ELECTENG 721 - Transmission Lines

Does not include smith charts since those are non examinable

Why is it important to choose the shortest line/stub length from a bandwidth perspective?

Choosing the shortest line/stub length minimizes how much the impedance changes with frequency, making the match less sensitive to frequency shifts. This results in a wider bandwidth and a more stable matching network.

Why is it important to use admittances instead of impedances in phasor amplitude calculations?

When calculating powers, we usually make use of voltages which appear across loads.

When the load is represented as an admittance, the voltage appears across the parallel combination of G and jB, and since jB does not contribute to power loss the power absorbed by a load is given by P = V²G.

When the load is represented as an impedance, the voltage appears across the series combination of R and jX and so

P =/= V²/R

What is an advantage of using an alternated line transformer to match transmission lines with different characteristic impedances

it makes use of sections of the 2 lines to be matched ( i.e. no intermediate Z₀ like the quarter-wave transformer)

Define broadband

the ability of a system to provide a specified level of performance over a range of frequencies

How can bandwidth performance be improved

Using multi-stage matching networks, e.g. cascaded quarter-wave transformers

In the case of stub-matching networks, using the shortest stubs possible

What is considered low frequencies and what components are preferred

(typically up to hundreds of kHz)

designs incorporating operational amplifiers are optimal

What is considered high frequencies and what components are preferred

typically up to hundreds of MHz)

lumped element solutions (containing discrete inductors and capacitors) are preferred

Why do really high frequencies have parasitic effects

At really high frequencies, the required inductances and capacitances become unrealistic as the performance of the physical components tend to be dominated by parasitic effects

What are the steps in designing a filter using transmission lines

1. Start with the filter specification (e.g. filter type, cutoff frequencies, attenuation in stopband, maximum ripple in passband, etc)

2. Use ‘standard’ filter design techniques5 to produce a lumped element prototype (consisting of inductors and capacitors)

3. Convert this lumped element prototype into a design incorporating transmission line elements

Why does a coaxial low pass filter work

The high-impedance lines (narrow centre conductor, characteristic impedance Z01) act as series inductors. The low-impedance line (thick centre conductor, characteristic impedance Z02) acts as a shunt capacitor

What is the objective of the NZS2772:Part 1:1999

to specify basic restrictions and reference levels for human exposure to radiofrequency fields in the frequency range 3 kHz to 300 GHz

Where is the NZS2772:Part 1:1999 applicable

wherever people may be exposed to RF fields in the course of their work and wherever the public may be exposed. Such as:

continuous wave (CW), pulsed and modulated fields

What is the purpose of the limits in NZS2772:Part 1:1999

intended to be used to as a basis for planning and procedures, designing protective facilities, the assessment of the efficacy of protective measures and practices, and guidance on health surveillance

What are the 2 types of limits on RF exposure that are used in the NZS2772:Part 1:1999

Basic restrictions — Developed from established health effects. Protection against established adverse health effects requires that these basic restrictions are not exceeded

Reference levels — More practical to measure and have been (conservatively) derived from the basic restrictions

What are the questions RF engineers ask to obtain a license and/or adopt measurement/design strategies which limit radiated emissions to acceptable levels

Will the designs/systems I am responsible for radiate in frequency bands (either intentionally or unintentionally) for which I (or my employer) have no license in which to operate?

Will the levels of radiated RF energy exceed the exposure standards in NZS2772:Part 1:1999?

What are the potential strategies for RF engineering in regards ot the NZS2772:Part1:1999

Potential strategies include the minimisation of unwanted out-of-band emissions via redesign, reducing power levels (where practicable), and performing measurements in screened enclosures (which contain any unwanted emissions

What is the ALARA principle

As Low As Reasonably Achievable

Do not engineer for the maximum power density (or SAR, E or H) as permitted by the standard, but design for what is needed for adequate system performance

What are the beneficial side effects of the ALARA principle

detrimental levels of interference will be reduced

Why should you never look down an open ended waveguide

Never look down an open-ended waveguide — it can emit concentrated microwave radiation, which can damage your eyes and tissues even without you feeling heat or pain.