HISTORY

What is the origin of the antenna? I'm ruling out such early devices as compasses, because while they in some sense receive a magnetic field, it is not an electromagnetic field. Ben Franklin's kite experiment wasn't quite an antenna, as that captured lightning discharge, which is a direct current path where the energy is not transferred independent of the medium it travels. The human eye of course receives high frequency electromagnetic waves (light, to the layman). Technically the eye could be classified as an antenna; however since it can't transmit waves, it is really a sensor, so I'll exclude that as well.

The first experiments that involved the coupling of electricity and magnetism and showed a definitive relationship was that done by Faraday somewhere around the 1830s. He slid a magnetic around the coils of a wire attached to a galvanometer. In moving the magnet, he was in effect creating a time-varying magnetic field, which as a result (from Maxwell's Equations), must have had a time-varying electric field. The coil acted as a loop antenna and received the electromagnetic radiation, which was received (detected) by the galvanometer - the work of an antenna. Interestingly, the concept of electromagnetic waves had not even been thought up at this point.

Heinrich Hertz developed a wireless communication system in which he forced an electrical spark to occur in the gap of a dipole antenna. He used a loop antenna as a receiver, and observed a similar disturbance. This was 1886. By 1901, Marconi was sending information across the atlantic. For a transmit antenna, he used several vertical wires attached to the ground. Across the Atlantic Ocean, the receive antenna was a 200 meter wire held up by a kite [1].

In 1906, Columbia University had an Experimental Wireless Station where they used a transmitting aerial cage. This was a cage made up of wires and suspended in the air, resembling a cage.

A rough outline of some major antennas and their discovery/fabrication dates are listed:

Yagi-Uda Antenna, 1920s Horn antennas, 1939. Interesting, the early antenna literature discussed waveguides as "hollow

metal pipes". Antenna Arrays, 1940s Parabolic Reflectors, late 1940s, early 1950s? Just a guess. Patch Antennas, 1970s. PIFA, 1980s.

Current research on antennas involves metamaterials (materials that have engineered dielectric and magnetic constants that can be simultaneously negative, allowing for interesting properties like a negative index of refraction). Other research focuses on making antennas smaller, particularly in communications for personal wireless communication devices (e.g. cell phones). A lot of work is being performed on numerical modeling of antennas, so that their properties can be predicted before they are built and tested.

DESIGN PARAMETERS

Radiation patterns of broadside array along the z-axis, d = λ/2.

Radiation patterns of broadside array along the z-axis, d = λ/2.

We can observe that increasing M has the following effects on the radiation pattern:

• The width of the main lobe decreases; in other words, it becomes narrower. This is crucial for the applications of smart antennas when a single narrow beam is required to track a mobile or cluster of mobiles. • The number of sidelobes increases. In addition, the level of the first and subsequent sidelobes decreases compared with the main lobe. Sidelobes represent power radiated or received in potentially unwanted directions. So in a wireless communications system, sidelobes will contribute to the level of interference spread in the cell or sector by a transmitter as well as the level of interference seen by a receiver when antenna arrays are used. • The number of nulls in the pattern increases. In interference cancellation applications, the directions of these nulls as well as the null depths have to be optimized.

Polar pattern, broadside array, M = 6, d = λ/2. Polar pattern, broadside array M = 6, d = λ

Polar pattern, broadside array, M = 2, d = λ/2. Polar pattern, broadside array, M = 2, d = 5λ.

Polar pattern, broadside array, M = 2, d = 10λ.

First Null Beamwidth

The null-to-null beamwidth (NNBW) of the array has a significant impact on the performance of a smart antenna system and is considered one of the important parameters that need to be considered in the antenna design. For a broadside array on the z-axis, the null-to-null beamwidth is given by

The behavior of the NNBW is shown in Figures 4.14 and 4.15 as a function of d and M, respectively. Note that the larger the array, the smaller the NNBW becomes and the narrower the main lobe gets.

NNBW as a function of element spacing d NNBW as a function of number of elements M

Half-Power Beamwidth

Another very important beamwidth measure to consider is the half-power or 3-dB beamwidth. The 3-dB beamwidth of a broadside array on the z-axis is given by

Array Directivity

For a broadside array and small element spacing (d < λ), the directivity can be approximated by

APPLICATIONS

Broadside array antenna is an antenna in the form of an array of radiators, most often balanced dipoles or slot radiators, that are excited in the same phase by high-frequency currents.

The maximum radiation intensity is in the direction perpendicular to the plane of the array because the fields of all theradiators are in phase in that direction. The directional pattern of a broadside array in any plane perpendicular to the plane of the arrayconsists of a main lobe and many side lobes with widths that depend on the linear dimensions of the array (seeANTENNA, Figure 8). In orderto obtain unidirectional radiation from a broadside array; the array is supplemented with a tuned or aperiodic reflector. In cases where it isnecessary to simplify the feed system of a broadside array, a unidirectional traveling-wave antenna having a small gain is used as a radiator;a director antenna, helical antenna, or log-periodic antenna may be used in such cases, obviating the need for a reflector. Broadside arraysare used for a wide range of radio waves. At decameter (short) wavelengths, they are used chiefly for radio broadcasting over long distances

If all of the elements are fed in-phase, there will always be a broadside radiation pattern. However, depending on the relative spacing, an end fire pattern can also be created

The radiating elements in the above illustration can be placed such that they reinforce one another along the array axis, or not. An end fire pattern is recreated when they reinforce.

By varying the space or phase shift between the elements, the size and direction of the side lobes can be adjusted between these two extremes. Increasing the number of radiating elements increases the overall array gain.

Determining the array factor is sometimes relatively straightforward. By definition, the signal strength for a broadside array is a maximum when   = 90o and a minimum when   = 0o

Since the array factor is a maximum when   = 0o we can determine the current phase shift  , required to create a broadside radiation pattern for a given frequency or element spacing:

THEORY OF OPERATION

Physically, a broadside array antenna looks somewhat like a ladder. When the array and the elements in it are polarized horizontally, it looks like an upright ladder. When the array is polarized vertically, it looks like a ladder lying on one side (view B). View C is an illustration of the radiation pattern of a broadside array. Horizontally polarized arrays using more than two elements are not common. This is because the requirement that the bottom of the array be a significant distance above the earth presents construction problems. Compared with collinear arrays, broadside arrays tune sharply, but lose efficiency rapidly when not operated on the frequencies for which they are designed.

RADIATION PATTERN - Figure shows an end view of two parallel half-wave antennas (A and B) operating in the same phase and located 1/2 wavelengths apart. At a point (P) far removed from the antennas, the antennas appear as a single point. Energy radiating toward P from antenna A starts out in phase with the energy radiating from antenna B in the same direction. Propagation from each antenna travels over the same distance to point P, arriving there in phase. The antennas reinforce each other in this direction, making a strong signal available at P. Field strength measured at P is greater than it would be if the total power supplied to both antennas had been fed to a single dipole. Radiation toward point P1 is built up in the same manner.

Next consider a wave front traveling toward point Q from antenna B. By the time it reaches antenna A, 1/2 wavelength away, 1/2 cycle has elapsed. Therefore energy from antenna B meets the energy from antenna A 180 degrees out of phase. As a result, the energy moving toward point Q from the two sources cancels. In a like manner, radiation from antenna A traveling toward point Q1 meets and cancels the radiation in the same direction from antenna B. As a result, little propagation takes place in either direction along the QQ1 axis. Most of the energy is concentrated in both directions along the PP1 axis. When both antenna elements are fed from the same source, the result is the basic broadside array.

When more than two elements are used in a broadside arrangement, they are all parallel and in the same plane, as shown in figure 4-26, view B. Current phase, indicated by the arrows, must be the same for all elements. The radiation pattern shown in figure 4-26, view C, is always bidirectional. This pattern is sharper than the one shown in figure 4-27 because of the additional two elements. Directivity and gain depend on the number of elements and the spacing between them.

GAIN AND DIRECTIVITY - The physical disposition of dipoles operated broadside to each other allows for much greater coupling between them than can occur between collinear elements. Moving the parallel antenna elements closer together or farther apart affects the actual impedance of the entire array and the overall radiation resistance as well. As the spacing between broadside elements increases, the effect on the radiation pattern is a sharpening of the major lobes. When the array consists of only two dipoles spaced exactly 1/2 wavelength apart, no minor lobes are generated at all. Increasing the distance between the elements beyond that point, however, tends to throw off the phase relationship between the original current in one element and the current induced in it by the other element. The result is that, although the major lobes are sharpened, minor lobes are introduced, even with two elements. These, however, are not large enough to be of concern.

If you add the same number of elements to both a broadside array and a collinear array, the gain of the broadside array will be greater. Reduced radiation resistance resulting from the efficient coupling between dipoles accounts for most of this gain. However, certain practical factors limit the number of elements that may be used. The construction problem increases with the number of elements, especially when they are polarized horizontally.

UNIVERSITY OF PERPETUAL HELP CALAMBABRGY. PACIANO, RIZAL CALAMBA CITY

COLLEGE OF ENGINEERING

Transmission lines and antenna system(Broadside array antenna)

Submitted by:

Paul Angelo MagnayeLeslie Ann Flores

Engr. Diomedes PataniInstructor