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
(b), (d), (c), (a)
Fix the two ends first.
Parallel wire is the worst. An open two-wire line is an unshielded structure, so above roughly 100 MHz it radiates freely — the spacing becomes an appreciable fraction of a wavelength and the line behaves as an antenna. It is used for HF and low VHF feeders and nothing higher, so it must come first.
Waveguide is the best. A hollow metal guide has no centre conductor and no dielectric, so there is neither conductor loss in a thin inner wire nor dielectric loss. It is the standard medium above about 3 GHz and works to hundreds of gigahertz. Note that it has a lower cut-off — it will not propagate below
\(f_c=\dfrac{c}{2a}\)
for the dominant TE10 mode — which is exactly why it is the high-frequency choice and useless at low frequencies. It must come last.
Now order the two coaxial cables. Both are shielded, so both beat parallel wire. Between them:
| Flexible coax | Rigid coax | |
|---|---|---|
| Dielectric | solid polyethylene or PTFE | air, with spaced supports |
| Outer conductor | braid, imperfect coverage | solid copper tube |
| Dimensional stability | varies as it flexes | held precisely |
| Loss at high frequency | higher | lower |
The solid dielectric of a flexible cable has a loss tangent that worsens with frequency, and a braided screen leaks increasingly as the wavelength shortens. A rigid air-dielectric line avoids both, so it handles higher frequencies. Flexible therefore comes before rigid.
Assemble. Parallel wire, flexible coax, rigid coax, waveguide — (b), (d), (c), (a), which is option 1. The near-miss is option 2, which correctly begins with parallel wire and ends with waveguide but reverses the two coaxial types.
The unifying principle. As frequency rises, the enemy is radiation and loss, so the structure must progressively enclose the field: open wires leak, braided coax leaks a little, rigid coax encloses the field in air, and waveguide encloses it completely in metal.
Hence, the increasing order is parallel wire, flexible coaxial, rigid coaxial, waveguide.
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 :
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 disadvantage of co-axial cable is/are :
(A) Support higher bandwidth than twisted pair cable
(B) Light weight
(C) Relatively expensive compared to twisted pair cable
(D) EMI resistant
(E) Doesn't at very high frequencyChoose the most appropriate answer from the options given below :
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
