Antenna Types PDF

## 1.2 Antenna Types Over the years, a very wide variety of antenna designs have evolved and broadly speaking they can be divided into single radiators and multiple radiators. Some of the most important types are listed in Table 1.1. The monopole and dipole antennas shown in Fig. 1.1 have been used...

## 1.2 Antenna Types Over the years, a very wide variety of antenna designs have evolved and broadly speaking they can be divided into single radiators and multiple radiators. Some of the most important types are listed in Table 1.1. The monopole and dipole antennas shown in Fig. 1.1 have been used extensively in the past and are still used individually or as components of more complex arrays such as the Yagi array. The long wire antenna has been used in many applications especially for receiving purposes. In a transmitting application, it tends to form a more composite structure such as the rhombic antenna. * **Table 1.1** | Single Radiators | Multiple Radiators | |:---|:---| | Monopole | Yagi Array | | Dipole | Rhombic Array | | Wire | Log-Periodic Array | | Helix | Linear Array | | Spiral | Planar Array | | Horn | Adaptive Array | | Reflector | * **Fig. 1.1** : A diagram showing the monopole and dipole antennas. * **Fig. 1.2** : A diagram showing the helix and spiral antennas. * **Fig. 1.3** : A diagram showing the horn and reflector antennas. **More recently, the helix and spiral antennas shown in Fig. 1.2 have evolved and found a particular use in satellite communications, because of their broadband properties.** **The microwave horn and parabolic reflector shown in Fig. 1.3 have been very popular antennas, which are used extensively in radar and satellite systems. Microwave horns very often are used as feeds for large parabolic reflectors.** * **Fig. 1.4** : A diagram showing the Yagi array and log-periodic array antennas. **The Yagi array shown in Fig. 1.4, is a very popular TV antenna for the VHF and UHF bands, due to its convenient size, useful gain, simplicity and low cost. In the transmitting case, the rhombic antenna and more recently, the log-periodic array shown in Fig. 1.4 are generally used in HF communications. They are particularly attractive, because of their broadband properties.** **In recent years, there has been a large amount of work on the design and development of several other types of linear arrays, which are capable of being scanned over a region of space. These arrays are either of the frequency-scanned type or of the phase-scanned type (phased arrays).** **The arrays are usually constructed using basic radiators, like the dipole or slotted waveguide. One-dimensional structures are called linear arrays. For certain applications especially in satellite communications, linear arrays are used as adaptive arrays, which have a signal processing capability. For example, by adjusting the amplitude and phase excitation of the array, a null is produced in the direction of an interfering signal to cancel it out.** ## **1.3 Wave Propagation** A typical antenna used for radiating electromagnetic waves is the vertical dipole shown in Fig. 1.5. The electric and magnetic field lines near the antenna produce the induction field, while farther away, the radiation field travels outwards with the velocity of light. * **Fig. 1.5** : A diagram showing the dipole antenna with the electric (E) field and magnetic (H) field. **The field pattern consists of electric lines in the vertical plane and magnetic lines in the horizontal plane which are symmetrical about the antenna. When the dipole is energised by a voltage at a frequency greater than about 16 kHz, the charge distribution on the antenna reverses sign more rapidly than the collapsing field lines near it. The collapsing field lines are pushed away by the reversing field, and they close up to form loops. The loops expand and travel away from the antenna.** **The electromagnetic waves propagated from an antenna, consist essentially of a ground wave and a sky wave. The ground wave travels along the earth’s surface and is used at the lower frequencies for medium distance communication. The sky wave travels upwards and is refracted back to earth by the ionosphere. It is used at the higher frequencies for long distance communication. Further details are given in Chapter 6.** ## 2 Elementary Antennas A very elementary, yet important theoretical antenna is the point source radiator or isotrope. It serves as a basis for comparison of many types of antennas, whose performance is best expressed in terms of such a basic radiator. ## **2.1 The Isotrope** **An isotrope or point source radiator radiates energy equally in all directions. Hence, its radiation is isotropic and the radiation pattern in any plane is a circle.** **Consider such an isotrope at point O in Fig. 2.1, fed with power P watts. The power flows outwards from O, and at any time t, it must flow through the spherical surface S of radius r. Hence, the power flow per unit area or power density Pa at point Q is given by** > $P_a = P/4πr²$ **watts/m²** * **Fig. 2.1** : A diagram showing an isotrope at point O with a spherical surface S at a radius r. **By Poynting’s theorem, the power density Pa is related to the E and H vectors by the equation** > $P_a = E * H$ **watts/m²** **or** > $|P_a| = E * H = E²/120π$ **since** > $E/H = 120π$ **Hence** > $E² / 120π = P/4πr²$ **or** > $E = √30P/r$ **V/m** **where E and H are r.m.s. values when P is the average power.** **The isotrope may be considered as a standard lossless reference antenna, with power gain G = 1. If any other practical antenna with a power gain G were placed at O, then the power received at Q would be increased G times to G x P watts. Hence, the field strength at Q will be increased to** > $E = √30GP/r $ **V/m** **The concept of power density is vividly illustrated by a radar system where it leads to the well-known radar equation.** ## **Radar Equation** **Suppose a radar transmitter at T in Fig. 2.2 has an antenna gain Gr which is fed by the transmitter with power Pr. The power density Pa at an object located at O distant r is** > $P_a = PTGT / 4πr²$ **watts/m²** **The target area of the object at O, is defined as σm² such that the intercepted power, which is assumed to be reflected back isotropically is Paσ. It therefore produces a power density Pa at the transmitter where** > $P = PTGTσ / (4πr²)²$ **watts/m²** * **Fig. 2.2** : A diagram showing the radar transmitter at T, with the power density Pa and the target area defined as σ at a distance r. **If a receiving antenna at T has a gain GR and effective area Ae, then the received power is** > $PR = PaAe = PTGTGROλ²/ (4πr²)² * 4π$ **or** > $PR = PTGTGROAe / (4π)³ r⁴$ **This is the basic radar equation relating transmitted and received powers. If the minimum detectable signal-to-noise ratio at the receiver is S/N, then PR = S and N=kTB where k is Boltzmann’s constant, T is the absolute temperature and B is the bandwidth of the system. The radar range r is then given by** > $r = [ PTGT GROA² / (4π)³kTB(S/N) ] ^1/4$ **or** > $r = [(PTGT GROA²) / (4π)⁴kTB (S/N) ] ^ 1/4$ **The range depends on the fourth root of Pr, and so accounts for the large powers used in radar systems.** ## **Example 2.1** **Calculate the minimum peak transmitter power needed in a pulsed radar required to-detect a target of 10 m² echoing area at a range of 120 km, given the following system parameters** - Operating frequency = 1.3 GHz - Receiver sensitivity = -105 dBm - Aerial gain = 34 dB - Atmospheric attenuation = 0.008 dB/km **How would the pulse length and pulse repetition frequency of the radar be determined?** **Solution** **From the previous analysis, we obtained** > $PT = (4π)³⁴ PR / (GT GROA²)$ **or** > $PT = (4π)³⁴ PR / (GT GROA²) $ **Due to atmospheric attenuation over the two-way distance there is a path loss given by** > **Path loss = 2 × 120 × 0.008 = 1.92 dB** **Hence, the transmitted power Pr must, be increased by 1.92 dB or 1.556 times.** **Hence** > $PT = (4π)³⁴ PR (1.556) / (GT GROA²) $ **Here** - r = 120 km = 1.2 × 10⁵ m - GT = GR = 34 db = 2512 - σ = 10 m² **Also** - f λ = 3 × 10⁸ **or** - λ = 3 × 10⁸ / 1.3 × 10⁹ = 0.23 m **Since the receiver sensitivity is - 105 dBm** - PR = -105 dBm = -135 dBW = 10⁻¹³.⁵ Watts **Hence** > $PT = (4π)³⁴ × (1.2 × 10⁵)⁴ × (10⁻¹³.⁵) × (1.556) / (2512)² × 10 × (0.23)²$ **or** > $PT = 6$ **kW** **In the last part of the question, the pulse length and pulse repetition frequency determine the minimum range and maximum range of the radar respectively.**