K= wave number = 2𝜋/𝜆
Radiated Fields: Element Factor, Space Factor, and Pattern Multiplication
لشكلshaping بيعمل
radiation pattern
As the length of the
dipole increases beyond
one wavelength (l > λ),
the number of lobes
begin to increase.
As was illustrated in Figure 4.6, the radiation pattern of a dipole becomes more directional as its length
increases. When the overall length is greater than one wavelength, the number of lobes increases, and the
antenna loses its directional properties.
Loss of Directionality: The antenna loses its omnidirectional behavior and
becomes more directional.
HALF-WAVELENGTH DIPOLE
CHAPTER4 Linear Wire Antennas
INTRODUCTION
Wire antennas, linear or curved, are some of the oldest, simplest, cheapest, and in many cases the most versatile for
many applications.
INFINITESIMAL DIPOLE
An infinitesimal linear wire (l ≪λ) is positioned symmetrically at the origin of the coordinate system and oriented
along the z axis,
Radiated Fields:
To find H:
Power Density and Radiation Resistance
بيكون كويس لما radiation resistanceكبير علشان ال powerبيعتمد عليه
علشان كده النوع ده مش شائع االستخدام بسبب ان radition resistanceصغير
عندي جزء يعتمد على الجزء التخيلي وجزء ثاني على الحقيقي بسبب كده هينشأ عندي حاالت اعتمادا على kr
الجزء التخيلي يعتمد على 1/𝑟 3يعني بيكون كبير في near fieldوصغير في far field
Intermediate-Field (kr > 1) Region
SMALL DIPOLE
A better approximation of the current distribution of wire antennas, whose lengths are usually λ/50 < l ≤ λ/10, is the
triangular variation. The current distribution of a small dipole (λ/50 < l ≤ λ/10). where 𝐼0 = constant.
REGION SEPARATION
The length R is defined as the distance from any point on the source to the observation point.
antennaLECTURES
ANTENNA lecture: antenna
BY Abdulmuttalib A. H. Aldouri parameters
& Mohammed Kamil
4. Half-Wave Dipole ( ⁄ )
One of the most commonly used antennas is the half-wavelength (l = λ/2) dipole.
Because its radiation resistance is 73Ω, which is very near the 75Ω characteristic impedances
of some transmission lines.
The electric and magnetic field components of a half-wavelength dipole can be obtained by
letting l = λ/2, thus:
( )
[ ]
( )
[ ]
The average power density and radiation intensity can be written, respectively, as:
( )
[ ]
( )
[ ] ( )
( )
( ) [ ]
* ( )+
∫
( ) ( )
and the radiation resistance is:
( )
( )
9
antennaLECTURES
ANTENNA lecture: antenna
BY Abdulmuttalib A. H. Aldouri parameters
& Mohammed Kamil
To find the HPBW:
( )
( )| [ ]
( )
( ) ( ( ))
By using trial and error, we can find the value of θh 51o
| | | |
Figure (13) shows the two and three dimensional radiation pattern for a λ/2 dipole.
0
330 30
300 60
1
0.5
270 90
240 120
210 150
180
Figure (13)
H.W. For a λ/4 dipole:
1. Find the radiation resistance.
2. Maximum directivity and maximum aperture.
3. HPBW and FNBW.
4. Draw the radiation pattern.
10
antennaLECTURES
ANTENNA lecture: antenna
BY Abdulmuttalib A. H. Aldouri parameters
& Mohammed Kamil
Example:
A λ/2 dipole radiates a time-averaged power of 600 W at a frequency of 300 MHz. A second
λ/2 dipole is placed at a point ( ), where r = 200 m, = 90◦, = 40◦. It is oriented
so that its axis is parallel to that of the transmitting antenna. What is the available power at the
terminals of the second (receiving) dipole?
Solution:
( )
( )
( ) 1.643
( )
H.W: Find prad at r = 200 m, = 50◦, = 20?
Example:
A dipole is radiating into free-space. Input power to the dipole is 100 W. Assuming an
overall efficiency of 50%, find the power density (in W/m2) at (r = 500 m, = 60◦, = 0)?
Solution:
11
