ANTENNA BASICS

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A transmitter needs an antenna to send its radio signal, and a receiver needs an antenna to pick up that radio signal. The simplest form of antenna is the “dipole”. Suppose the electrical output of an oscillator is directed down two conductors, not connected at the ends. This will radiate EM energy from the open-circuit ends. It radiates energy much more effectively if the conductors are bent at the ends to form a right angle, with each bend being a quarter-wavelength long relative to the oscillator output.

This is a “half-wave” dipole. It is not only effective in generating radio waves at a particular frequency, it is also effective in picking them up. This is true in general of all antennas: they are “reciprocal”, working much the same in transmission or reception, just in different directions. A single conductor can be used as well; this is the “monopole” antenna used in portable radio receivers and the like.

By itself, a dipole or monopole antenna “broadcasts” in all radial directions evenly, or in other words it is an “omnidirectional” antenna. It could be turned into a “directional” antenna by placing it in the center of metal parabolic dish, with a small reflector above the dipole to bounce the signal back into the dish for transmission in one direction. This configuration is familiar from the modern satellite-TV receiver, though instead of a dipole radio energy is usually just dumped into the dish through an open “horn”, either fed through the dish or under the bottom of the dish. It’s really very much the same as using a parabolic mirror to focus light, only the wavelength of radio signals is longer.

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While parabolic dishes are usually circular, creating a focused “pencil” beam, elliptical or cylindrical dishes with parabolic curvature can also be used if the radio beam needs to be focused along one axis but not along the other, or in other words has a “fan” configuration. The beam width is normally defined by a “3 dB” law, with the boundary of the beam defined as the surface where the power of the beam at the center falls by 3 dB.

Another simple way to create a “directional” antenna with a dipole is to mount it within a row of parallel conductive rods, with the rods of decreasing length to the “front” of the dipole (relative to the direction of focus) and of increasing length to the “back” of the dipole. This type of antenna is known as a “Yagi-Uda” or just “Yagi” antenna. Such antennas are referred to as “end-fire antennas”, since they are directional along their long axis. Dipole antennas, in contrast, are directional at a right angle to their plane.

A more sophisticated approach is to obtain a directed focus by using an antenna with multiple dipoles in a grid arrangement, with the focus obtained by interference effects. Such “dipole arrays” were common with early longwave radars. Arrays can be made with end-fire antennas as well, and very significantly as slotted plates, with radio energy fed through the slots. The slots act as dipoles, though while the polarization of a radio wave generated by a dipole is in line with the long axis of the dipole, it’s at a right angle to the long axis of the slot. Slotted planar arrays are very popular these days, since shrewd slot arrangements allow them to be much more efficient than simple parabolic antennas, which will waste about two-thirds of the energy pumped into them. A well-designed slotted planar array will waste less than a third of the energy.

* Directional antennas are characterized by a factor known as “antenna gain”. This is simply the ratio of the focused beam power to the same broadcast power sent through an omnidirectional antenna. For example, if the focused beam has 50 times the power of an omnidirectional antenna with the same transmitter power, the directional antenna has a gain of 50, or 17 dB.

The larger the receiver dish, the greater the receiver sensitivity, since it creates a bigger “bucket” or “eye” to collect radio waves. However, the longer the wavelength, the bigger the dish has to be to focus the radio waves, and conversely the more focused the beam, the bigger the antenna. One simple rule of thumb gives the width of the antenna as:

width_in_meters = 51 * wavelength_in_meters / beamwidth in degrees

For example, to obtain a beamwidth of 2 degrees at a low frequency of 5 MHz (corresponding to 60 meters), the antenna width is:

51 * 60 / 2 = 1,530 meters

This is not a very reasonable size for an antenna. At 150 MHz, with a wavelength 30 times shorter, the antenna size shrinks to 51 meters, and at 1 GHz, with a wavelength 200 times shorter, the antenna size shrinks to 7.65 meters. Big antennas don’t necessarily mean high power or sensitivity: they may indicate low frequency operation, or narrow beamwidths, or both.

Another minor related fact is that the dish doesn’t have to be solid. It can be a mesh, just as long as the mesh grid spacing is less than that of the radar operating wavelengths. This makes for a lighter antenna, and also one not so easily disturbed by the wind.

* Directional antennas don’t always generate all their radio output in a nice neat directional beam. Interference between transmit signals may generate “sidelobes” that cause unwanted transmissions to the sides of the beam, or a “backlobe” in the reverse direction. The sidelobes and backlobe can rob the main lobe of energy and of course corrupt the directionality of the beam, generating and receiving signals in unwanted directions. Proper antenna design minimizes the power lost by sidelobes and backlobes.

Sidelobes are a particular nuisance in radars, since they can produce false returns and can pick up radio interference, including deliberate interference produced by countermeasures systems. In radar systems, the ratio of sidelobe to main beam power is generally kept to less than -40 dB, or 1:10,000. This is done in arrays by carefully arranging the power levels of the array elements, with more power in the center elements than at the elements along the edge. There are a number of “aperture tapering” schemes to define the proper arrangements of power levels.