CHAPTER 1: ANTENNA FUNDAMENTALS I ANTENNA BASICS

Here you can study about the basics of antenna

ANTENNA BASICS

Antenna gain is essential for microwave communication - since it helps both transmitting and receiving, it is doubly valuable.  Practical microwave antennas provide high gain within the range of amateur fabrication skills and budgets. Three types of microwave antennas meet these criteria: horns, lenses, and dishes.  Horns are simple, foolproof, and easy to build; a 10 GHz horn with 17 dB of gain fits in the palm of a hand.  Metal-plate lenses are easy to build, light in weight, and non-critical to adjust. Finally, dishes can provide extremely high gain; a 2-foot dish at 10 GHz has more than 30 dB of gain, and much larger dishes are available. These high gains are only achievable if the antennas are properly implemented before we talk about specific microwave antennas, there a few common terms that must be defined and explained as follows.

 Aperture

The aperture of an antenna is the area that captures energy from a passing radio wave. For a dish antenna, it is not surprising that the aperture is the size of the reflector, and for a horn, the aperture is the area of the mouth of the horn.  Wire antennas are not so simple a thin dipole has almost no area, but its aperture is roughly an ellipse with an area of about 0.13λ² and Yagi-Uda antennas have even larger apertures.

 Gain

The hypothetical isotropic antenna is a point source that radiates equally in all directions. Any real antenna will radiate more energy in some directions than in others.  Since it cannot create energy, the total power radiated is the same as an isotropic antenna driven from the same transmitter; in some directions it radiates more energy than an isotropic antenna, so in others it must radiate less energy. The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. 

 Usually we are only interested in the maximum gain the direction in which the antenna is radiating most of the power. An antenna with a large aperture has more gain than a smaller one; just as it captures more energy from a passing radio wave, it also radiates more energy in that direction. Gain may be calculated as following equation 1.1.

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Where η is the efficiency of the antenna.

 Efficiency

Consider a dish antenna pointed at an isotropic antenna transmitting some distance away. We know that the isotropic antenna radiates uniformly in all directions so it is a simple matter of spherical geometry to calculate how much of that power should be arriving at the dish over its whole aperture.  Now we measure how much power is being received from the dish (at the electrical connection to the feed) never greater than is arriving at the aperture.  The ratio of power received to power arriving is the aperture efficiency.

How much efficiency should we expect?  For dishes, all the books say that 55% is reasonable, and 70 to 80% is possible with very good feeds. To achieve this efficiency we need small modification takes place on there.  

Reciprocity

If we transmit alternately with a smaller and a larger dish, is there any reason that the relative power received at a distant antenna would be any different than the relative power received by the two dishes?  No, but a mathematical proof is surprisingly difficult. Transmitting and receiving gains and antenna patterns are identical. However, the relative noise received by different types of antennas may differ, even with identical antenna gains.  Thus, the received signal-to-noise ratio may be better with one type of antenna compared to another.

Reciprocity

Reciprocity principle says “both transmitting and receiving it follows same principles”.

If we transmit alternately with a smaller and a larger dish, is there any reason that the relative power received at a distant antenna would be any different than the relative power received by the two dishes?  No, but a mathematical proof is difficult. Transmitting and receiving gains and antenna patterns are identical. However, the relative noise received by different types of antennas may differ, even with identical antenna gains.  Thus, the received signal-to-noise ratio may be better with one type of antenna compared to another.

Directivity and Beam width

Suppose an antenna has 20 dB of gain in some direction.  That means it is radiating 100 times as much power in that direction compared to radiation from an isotropic source, which is uniformly distributed over the surface of an arbitrarily large sphere which encloses it.  If all the energy from the 20 dB gain antenna were beamed from the center of that same sphere, then it would pass through an area 100 times smaller than the total surface of the sphere.

 Since there are 41,253 solid degrees in a sphere, the radiation must be concentrated in 1/100^th of that, or roughly 20 degrees beam width. The larger the gain, the smaller the beam width. The directivity of an antenna is the maximum gain of the antenna compared with its gain averaged in all directions.  It is calculated by calculating the gain, using the previous formula, with 100% efficiency.

Side lobes

No antenna is able to radiate all the energy in one preferred direction.  Some is inevitably radiated in other directions.  Often there are small peaks and valleys in the radiated energy as we look in different directions (Figure 1.1). The peaks are referred to as sidelobes, commonly specified in dB down from the main lobe, or preferred direction. Are sidelobes important?  Let's suppose that we could make an antenna with a 1 degree beamwidth, and in all other directions the average radiation was 40 dB down from the main lobe.  

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Fig no1:  Typical antenna pattern with side lobes

E-plane and H-plane

An antenna is a transducer which converts voltage and current on a transmission line into an electromagnetic field in space, consisting of an electric field and a magnetic field travelling at right angles to each other.  An ordinary dipole creates electric field, creating a pattern with larger amplitude in planes which include the dipole.  The electric field travels in the E-plane; the H-plane, perpendicular to it, is the field in which the magnetic field travels.  When we refer to polarization of an antenna, we are referring to the E-plane. However, for three-dimensional antennas like horns, dishes, and lenses, it is important to consider both the E-plane and the H-plane, in order to fully utilize the antenna and achieve maximum gain.

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Where  the path length d is the distance between the two antennas.