Experiment 1 Principles of Microwave Propagation In Wave...

Faculty of Information Engineering and Technology Communications Department Communication Lab COMM-606

Radio Frequency and Microwave Engineering

Experiment 1

Principles of Microwave Propagation

In Wave Guides

Lecturer:

Associate Prof. Hany Hammad

Assistants:

Eng. Mohamed Salah

Eng. Shoukry Ibrahim

Eng. Walid Galal

Eng. Abdelfattah Fawky

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TABLE OF CONTENTS

Preface...................................................................................................................... 3

Safety Instructions ..................................................................................................... 4

The Microwave Source............................................................................................... 5

Part I. Principles of microwave propagation in wave guides .......................................... 7

Radio Frequency & Microwave Engineering Experiment 1

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Preface:

The experiments in the present training system are intended to achieve various training

objectives in parallel:

a) The understanding of the physical effects which are of significance in microwave

technology (for example, diffraction and interference of electromagnetic waves, Gunn

effect).

b) Acquiring knowledge of the function of important components and systems of

microwave technology and the principles behind the methods of realizing these functions

by exploiting physical phenomena (such as the function of a waveguide directional

coupler in microwave circuits in “black-box” representation and the realization of the

directional coupler using effects in electromagnetic coupling through holes).

c) Acquiring skills in measuring techniques and principles to determine the properties of

microwave devices (example: measuring the reflection coefficient using the reflectometer

principle).

d) Becoming familiar with the actual technical design of various components and using

these in practical applications (examples: learning to correctly install a ferrite

waveguide isolator in a microwave circuit, operation of a slide screw transformer for

matching of a load).

The components for carrying out experiments with the above objectives are all designed in

Waveguide Technology. One discrepancy, however, had to be considered when designing this

training system: on the one hand, waveguide technology is the most suitable of all technologies

for experiment purposes; on the other hand, in the field of radio-frequency circuit technology, it

is with few exceptions, being increasingly overtaken by microwave integrated circuits (MIC)

utilizing microstrip line or coplanar line technology. The result of this discrepancy is the concept

of a teaching and training system based primarily on waveguide technology, in which the

physical phenomena and the fundamental principles of technical elements and measuring

methods are given priority, independent of the particular transmission line forms waveguide,

coaxial line, microstrip line, etc.).


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Safety Instructions:

Important! Read this before putting the equipment into operation!

Due to the low power level of the Gunn oscillator (approx. ~ 10 mW) there is absolutely no

danger for persons conducting the experiment, even in the case of free space experiments.

Nevertheless, the following rules should also be observed in view of later work with stronger

RF-sources:

– It is imperative that any direct viewing into the radiating aperture be avoided in all of the

experiments in which RF-power is radiated. This applies for open waveguides, for

example, and especially all types of antennas.

– Disconnect the supply voltage whenever modifications are made to experiment setups in

which waveguide components are interchanged.

Active elements in microwave components can be destroyed by electrostatic discharge.

Detectors and varactor components are especially endangered! DO NOT TOUCH! Discharge

any long cables before connecting them to these components. This is carried out by

connecting them to the power supply unit.

Components which operate with strong permanent magnets, e.g. isolators or circulators, must

be kept at a distance from magnetically conductive materials. Avoid shaking or bumping the

equipment.

The flange surfaces should be treated with care. Mechanically movable parts are to be

carefully lubricated from time to time. Do not allow any oil or grease on electrical contacts or

in the waveguide.


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The Microwave Source:

In this training system, the microwave source is a GUNN oscillator. This source generates the

microwave field needed for the load connected downstream. Only through modulation, it is

possible to perform frequency-selective detection of the demodulated receiving signal and thus

carry out effective noise suppression. For the design of a modulated microwave source there are

two options shown in Fig.1 and Fig.2. These options are:

1. The direct modulation of the GUNN oscillator:

Direct modulation of the Gunn oscillator is one way of performing modulation without additional

equipment but also without the best results. The emitted microwave power (and the emitted

spectrum) is severely dependent on the GUNN voltage. Furthermore, these variables are also

subject to considerable manufacturer tolerance involving the GUNN diode. Due to its

dependency on the GUNN voltage, the emitted microwave power demonstrates discontinuities or

irregularities. Also, in the case of amplitude modulation of the Gunn voltage where the operating

point (supply voltage) is in the proximity of a discontinuity (approx. 4V), a slight variation in the

Gunn voltage leads to a considerable variation in the emitted power and thus to a large

demodulated signal. This is done as follows:

– Connect the GUNN oscillator to the GUNN socket in the GUNN POWER SUPPLY

section of the basic unit (737021), see Fig.1.

– Set the toggle switch in the PIN MODULATOR section of the basic unit to “GUNN

INT”.

– Set the Gunn voltage to approx. 4 V using the UG controller.

The Gunn voltage is then superimposed by a square-wave signal with a frequency of 976 Hz and

amplitude of 300 mV.

Figure 1 Connection sketch for direct modulation of the


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2. Modulation with an external PIN modulator

The experiment setup is performed as depicted in Fig.2. The Gunn oscillator generates a

continuous wave power, i.e. its power output is constant in time. The modulation is performed by

a PIN modulator connected downstream. Depending on the control voltage this can have a

variably high reflection and transmission coefficient. In order to prevent any undesired reflections

from reflecting back to the Gunn oscillator, this is isolated from the rest of the circuit by an

isolator. The unit consisting of the Gunn oscillator, isolator and PIN modulator constitutes a

typical configuration frequently used in practical applications and is known for its easy handling.

Thus you avoid the disadvantages of direct modulation performed along a Gunn diode's

characteristic (subject to discontinuity) which is severely dependent on manufacturer's tolerance.

The modulated signal is considerably better than direct modulation because in this case the

amplitude range only depends on the control voltage of the PIN modulator. As the Gunn diode

can operate at a fixed operating point, the emitted microwave spectrum is not affected by the

modulation. Additional information can be taken from the instruction sheet for the PIN

modulator. If you have a PIN modulator at your disposal, then we recommend using it for

modulation to obtain better measurement findings. The PIN modulator can be modulated

internally or externally. In this experiment it is modulated internally.

Figure 2 Connection sketch for modulation with external PIN modulator and internal modulation signal.

Switch set to “PIN INT”


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Part I. Principles of microwave propagation in wave guides:

This part of the experiment aims to investigate the main principles of electromagnetic waves

propagation in transmission lines terminated with different loads. The SWR and complex

reflection coefficient is measured practically using the slotted measuring line. This part is taken

form experiments 5 and 6 from MTS. 7.4.4. Microwave Propagation in Wave Guides.


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