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).
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Important Questions from Transmission Lines
A characteristic impedance does NOT satisfy which of the following statements?
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List I (Quantity)
List II (Range of values)
a.
input impedance
1.
-1 to +1
b.
reflection co-efficient
2.
1 to ∞
c.
VSWR
3.
0 to ∞
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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
