UNIT 5 Transmission Line and Guided Waves

 

What is a Transmission Line?

Transmission line ek aisa system hota hai jo high-frequency signals ko ek jagah se dusri jagah carry karta hai.

👉 Jab conductor ki length signal wavelength ke comparable ho jaati hai, tab lumped circuit theory fail ho jaati hai aur transmission line theory apply hoti hai.

Examples:

• Coaxial cable

• Open wire line

• Microstrip line

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Distributed Parameter Model of Transmission Line

Transmission line ko distributed parameters ke form me represent kiya jaata hai:

• R → Resistance per unit length

• L → Inductance per unit length

• C → Capacitance per unit length

• G → Conductance per unit length

👉 Ye parameters poori length me continuously distributed hote hain.

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Types of Transmission Lines

1. Open Wire Transmission Line

• Do parallel conductors

• Low cost

• Noise interference zyada

2. Coaxial Cable

• Inner conductor + outer shield

• High noise immunity

• TV cable, RF communication me use

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Transmission Line Equations (Telegrapher’s Equations)

Voltage aur current ka variation:

$$\frac{dV}{dz} = -(R + j\omega L)I$$ $$\frac{dI}{dz} = -(G + j\omega C)V$$

👉 Ye equations batati hain ki V aur I line ke along kaise change hote hain.

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Lossless Transmission Line

Lossless line ke liye:

• R = 0

• G = 0

Equations simplify ho jaati hain:

$$\frac{d^2 V}{dz^2} = -\omega^2 LC V$$

👉 Mostly high-frequency communication me assume kiya jaata hai.

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Voltage Standing Wave Ratio (VSWR)

VSWR batata hai ki line kitni achhi match hui hai load ke sath.

$$VSWR = \frac{V_{max}}{V_{min}}$$

• VSWR = 1 → Perfect matching

• VSWR > 1 → Mismatch present

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Transmission Line as a Circuit Element

Transmission line ko:

• Inductor

• Capacitor

• Resonator

ke jaise use kiya ja sakta hai, specially microwave circuits me.

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Quarter Wave Transformer

Quarter wave transformer impedance matching ke liye use hota hai.

Length:

$$l = \frac{\lambda}{4}$$

Characteristic impedance:

$$Z_0 = \sqrt{Z_L Z_S}$$

👉 RF & microwave circuits me bahut popular technique.

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Impedance Matching

Impedance matching ka goal:

• Maximum power transfer

• Reflection kam karna

Single Stub Matching

• Short-circuited ya open-circuited stub

• Main line ke parallel ya series me connect

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Guided Waves

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Wave Between Parallel Planes

Jab EM wave do parallel conducting plates ke beech propagate karti hai, to use guided wave kehte hain.

👉 Wave boundaries ke andar confined hoti hai.

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TE (Transverse Electric) Waves

• Electric field ka longitudinal component zero hota hai

• Magnetic field present hoti hai

👉 Common in rectangular waveguides

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TM (Transverse Magnetic) Waves

• Magnetic field ka longitudinal component zero hota hai

• Electric field present hoti hai

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Characteristics of TE and TM Waves

Property	TE Wave	TM Wave
Ez	0	Present
Hz	Present	0
Cut-off frequency	Yes	Yes
Used in	Waveguides	Waveguides

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TEM (Transverse Electromagnetic) Waves

• Electric aur Magnetic fields dono transverse

• Longitudinal component = 0

Examples:

• Coaxial cable

• Two-wire line

Properties of TEM Waves

• No cut-off frequency

• Can propagate at any frequency

• Used in transmission lines

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Difference Between TE, TM and TEM Waves

Feature	TE	TM	TEM
Ez	0	≠0	0
Hz	≠0	0	0
Cut-off frequency	Yes	Yes	No
Example	Waveguide	Waveguide	Coaxial cable

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Conclusion

Transmission Line aur Guided Waves concepts:

• High-frequency signal behavior

• Microwave communication

• Antennas & RF systems

samajhne ke liye foundation topics hain.