Helical antenna It is a simple Broad band VHF and UHF antenna. The concept of Helical antenna is first introduced by J.D.Kraus. It consists of a helix of thick copper wire or tubing wound in the shape of a screw thread and used in conjunction with a flat metal plate called a ground plane. It is a three dimensional structure of which Linear and Loop antennas are the special cases. It provides circularly polarized waves.

Prof J D Kraus

Modes of operation of helical antenna Helical antenna is operated in various modes. 1) Axial mode of operation. 2) Normal mode of operation. In axial mode, the maximum radiation is along the helix axis under the condition that the circumference is of the order of one wavelength. In normal mode, the radiation is maximum perpendicular to the helix axis under the condition that circumference is smaller w.r.t one wavelength.

Helical geometry Kraus: Fig. 8-8 Kraus: Fig Relation between C, S & L D = diameter of helix C = circumference of helix = πD S = spacing between turns L = Length of 1 turn n or N = number of turns A = axial length = nS d = diameter of helix conductor α = pitch angle From fig. 8-9,

Also, pitch angle α can be found as follows. or

Geometry of helical antenna Helical antenna is a basic broadband VHF and UHF antenna which provides circular polarization. It consists of a thick copper wire wound in the form of a screw thread forming helix. In general helix is used with ground planes There are different forms ground planes such as Flat ground plane, cylindrical cavity. In general helical antenna is fed with Co-axial transmission line in which the central conductor is connected to helix at the feed point, while the outer conductor is connected to the ground plane.

 The helical antenna can operate in many modes, but two important modes are Axial mode of operation, Normal mode of operation  In normal mode, the field radiated by the antenna is maximum in a plane normal to the axis of the helix.  This mode id also called broadside mode.  In axial mode, the field radiated by the antenna is maximum in a plane along the axis.  This is also called End-fire mode.  This end-fire mode is used to achieve circular polarisation.  For space communication, the helical antennas are most suitable as they have wider bandwidth, higher directivity and circular polarization.  To transmit or receive VHF signals through Ionosphere generally an array of helical antennas are used. These are also used for space and satellite communication.

The Helix Modes (i) Normal or Omni mode: It is also called perpendicular mode of radiation. In this, the radiation beam is normal to the direction of helix axis. This mode of radiation is obtained if dimensions of helix are small compared with wavelength i.e. nL << λ

Fig. Normal mode radiation pattern (ii) Axial or beam mode: It is also called end- fire mode of radiation. It is denoted by R 1. In this, radiation beam is parallel to the helix axis. This mode of radiation is obtained if the helix circumference C is of the order of 1 λ ( C λ = 1). The radiation in this mode is circularly polarized.

Parameters for mono filar axial-mode helical antenna: Far field pattern where Terminal impedance (resistive) Ω (axial feed)

Directivity (or Gain) Axial Ratio All above formulae apply for 0.8 3

Axial mode radiation pattern

Fig. Helical antenna used in Normal or omni mode Practical helical antennas

Fig. Arrays of helical antennas

Fig. Quadrifilar helical antenna

Microstrip Antennas They are also called printed antennas or microstrip patch antennas They are used wherever size, weight, cost, performance are constraints. They are popular for low profile applications at frequencies above 100 MHz. Linear & circular polarizations can be achieved.

Structure of microstrip antenna It consists of a metal patch on a dielectric substrate with ground plane on the other side. Kraus: Fig. 14-4

Radiating patch and feed lines are photo etched on the dielectric substrate with a continuous metal layer bonded on opposite side of substrate to form the ground plane Shape of radiating metal patch may be square, rectangular, circular, triangular or elliptical.

Different shapes of patch antennas Types of patches

E-field distribution Radiation pattern (for linearly polarized MSA) Kraus: Fig Kraus: Fig. 14-5

Advantages Small size, light weight and less volume Can be easily molded to any desired shape and so can be attached to any surface Their fabrication processes are simple, production is easy and fabrication cost is low, so can be manufactured in large quantities. The MSA can be designed to produce variety of patterns and polarizations. In general they support linear as well as circular polarization. They are capable of dual & triple frequency operations Easy to form large arrays.

Contd… No cavity backing is required. Feed lines and matching networks can be fabricated simultaneously with the antenna structure. They can be easily integrated with microwave integrated circuits. Mechanically robust when mounted on rigid surfaces.

Disadvantages Contd… They suffer from effects of radiation from feeds and junctions. Surface waves are excited in the substrate which is a loss. Polarization purity is difficult to achieve. Poor end fire radiator. The size of the antenna is inversely proportional to frequency, they can be used only for very high frequencies only. Because at low frequencies the size of antenna is large.

Aperture Feed

Applications of MSA Military applications : In high velocity aircrafts Space crafts Missile and rockets Other areas where MSA used are missile guiding,fuzing, telemetry, satellite communication,radars.altimeters etc Space applications: Earth Limb measurement satellite International sun earth explorer Shuttle imaging Radar Solar mesopheric Explorer.

Applications contd… Commercial applications: Mobile satellite communication Direct broadcast Satellite services Global positioning system Aeronautical and marine radars Remote sensing

Practical Microstrip (Patch) antennas