Chapter 14: Transmission Lines

TRUE/FALSE

1.A transmission line is a metallic cable.

ANS:T

2.Coaxial cables are referred to as "unbalanced".

ANS:T

3."Balanced" means that both conductors are the same size.

ANS:F

4.Twisted-pair telephone wire is a kind of transmission line.

ANS:T

5.Cable resistance does not depend on frequency.

ANS:F

6.Losses occur in the plastic dielectric of a transmission line.

ANS:T

7.Basically, a transmission line looks like an inductor in series with a small resistor.

ANS:F

8.Distributed parameters (inductance and capacitance) are characteristic of transmission lines.

ANS:T

9.Transmission lines are often considered to be "lossless" at higher frequencies.

ANS:T

10.The characteristic impedance of a cable depends mostly on the resistance of the wires.

ANS:F

11.For coaxial cables, there are only a few standard values in common use.

ANS:T

12.The value of the load at the end of a transmission line must be equal to or less than Z0.

ANS:F

13.RF signals travel slower on a transmission line than they would through free space.

ANS:T

14.Energy can "reflect" from a load at the end of a cable and travel back to the source.

ANS:T

15.A "mismatched" line will cause reflections.

ANS:T

16.The typical load on a cable, such as an antenna, has the same value at any frequency.

ANS:F

17.A mismatched line between a transmitter and an antenna could actually damage the transmitter.

ANS:T

18.A matched transmission line will exhibit "standing" waves.

ANS:F

19.The optimum value for SWR is one.

ANS:T

20."Ghost" images on a cable TV indicate an impedance mismatch.

ANS:T

21.If a cable is shorter than 1/16 of the signal's wavelength, it does not really behave as a transmission line.

ANS:T

22.Shorted stubs radiate more energy than open stubs do.

ANS:F

23.It is often better to measure SWR at the load rather than the source.

ANS:T

24.The only limit to the power that a transmission line can carry is the heat from I2R losses.

ANS:F

25.A length of transmission line can be used to match impedances at very high frequencies.

ANS:T

26.A circular graph called a "Jones" chart is commonly used to analyze transmission lines.

ANS:F

27.There is no practical way to connect a balanced line to an unbalanced line.

ANS:F

28.A 1/4 wavelength transmission line can be used as a transformer.

ANS:T

29.When used, stubs are usually inserted in series with a transmission line.

ANS:F

30.Slotted-lines are only useful at lower frequencies.

ANS:F

MULTIPLE CHOICE

1.SWR stands for:a.Shorted Wave Radiationc.Shorted Wire Regionb.Sine Wave Responsed.none of the above

ANS:D

2.TDR stands for:a.Total Distance of Reflectionc.Time-Domain Responseb.Time-Domain Reflectometerd.Transmission Delay Ratio

ANS:B

3.An example of an unbalanced line is:a.a coaxial cablec.an open-wire-line cableb.300-ohm twin-lead TV cabled.all of the above

ANS:A

4.When analyzing a transmission line, its inductance and capacitance are considered to be:a.lumpedc.equal reactancesb.distributedd.ideal elements

ANS:B

5.As frequency increases, the resistance of a wire:a.increasesc.stays the sameb.decreasesd.changes periodically

ANS:A

6.The effect of frequency on the resistance of a wire is called:a.I2R lossc.the skin effectb.the Ohmic effectd.there is no such effect

ANS:C

7.As frequency increases, the loss in a cable's dielectric:a.increasesc.stays the sameb.decreasesd.there is no loss in a dielectric

ANS:A

8.The characteristic impedance of a cable depends on:a.the resistance per foot of the wire usedb.the resistance per foot and the inductance per footc.the resistance per foot and the capacitance per footd.the inductance per foot and the capacitance per foot

ANS:D

9.For best matching, the load on a cable should be:a.lower than Z0c.equal to Z0b.higher than Z0d.50 ohms

ANS:C

10.The characteristic impedance of a cable:a.increases with lengthc.increases with voltageb.increases with frequencyd.none of the above

ANS:D

11.The velocity factor of a cable depends mostly on:a.the wire resistancec.the inductance per footb.the dielectric constantd.all of the above

ANS:B

12.A positive voltage pulse sent down a transmission line terminated in a short-circuit:a.would reflect as a positive pulseb.would reflect as a negative pulsec.would reflect as a positive pulse followed by a negative pulsed.would not reflect at all

ANS:B

13.A positive voltage pulse sent down a transmission line terminated with its characteristic impedance:a.would reflect as a positive pulseb.would reflect as a negative pulsec.would reflect as a positive pulse followed by a negative pulsed.would not reflect at all

ANS:D

14.A positive voltage-pulse sent down a transmission line terminated in an open-circuit:a.would reflect as a positive pulseb.would reflect as a negative pulsec.would reflect as a positive pulse followed by a negative pulsed.would not reflect at all

ANS:A

15.The optimum value for SWR is:a.zeroc.as large as possibleb.oned.there is no optimum value

ANS:B

16.A non-optimum value for SWR will cause:a.standing wavesc.higher voltage peaks on cableb.loss of power to loadd.all of the above

ANS:D

17.VSWR stands for:a.variable SWRc.voltage SWRb.vacuum SWRd.none of the above

ANS:C

18.The impedance "looking into" a matched line:a.is infinitec.is the characteristic impedanceb.is zerod.50 ohms

ANS:C

19.A Smith Chart is used to calculate:a.transmission line impedancesc.optimum length of a transmission lineb.propagation velocityd.transmission line losses

ANS:A

20.Compared to a 300-ohm line, the loss of a 50-ohm cable carrying the same power:a.would be lessc.would be the sameb.would be mored.cannot be compared

ANS:B

21.A balanced load can be connected to an unbalanced cable:a.directlyc.by using a "balun"b.by using a filterd.cannot be connected

ANS:C

22.On a Smith Chart, you "normalize" the impedance by:a.assuming it to be zeroc.multiplying it by 2pb.dividing it by 2pd.dividing it by Z0

ANS:D

23.The radius of the circle you draw on a Smith Chart represents:a.the voltagec.the impedanceb.the currentd.none of the above

ANS:D

24.The center of the Smith Chart always represents:a.zeroc.the characteristic impedanceb.oned.none of the above

ANS:C

25.A TDR is commonly used to:a.measure the characteristic impedance of a cableb.find the position of a defect in a cablec.replace a slotted-lined.all of the above

ANS:B

COMPLETION

1.A cable that lacks symmetry with respect to ground is called ____________________.

ANS:unbalanced

2.Parallel lines are usually operated as ____________________ lines since both wires are symmetrical with respect to ground.

ANS:balanced

3.Normally, a transmission line is terminated with a load equal to its ____________________ impedance.

ANS:characteristic

4.Twisted-pair cables are transmission lines for relatively ____________________ frequencies.

ANS:low

5.To analyze a transmission line, it is necessary to use ____________________ parameters instead of lumped ones.

ANS:distributed

6.The increase of a wire's resistance with frequency is called the ____________________ effect.

ANS:skin

7.The increase of a wire's resistance with frequency is caused by the ____________________ field inside the wire.

ANS:magnetic

8.Dielectrics become more ____________________ as the frequency increases.

ANS:lossy

9.The inductance and capacitance of a cable are given per unit ____________________.

ANS:length

10.Characteristic impedance is sometimes called ____________________ impedance.

ANS:surge

11.A cable that is terminated in its characteristic impedance is called a ____________________ line.

ANS:matched

12.A pulse sent down a cable terminated in a short-circuit will reflect with the ____________________ polarity.

ANS:opposite

13.The apparently stationary pattern of waves on a mismatched cable is called a ____________________ wave.

ANS:standing

14.SWR stands for ____________________-wave ratio.

ANS:standing

15.The ideal value for SWR is ____________________.

ANS:one

16.Transmission line impedances can be found using a ____________________ chart.

ANS:Smith

17.Short transmission-line sections called ____________________ can be used as capacitors or inductors.

ANS:stubs

18.Any cable that radiates energy can also ____________________ energy.

ANS:absorb

19.A ____________________-dB loss in a cable means only half the power sent reaches the load.

ANS:3

20.It is often best to measure SWR at the ____________________ end of a cable.

ANS:load

21.Besides heat from I2R, the power a cable can carry is limited by the ____________________ voltage of its dielectric.

ANS:breakdown

22.To normalize an impedance on a Smith Chart, you divide it by ____________________.

ANS:Z0

23.The ____________________ of a Smith Chart always represents the characteristic impedance.

ANS:center

24.A ____________________ wavelength transmission line can be used a transformer.

ANS:one-quarter

25.A slotted line is used to make measurements in the ____________________ domain.

ANS:frequency

SHORT ANSWER

1.A transmission line has 2.5 pF of capacitance per foot and 100 nH of inductance per foot. Calculate its characteristic impedance.

ANS:Z0 = 200 ohms

2.Two wires with air as a dielectric are one inch apart. The diameter of the wire is .04 inch. Calculate, approximately, its characteristic impedance.

ANS:386 ohms

3.If a coaxial cable uses plastic insulation with a dielectric constant r = 2.6 , what is the velocity factor for the cable?

ANS:0.62

4.If a cable has a velocity factor of 0.8, how long would it take a signal to travel 3000 kilometers along the cable?

ANS:12.5 ms

5.If a cable has a velocity factor of 0.8, what length of cable is required for a 90 phase shift at 100 MHz?

ANS:0.6 meters

6.A cable has a VSWR of 10. If the minimum voltage along the cable is 20 volts, what is the maximum voltage along the cable?

ANS:200 volts

7.A lossless line has a characteristic impedance of 50 ohms, but is terminated with a 75-ohm resistive load. What SWR do you expect to measure?

ANS:1.5

8.If a cable has an SWR of 1.5, what will be the absolute value of its voltage coefficient of reflection?

ANS:0.2

9.A generator matched to a line with a voltage coefficient of reflection equal to 0.2 transmits 100 watts into the line. How much power is actually absorbed by the load?

ANS:96 watts

10.Using a Smith Chart to analyze a 50-ohm cable, what would be the normalized value of an impedance equal to 200 + j50 ohms?

ANS:4 + j1