Concept Notes
1. What is an Antenna?
- An antenna is a device that converts electrical signals (current/voltage) into electromagnetic waves (radiation) for transmission, and vice versa for reception.
- In simple terms: Transmitter end → antenna radiates; Receiver end → antenna collects.
2. Basic Principle
- Antenna works on reciprocity theorem:
- If it can transmit effectively, it can also receive effectively.
- Based on Maxwell’s equations, a time-varying current produces radiation.
3. Important Parameters of Antennas
- Radiation Pattern
- Spatial distribution of power radiated.
- Main lobe → strongest radiation direction.
- Radiation Intensity (U):
Gain (G):
where 
= efficiency, 
= directivity.
Directivity (D):
Efficiency (η):
Effective Aperture (Ae):
Bandwidth of Antenna: range of frequencies over which antenna works effectively.
Polarization: orientation of electric field vector (linear, circular, elliptical).
Input Impedance (Zin):
where 
= radiation resistance, 
= loss resistance.
4. Types of Antennas
- Wire antennas: Dipole, Monopole, Loop.
- Aperture antennas: Horn antennas.
- Array antennas: Yagi-Uda, Phased Array.
- Reflector antennas: Parabolic dish.
- Microstrip antennas: Patch antennas (used in mobiles, satellites).
5. Examples
Example 1:
A transmitting antenna has 
and operates at 
. Find effective aperture.
Example 2:
If antenna efficiency = 
and directivity = 
, then gain =
⚙️ Key Formulas
Radiation Intensity:
Directivity:
Gain:
Effective Aperture:
Efficiency:
Input Impedance:
🔟 10 MCQs
Q1. The function of an antenna is to:
a) Amplify signal
b) Convert AC to DC
c) Convert electrical signals to EM waves
d) Filter frequencies
Q2. The antenna property that defines directional concentration of radiation is:
a) Gain
b) Bandwidth
c) Polarization
d) Efficiency
Q3. Effective aperture formula is:
a) 
b)
c) 
d) 
Q4. Reciprocity theorem in antennas states:
a) Antenna can only transmit
b) Antenna can only receive
c) Transmission and reception characteristics are same
d) None
Q5. If 
, 
, effective aperture is:
a) 
b) 
c) 
d) 
Q6. Antenna efficiency is defined as:
a) 
b) 
c) 
d) 
Q7. A dipole antenna is a type of:
a) Wire antenna
b) Aperture antenna
c) Reflector antenna
d) Patch antenna
Q8. Gain is product of:
a) Efficiency × Polarization
b) Efficiency × Directivity
c) Aperture × Bandwidth
d) None
Q9. Polarization refers to:
a) Orientation of electric field
b) Orientation of magnetic field
c) Orientation of radiation pattern
d) Antenna resistance
Q10. Bandwidth of antenna is defined as:
a) Range of frequencies over which antenna resonates
b) Range of frequencies over which antenna works effectively
c) Minimum frequency antenna works at
d) Maximum frequency antenna works at
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | c |
| Q2 | a |
| Q3 | b |
| Q4 | c |
| Q5 | b |
| Q6 | b |
| Q7 | a |
| Q8 | b |
| Q9 | a |
| Q10 | b |
🧠 Explanations
- Q1: Antenna converts electrical energy ↔ EM waves.
- Q2: Directionality = Gain.
- Q3: Effective aperture = .
- Q4: Reciprocity → Tx and Rx properties are identical.
- Q5:
. - Q6: Efficiency =
. - Q7: Dipole = basic wire antenna.
- Q8: Gain = Directivity × Efficiency.
- Q9: Polarization = orientation of electric field vector.
- Q10: Antenna bandwidth = range of frequencies where antenna performs satisfactorily.
🎯 Motivation
Antennas are the backbone of wireless communication – mobiles, satellites, radar, 5G all depend on antenna design.
👉 Mastering parameters (Gain, Directivity, Aperture) is crucial for ECET numericals.
📲 CTA
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