FOLDED DIPOLE AND SLEEVE DIPOLE ANTENNAE

Why the term folded dipole?

It consists of two parallel dipoles connected at the ends forming a narrow wire loop.

When length is equal to /2, the structure is called Half Wave Folded Dipole.

Folded Dipole Antenna

Its properties are:

High input impedance It increases the bandwidth of antenna which is very essential in FM and TV applications.

INPUT IMPEDANCE OF FOLDED DIPOLE

Operation of folded dipole is in two modes;

1. Transmission line mode: antenna is considered to be a part of transmission line. Its current is

2. Antenna mode: second arm has a induced curent

of the arm which is connected to transmission line

Transmission mode has cancelling currents and antenna mode has two arms in same direction increasing the radiation.

=

For a short circuited transmission line,

Substituting value of

Thus input impedance of folded dipole is 4 times more than half wave dipole and thus helps in optimum power transfer.

Thus it is most suited for TV applications.

For half wave dipole, simple compensating network is as follows:

FOR PARALLEL LC RESONANT CIRCUIT

IMPEDENCE AT

• INFINITEf = f0• INDUCTIVEf < f0• CAPACITIVEf > f0

LC parallel circuit connected across input terminals of half wave dipole to achieve the bandwidth compensation.

ANTENNA IMPEDENCECAPACITIVE

PARALLEL LCIMPEDENCE

INDUTIVE

BELOW RESONANT FREQUENCY

ANTENNA IMPEDENC INDUCTIVE

PARALLEL LCIMPEDENCECAPACITIVE

ABOVE RESONANT FREQUENCY

Sleeve Antenna A sleeve antenna is a specific design of radio wave antenna used for both the reception and transmission of radio frequency signals.The two-element construction of the sleeve antenna is what gives the device its name.In its simplest and most common form, a sleeve antenna has a straight metal element, often a rod or wire, placed and centered inside a straight metal tube, which acts as a sleeve over the center element.

ClassificationsHaving two separate elements, a sleeve antenna is classified as a dipole-type antenna meaning it has two poles.

It is also classified as a half-wave antenna, meaning that the antenna is best suited to receive or transmit at a radio wavelength twice the antenna’s physical length.

These antennas, which are mounted vertically, can receive and broadcast in all directions simultaneously, making them members of the omnidirectional class of antennas.

Finally, because both elements of the antenna share the same centerline, it can be described as a coaxial antenna, meaning that the two poles have a common center axis.

Principle of operationLike all radio antennas, a sleeve antenna operates on the principle of frequency resonance.

Frequency resonance dictates that a specific radio frequency transmission will cause a specific length of metal to resonate at an atomic level and generate an AC electrical current.

This generated current, identical in structure and modulation to the original radio signal, is then passed through other electronics, like a radio, so the signal can be heard

EXAMPLES OF SLEEVE ANTENNAS

Sleeve Monopole: Sleeve Dipole:

STANDARD vs SLEEVE MONOPOLE

STANDARD vs SLEEVE DIPOLE

ADVANTAGES OF SLEEVE ANTENNA

•Bandwidth increases to a large extent

•Provides better pattern stability.

•Cylindrical shape is maintained, i.e. conical or further more complex 3D structures are not required.

DISADVANTAGES OF SLEEVE ANTENNA

•Efficiency in the operating frequency range is less compared to that of the standard antenna.

•The manufacturing complexity increases.

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