Question

A travelling wave of magnituide 100 V in transmission line of surge impedance 400 Ω passes into a cable of surge impedance 50 Ω. What will be the magnitude of the wave while travelling in the cable?
1. 800 V
2. 100 V
3. 20 V
4. 12.5 V

Answer 1

Correct Answer - Option 3 : 20 V

Concept:

Transmitted voltage V" = V + V’

Transmitted current I" = I + I’

Where,

V’ = Reflected voltage

V = Incident voltage

I’ = Reflected current

I = Incident current

Transmission or reflection coefficient of voltage TV = V"/V

Transmission or reflection coefficient of current Ti = I”/I

Tv = V”/V = 2Zs/(Z1 + Zs)

Where,

Z1 = Surge impedance of load

Zs = Surge impedance of cable

Calculation:

Z1 = Surge impedance of load = 400 Ω

Zs = Surge impedance of cable = 50 Ω

V" = 100 × 2 × 50/450 = 22.22 V ≈ 20 V