Hertz Antenna & Discone AntennaUsman M. Nooruddin (07-0039) | Wave Propagation & Antenna Theory Lab

Introduction:

An antenna is an electrical device which couples radio waves in free space to an electrical current used by a radio receiver or transmitter. In reception, the antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage that the radio receiver can amplify. Alternatively, a radio transmitter will produce a large radio frequency current that may be applied to the terminals of the same antenna in order to convert it into an electromagnetic wave (radio wave) radiated into free space. Antennas are thus essential to the operation of all radio equipment, both transmitters and receivers. They are used in systems such as radio and television broadcasting, two-way radio, wireless LAN, mobile telephony, radar, and satellite communications.

Typically an antenna consists of an arrangement of metallic conductors (or "elements") with an electrical connection (often through a transmission line) to the receiver or transmitter. A current forced through such a conductor by a radio transmitter will create an alternating magnetic field according to Ampère's law. Or the alternating magnetic field due to a distant radio transmitter will induce a voltage at the antenna terminals, according to Faraday's law, which is connected to the input of a receiver. In the so-called far field, at a considerable distance away from the antenna, the oscillating magnetic field is coupled with a similarly oscillating electric field; together these define an electromagnetic wave which is capable of propagating great distances.

Hertz Antenna (Horizontal End Fed):

Introduction & History:Invented by Heinrich Rudolf Hertz in 1886, the Hertz Antenna is defined as a dipole antenna with a length of ½ wavelength (or) a half wave dipole antenna. The Hertzian Dipole is the name given to a dipole, which is very small, compared to its wavelength that is about one-hundredths of the wavelength at its operating frequency. Even in this case, the radiation pattern is not truly isotropic.

Construction:It consists of a conductor that is broken in the center so that RF power can be applied to it. One can think of the half wave dipole as an open circuited transmission line that has been spread out, so that the transmission line can radiate a signal into space.

A dipole can be any length, but it most commonly is just under 1/2 wavelength long. A dipole with this length, known as a resonant or half wave dipole, has an input impedance that is purely resistive and lies between 30 and 80 Ω, which provides a good match to commercially available 50 Ω coaxial cables as well as commercial transmitters and receivers, most of which have 50 Ω output and input impedances.

Radiation Pattern:The 3-dimensional radiation pattern in free space is a fat doughnut with the dipole piercing its central hole.

Notice that unlike an isotropic radiator that radiates equally well in all directions, the dipole radiates more RF in some directions than others.

This means that the dipole has a gain or directivity over an isotropic radiator of approximately 2.1 dBs.

That means that the radiation from the dipole is 2.1 dB stronger in the direction of maximum radiation than the radiation from an isotropic radiator in the same direction, when both antennas are fed with the same amount of RF power.

Gain, Directivity & Polarization:The gain of the Hertz Antenna is 2.1dBs. Since the Directivity Gain, the directivity increases as the gain increases. The Hertz Antenna is generally horizontally polarized, that is to say that the antenna is placed horizontal to the Earth’s surface. Although the Hertz Antenna can also be vertically polarized, that is, that the antenna is placed perpendicular, or vertically with respect to the Earth’s surface.

The advantage of horizontal polarization over vertical polarization is that it provides directivity. It is normally used in reducing the interference between the transmission line and the antenna.

Applications:The Hertz Antenna is normally used for small applications and was used in set top TV antennas.

Discone Antenna:

Introduction & History:The discone antenna receives its name from its distinctive shape. The RF antenna design consists of a top "disc" formulated from a number of elements arranged in a disc at the top, and further elements pointing downwards in the shape of a cone.

Construction:A discone may be made from solid metal sheet (often copper), which is practical for small indoor UHF antennas, such as for Wi-Fi.

At lower frequencies a sufficient number of metal wires or rods in a spoke configuration is often used to approximate a solid surface. This simplifies construction and reduces wind loading.

The spokes may be made of stiff wire, brazing rods or even coat hanger wire. The optimal number of rods comprising the disc and cone is often quoted as being from 8 to 16.

A discone antenna typically has at least three major components:

The discThe disc should have an overall diameter of 0.7 times a quarter wavelength of the antenna's minimum frequency. The antenna's feed point is at the center of the disc. It is usually fed with 50 Ω coaxial cable, with the center conductor connected to the disc, and the outer conductor to the cone.

The coneThe length of the cone should be a quarter wavelength of the antenna's minimum operating frequency. The cone angle is generally from 25 to 40 degrees.

The insulatorThe disc and cone must be separated by an insulator, the dimensions of which determine some of the antenna's properties.

Radiation Pattern:The way in which the discone operates is relatively complicated, but it can be visualized in a simplified manner. The disc and cone elements sufficiently simulate an electrically complete disc and cone from which the energy is radiated. As a result the greater the number of elements, the better the simulation, although in reality there is a balance between performance, cost and wind resistance. Often around six elements are used, but the number is not critical.

The RF antenna radiates and receives energy that is vertically polarized, and the radiation pattern is omnidirectional in the horizontal plane. The antenna radiates most of the energy at a low angle which it maintains over the most of the operating range.

Gain, Directivity & Polarization:Omnidirectional, vertically polarized and exhibiting unity gain, it is exceptionally wideband, offering a frequency range ratio of up to ~10:1. The radiation pattern in the horizontal plane is quite narrow, making its sensitivity highest in the plane tangent to the Earth's surface.

Applications:Although it is widely used for receiving applications, the discone antenna is less commonly used for transmitting.

The discone's wideband coverage makes it attractive in commercial, military, amateur radio and radio scanner applications.When employed as a transmitting antenna, it is often less efficient than an antenna designed for a more limited frequency range. SWR (standing wave ratio) is typically ~2:1 over the range of the design frequency to the second harmonic and ~3.1 thereafter.The discone's inherently wideband nature permits it to broadcast undesirable spurious emissions from faulty or improperly filtered transmitters.

References:

Wikipediao Antennaso Heinrich Hertzo Dipole Antennao Discone Antenna

YCARSo Electronics for Radio Amateurso Module C - Antennas, Propagation, and Transmission Lines