The disadvantage of co-axial cable is/are : (A) Support higher bandwidth than twisted pair cable Choose the most appropriate answer from the options given below :
(B) Light weight
(C) Relatively expensive compared to twisted pair cable
(D) EMI resistant
(E) Doesn't at very high frequency
(C), (E) Only
The question asks only for disadvantages, and three of the five statements are plainly advantages — so the answer is (C) and (E), option 2.
| Statement | Nature |
|---|---|
| (A) Higher bandwidth than twisted pair | Advantage |
| (B) Light weight | Advantage — and doubtful for coax |
| (C) More expensive than twisted pair | Disadvantage |
| (D) EMI resistant | Advantage |
| (E) Does not work at very high frequency | Disadvantage |
(C) — the cost. A coaxial cable needs a solid centre conductor, a precisely dimensioned dielectric, a braided or foil shield and an outer jacket, all held concentric to a tight tolerance, since the impedance depends on the ratio of the diameters:
\(Z_{0}=\dfrac{138}{\sqrt{\varepsilon_{r}}}\log_{10}\dfrac{D}{d}\)
Twisted pair is two insulated wires twisted together, and costs a fraction as much. The connectors are more expensive too, needing to maintain the same impedance through the joint.
(E) — the high-frequency limit. Coax is excellent through VHF and UHF, but two effects end its usefulness in the microwave region. Attenuation rises as \(\sqrt{f}\) through the skin effect, since current crowds into an ever thinner layer of the conductors. And above a frequency set by the cable's circumference, higher-order modes begin to propagate:
\(f_{c}\approx\dfrac{2c}{\pi\left(D+d\right)\sqrt{\varepsilon_{r}}}\)
Once the pure TEM mode is no longer alone, the propagation is unpredictable and the cable ceases to behave as a transmission line. This is why waveguide replaces coax above a few gigahertz.
Why (D) is firmly an advantage. The outer conductor completely surrounds the inner one, so the field is confined within the cable: it neither radiates out nor admits interference in. That shielding is the single main reason coax is chosen over twisted pair, and calling it a disadvantage inverts the point.
(B) is doubtful even as a description. Coax is heavier than twisted pair, being a solid conductor plus a braid plus a thick dielectric — so it is neither a disadvantage as phrased nor a true property.
Hence, the disadvantages are (C) and (E).
Similar Questions
Consider the following statements
A. Reflection coefficient is change in the magnitude of reflected wave with constant phase with respect to incident wave.
B. Transmission coefficient is change in the magnitude and phase of transmitted wave with respect to incident wave
C. On smith chart, \(\dfrac{\lambda}{2}\) distance is equivalent to 2π.
D. For distortion less line, RL = GC
E. Directivity of an antenna can be less than 1 (unity).
Choose the most appropriate answer from the options given below :
In a loss less transmission line of length 50 cm with L = 10 μH/m, C = 40 pF/m is operated at 25 MHz. Its electrical path length is
The normalized impedance of a transmission line is given by expression :
A lossless line has a characteristic impedance of 50 ohms. It is terminated in a load resistance of 75 ohms. The line is energised by a generator which has an output impedance of 50 ohms and an output voltage of 30 V (rms). The line is assumed to be 2.25 wavelength long. The input impedance and instantaneous load voltages are given :
(a) Zin = 50 ohms (b) VL (instantaneous) = 36 V (c) Zin = 33.33 ohms (d) VL (instantaneous) = 12 V
Which of the above are correct :
Arrange the below mentioned transmission lines in order of their increasing frequency handling capabilities :
(a) wave guide (b) Parallel wire (c) Rigid co-axial cables (d) Flexible co-axial cables
For a quarter wave ideal transmission line of characteristic impedance of 50 Ω and load impedance of 100 Ω, the input impedance of line will be :
The propagation constant for uniform plane wave is given by the expression given below :
Important Questions from Transmission Lines
A characteristic impedance does NOT satisfy which of the following statements?
The dielectric constant of the material used in a transmission line is 2. What is the velocity factor of this line if its characteristic impedance is 300 Ω?
A transmission line of \(50{\rm{\;\Omega }}\) characteristic impedance is terminated with a \(\rm 100 \ Ω\) resistance. The minimum impedance measured on the line is equal to
Twisting of live and return lines in long signal lines is done to reduce the effect of
The input impedance of short circuited lossless transmission line quarter wavelength is
