microwave comms final

7/31/2019 Microwave Comms Final

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College of Engineering

University of St. La Salle


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I. DECIBEL MEASUREMENTS

Decibel (dB)

measurement unit widely used in transmission

systems; it is logarithmic with the number base 10 unit that describes a ratio

express the magnitude of a change in signal or soundlevel

+ dB means a gain- dB - means a loss


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Relationship between L and G:

G dB = - L dB

Ex. 6 dB lossL dB = 6 dBG dB = -- 6 dB

useful for calculating gains, losses and power levels

Power:dB = 10 log P out

P inVoltage:

dB = 20 log E outE in

Current:dB = 20 log I out

I in

o


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Amplifiers in Cascade

P1 P2 P3 P4

Over-all gain, GT

GT = G1 G2 G3 .. Gn

In decibel,

GTdB = G1dB + G2dB + G3dB + + GndB

GidB G2dB G3dB


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Reference Powers

dB = 6 mW reference

dBm = 1 mW ref. ( most common )

dBW = 1 W ref. ( used in microwave ) dBf = 10 -15 W ref.

dBrn = 10 -12 W ref.

dBa = 10 11.5 W ref.

dBrap = 10 -16 W ref. (threshold of hearing)

dBrnC = 10 -12 W ref.


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Example:

1) dBm = 10 log P___

1 mW

2) dB = 10 log P___

6 mW

3) dBr n = 10 log P___

10 -12 W


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Comparison of +, -- , and 0 dB values:

+ dB = power level is greater than the reference

-- dB = power level is less than the reference0 dB = power level is equal to the reference

Then,

0 dBm = 1 mW0 dBr n = 10 -12 W


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4) dBW

P (dBW ) = 10 log P__

1 W

+ 30 dBm = 0 dBW

-- 30 dBW = 0 dBm


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Sample Problems:

1. An amplifier has an output of 20 watts. What is the output in dBm? P (dBm) = 10 log 20 W = + 43 dBm

1 mW 2. Find Pi and Po in dBm and get the gain. G

Pi Po 10 uW 1 W

GidB


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Relationships:

dBm + dB = dBm

dBmV + dB = dBmV

dBW + dB = dBW


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Decibel Sample Problems1. ( ECE November 1992 )

A TV antenna receives a signal measured at

200 uV and is immediately amplified by a pre-amplifier with a 15 dB gain. This amplified

signal then passes through a coaxial cable with

3 dB loss, what is the resulting input to the TVset, in dBmV?


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2. ( ECE September 1984 )

An amplifier has an input resistance of 200

ohms and output resistance of 6400 ohms.

When 0.5 volt is applied across the input, a

voltage of 400 volts appears across the output.

a. What is the power output of the

amplifier?

b. What is the power and voltage gain

measured in decibels?


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3. ( ECE November 1989 )

If you have available number of power

amplifiers with a gain of 100 each, how manysuch amplifiers do you use to cascade to give

an over-all gain of 60 dB?


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Transmission Problems4. Find the ERP in watts of an antenna having a

transmitter output power of 25 W, a line loss

of 6 dB and a connector loss of 0.5 dB and anantenna gain of 25 dB.

5. What is the ERP in dBw of a 50 dB antennaconnected to a transmitter with an output of10 kW through a transmission line loss of 5dB?


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Seatwork Sample Problems

1. ( ECE February 1975 )

An amplifier has an input resistance of 500 ohms

and output resistance of 8600 ohms. When 2.5

volt is applied across the input, a voltage of 600volts appears across the output.

a. What is the power output of the amplifier in

watts?

b. What is the power gain measured in

decibels?


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2. ( ECE August 1976 )

An amplifier has an output level of 43 dBm

connected to a load of 8 ohms. If the input is

supplied by a microphone with an output of 60

dBm and 200 ohm impedance, determine thefollowing:

a. power amplifier gain in dB

b. power amplifier output in watts, and the load

voltage and current

Note: Use as reference P1 = 0.001 watts


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3. Transmission Problems

What is the ERP in dBw of a 50 dB antenna

connected to a transmitter with an output of

10 kW through a transmission line loss of 5

dB?


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II. Radio Wave Propagation - 1 History

1865 predicted mathematically by James C. Maxwell

1887 - demonstrated experimentally by Heinrich R.

Hertz Introduction

Radio waves are one form of electromagnetic radiation( Ex. infrared, visible light, UV, X-rays, gamma rays )

Electromagnetic radiation has a dual nature: In some cases, it behaves as waves

In other cases, it behaves as particles (photons)


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For radio frequencies the wave model is generally more

appropriate Electromagnetic waves can be generated by many

means, but all them involve the movement of electricalcharges

Propagation: how radio waves get from point A topoint B. The events occurring in the transmission pathbetween two stations that affect the communications

between the stations.


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When the electrons in a conductor, (antenna wire) are

made to oscillate back and forth, ElectromagneticWaves (EM waves) are produced.

These waves radiate outwards from the source at thespeed of light, 300 million meters per second.

Light waves and radio waves are both EM waves,differing only in frequency and wavelength.

EM waves travel in straight lines, unless acted upon bysome outside force. They travel faster through a

vacuum than through any other medium.


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Electromagnetic spectrum


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Elec. Spec.

E M W


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E. M. Waves

Electromagnetic radiation comprises both an Electricand a Magnetic Field.

The two fields are at right-angles to each other andthe direction of propagation is at right-angles to bothfields.

The Plane of the Electric Field defines the Polarisation

of the wave.


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Two types of waves:

Transverse waves and Longitudinal Waves

Transverse Wavevibration is from side to side; that is, at right angles to

the direction in which they travel

Guitar string vibrates with transverse motion.

Electromagnetic transmissions move in space asTransverse waves


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Longitudinal waves:

Vibration is parallel to the direction of propagation.Sound waves, Pressure waves are

longitudinal.Oscillate back and forth, vibrations

along or parallel to their direction of travel

Ex. A wave in a "slinky" is a good visualization.


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Polarization: The polarization of an antenna is the orientation of

the electric field with respect to the Earth's surface andis determined by the physical structure of the antennaand by its orientation

Radio waves from a vertical antenna will usually be

vertically polarized. Radio waves from a horizontal antenna are usually

horizontally polarized.


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For both reflection and refraction, it is assumed thatthe surfaces involved are much larger than thewavelength; if not, diffraction will occur

Reflection of waves from a smooth surface (specularreflection) results in the angle of reflection beingequal to the angle of incidence


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Refraction

A transition from one medium to another results inthe bending of radio waves, just as it does with light

Snells Law governs the behavior of electromagnetic

waves being refracted:


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