
How do dielectrics work?
Dielectrics are poor conductors of current but support electric fields which in turn allow propagation of EM waves.
Good dielectrics have high permittivity, the ability of a material to store energy in an electric field. Permittivity represents the ability for the medium to be polarized. A medium which is not easily polarized will not support an electric field well.
High permittivity: High power rating.
High permittivity will result in dielectric heating loss, conversion into heat. When an alternating electric field is applied to the dielectric, the dipoles change direction continuously to orient themselves with the field, which abosrbs energy and produces heat.
You do not need a medium to support an EM.
You dont want it to conduct easily or you would get dielectric breakdown.
What are pros and cons to a ridged waveguide?
Better bandwidth: Increases the effective width, lowering the cutoff frequency
Worse power rating: Can introduce reflections and uneven field distributions, affecting power dielectric losses
What is an E-Plane Bend?
Bends in plane of the electric field.
Describe H-Plane Bends
Bends in plane of magnetic field.
How do we make tight radius bends that minimize losses?
The bend radius should be at least 2 wavelengths, but by making at integral number of wavelengths, reflections will cancel.
Why are double mitered corners used?
For large wavelengths, a simple miter bend may reduce the distance between walls, risking dielectric breakdown.
How do we prevent losses when twisting a waveguide?
A 90 degree twist should be undertaken over a distance greater than 2 wavelengths.
What is the Directivity of a coupler detector?
The ratio of the forward power to the reverse power measured by the detector
What is the Coupling Coefficient of a coupler detector?
The fraction of power from the main waveguide that makes it into the axiliary waveguide.
How does a Magic T Junction work?
What is a Cavity Resonator?
EM waves will bounce back and forth between the walls of the cavity and construct until they reach the resonant frequency, at which point they will form a standing wave.
The resonant frequency is determined by the physical dimensions.
How does the size of the waveguide affect losses?
Smaller waveguide -> Higher cutoff frequency -> more losses
How does waveguide size affect bandwidth?
Larger waveguide -> More possible modes and lower cutoff frequency -> Greater bandwidth
State 2 advantages and disadvantages of waveguides compared to transmission lines.
Higher power rating
Lower losses
Expensive
Difficult to install
What are 3 sources of loss in a waveguide?
Reflections from obstacles, discontinuities, or misaligned sections
Currents in the waveguide walls
Dielectric losses =
What effect do ridged waveguides have on BW, Power handling, attenuation?
Geater BW
Lower Power Handling
Greater Attenutation
