In its most common configuration , the dipole is supported at each end by rope and end insulators. The rope supports are tied to trees, buildings, masts, or some combination of such structures. Ads Link
The length of the antenna is a half-wavelength. Keep in mind that the physical length of the antenna, and the theoretical electrical length, are often different by about 5 percent. A free-space half-wavelength is found from :
L= ( 492 / F(MHz) ) ft
In a perfect antenna, that is self-supported many wavelengths away from any object,The Equation will yield the physical length. But in real antennas, the length calculated above is too long. The average physical length is shortened by up to about 5 percent because of the velocity factor of the wire and capacitive effects of the end insulators. A more nearly correct approximation (remember that word, it's important) of a half-wavelength antenna is
L= ( 468/ F(MHz) ) ft
where :
L is the length of a half-wavelength radiator, in feet
FMHz is the operating frequency, in megahertz
For example : Calculate the approximate physical length for a half-wavelength dipole
operating on a frequency of 7.25 MHz.
Solution:
L=( 468/F(MHz) ) ft
= 468/7.25 ft = 64.55 ft
or, restated another way:
L = 64 ft 6.6 in
It is unfortunate that a lot of people accept Equation above as a universal truth, a kind of immutable law of The Universe. Perhaps abetted by books and articles on antennas that fail to reveal the full story, too many people install dipoles without regard for reality.
We will design Dipole antenna for Digital TV reception , as We know in literature the band frequency for TV reception between 470 MHz - 800 MHz. for this band frequency, we can calculate the centre frequency. The centre frequency for this band is 635 MHz.
The design of antenna , for balance the power reception of this antenna, by calculating design in centre frequency .
As the formula :
L = 468 /F
So,
L = 468/635 feet
L = 0.737 feet , this is 1/2 Lambda for centre frequency band of Digital TV reception.
For Dipole antenna design , between 2 arms of dipole antenna should be 1/2 lambda, so one arm of dipole antenna will be 1/4 Lambda .
So the calculation will be :
L = 0.737 feet (1/2 lamda)
1/4 lambda of L = 1/2 X 0.737 = 0.3685 feet , convert to cm , will be :
1/4 Lambda , L = 0.3685 feet X 30.48 cm/feet = 11.23 cm
