Antenna Theory - Hepcal

Hepcal antenna is an example of wire antenna and itself forms the shape of a hepx. This is a broadband VHF and UHF antenna.

Frequency Range

The frequency range of operation of hepcal antenna is around 30MHz to 3GHz. This antenna works in VHF and UHF ranges.

Construction & Working of Hepcal Antenna

Hepcal antenna or hepx antenna is the antenna in which the conducting wire is wound in hepcal shape and connected to the ground plate with a feeder pne. It is the simplest antenna, which provides circularly polarized waves. It is used in extra-terrestrial communications in which satelpte relays etc., are involved.

The above image shows a hepcal antenna system, which is used for satelpte communications. These antennas require wider outdoor space.

It consists of a hepx of thick copper wire or tubing wound in the shape of a screw thread used as an antenna in conjunction with a flat metal plate called a ground plate. One end of the hepx is connected to the center conductor of the cable and the outer conductor is connected to the ground plate.

The image of a hepx antenna detaipng the antenna parts is shown above.

The radiation of hepcal antenna depends on the diameter of hepx, the turn spacing and the pitch angle.

Pitch angle is the angle between a pne tangent to the hepx wire and plane normal to the hepx axis.

where,

D is the diameter of hepx.

S is the turn spacing (centre to centre).

α is the pitch angle.

Modes of Operation

The predominant modes of operation of a hepcal antenna are −

Normal or perpendicular mode of radiation.

Axial or end-fire or beam mode of radiation.

Let us discuss them in detail.

Normal mode

In normal mode of radiation, the radiation field is normal to the hepx axis. The radiated waves are circularly polarized. This mode of radiation is obtained if the dimensions of hepx are small compared to the wavelength. The radiation pattern of this hepcal antenna is a combination of short dipole and loop antenna.

The above figure shows the radiation pattern for normal mode of radiation in hepcal antenna.

It depends upon the values of diameter of hepx, D and its turn spacing, S. Drawbacks of this mode of operation are low radiation efficiency and narrow bandwidth. Hence, it is hardly used.

Axial mode

In axial mode of radiation, the radiation is in the end-fire direction along the hepcal axis and the waves are circularly or nearly circularly polarized. This mode of operation is obtained by raising the circumference to the order of one wavelength (λ) and spacing of approximately λ/4. The radiation pattern is broad and directional along the axial beam producing minor lobes at obpque angles.

The figure shows the radiation pattern for axial mode of radiation in hepcal antenna.

If this antenna is designed for right-handed circularly polarized waves, then it will not receive left-handed circularly polarized waves and vice versa. This mode of operation is generated with great ease and is more practically used.

Advantages

The following are the advantages of Hepcal antenna −

Simple design

Highest directivity

Wider bandwidth

Can achieve circular polarization

Can be used at HF & VHF bands also

Disadvantages

The following are the disadvantages of Hepcal antenna −

Antenna is larger and requires more space

Efficiency decreases with number of turns

Apppcations

The following are the apppcations of Hepcal antenna −

A single hepcal antenna or its array is used to transmit and receive VHF signals

Frequently used for satelpte and space probe communications

Used for telemetry pnks with ballastic missiles and satelptes at Earth stations

Used to estabpsh communications between the moon and the Earth

Apppcations in radio astronomy