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Helical antenna

Combination of loop antenna and linear antenna

If dimensions much smaller than wavelength,behaves like linear antenna

Bandwidth, efficiency, and radiation resistanceincrease with increasing h

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Helical Antenna

simple, and practical configuration of anelectromagnetic radiator is that of aconducting wire wound in the form of a screw

thread forming a helix. helix is used with a ground plane.

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The ground plane can take different forms. One is forthe ground to be flat.

Typically the diameter of the ground plane should be atleast 3/4.

The helix is usually connected to the center conductorof a coaxial transmission line at the feed point with theouter conductor of the line attached to the groundplane.

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The geometrical configuration of a helix consistsusually of N turns, diameter D and spacing S betweeneach turn.

The total length of antenna is L =NS while

The total length of wire is Ln=NLo =N sqrt(S2 +C2)

where ,

Lo =sqrt(S2 +C2) is the length of wire between each turn

C=D is the circumference of the helix.

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is the pitch angle , which is the angle formed by a linetangent to the helix wire and a plane perpendicular to thehelical axis.

= tan-1 (S/D) = tan-1 (S/C) When = 0 , then the winding is flattened and the helix

reduces to a loop antenna of N turns.

When = 90 then the helix reduces to a linear wire.

When 0 < < 90 , then a true helix is formed with acircumference greater than zero but less than thecircumference when the helix is reduced to a loop

( = 0 ).

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Radiation characteristics

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The radiation characteristics of the antenna can be varied bycontrolling the size of its geometrical properties compared to thewavelength.

The input impedance is critically dependent upon the pitch angleand the size of the conducting wire, especially near the feed point,and it can be adjusted by controlling their values.

The general polarization of the antenna is elliptical. Howevercircular and linear polarizations can be achieved over different

frequency ranges.

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Normal mode

The one representing the normal mode has its maximum in a planenormal to the axis and is nearly null along the axis. The pattern is similarin shape to that of a small dipole or circular loop.

Because an elliptically polarized antenna can be represented as the sumof two orthogonal linear components in time-phase quadrature, a helixcan always receive a signal transmitted from a rotating linearly polarizedantenna. Therefore helices are usually positioned on the ground forspace telemetry applications of satellites, space probes, and ballisticmissiles to transmit or receive signals that have undergone Faradayrotation by traveling through the ionosphere

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A. Normal Mode

In the normal mode of operationthe field radiated by the antennais maximum in a plane normal tothe helix axis and minimum alongits axis.

figure-eight rotated about its axissimilar to that of a linear dipole ofl < 0 or a small loop (a

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B. Axial Mode

Amore practical mode of operation, which can be generated with greatease ,is the axial or end-fire mode.

In this mode of operation, there is only one major lobe and its

maximum radiation intensity is along the axis of the helix, The minorlobes are at oblique angles to the axis.

To excite this mode, the diameter D and spacing S must be largefractions of the wavelength. To achieve circular polarization, primarily inthe major lobe, the cir-cumference of the helix must be in the

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