Fundamentals of Antennas.

About radio energy

• A magnetic field forms around a magnet. An electrostatic field might form around your body when you shuffle your feet across a rug in the winter.
• When a magnetic field cuts across a wire, it generates a bit of electricity. An electrostatic field can also generate a bit of current in a wire. Radio waves are composed of alternating magnetic and electrostatic fields. When AC voltage, such as that in your house wiring, is fed into a sufficiently long wire, alternating current and voltage in the wire turns into alternating magnetic and electrostatic fields --- radio waves! Likewise, any wire intercepting the radio wave will generate a bit of electricity at the same frequency as the original alternating current and voltage.

  Antenna Parts.

• Anything that conducts electricity is an antenna, (usually a wire, but it could be your body) but the basic unit from which we build antennas is a wire, 1/2 wave length long having a connection in the middle to equipment. Called a dipole, it is an efficient radiator of alternating current electrical energy and is a standard of reference for other antennas.
• Radio and light are the same except for wavelength, i.e. frequency. Light and radio are both electromagnetic energy. The speed of light is about 300,000 kilometers per second. If a radio signal in free space has a frequency of one second, the length of the wave will be 300,000 kilometers, or 186,000 miles.
  If we were to increase the frequency to 10 cycles per second (Hertz) then each complete cycle would have traveled a length of only 30,000 kilometers, or 18,600 miles. The wave would be graphed like this:

  NOTE: above graphics courtesy Harry Lythall - SM0VPO. Click on graphic to see more on antenna basics.

• So, to find wave length in free space, divide the speed of light by the number of cycles per second (Hertz). Since the speed of light is 300,000,000 meters per second, a 100,000,000 (100 MegaHertz) wave is 3 meters (about 10 feet) in length.
• What we call radio is simply a part of the spectrum, which includes "microwave", infrared, light, xrays, etc. Click for a chart.
• Radio energy, being just like light except for the longer wavelength, can be collected by optics, as with the lens of a telescope. The bigger the lens, the more energy collected. For example, the dish antenna used for home satellite TV collects energy exactly as a telescope does. The ability of any antenna to capture energy is expressed as its aperture or effective area.
• The larger the antennas area, the stronger the signal. Double the area, and the signal is doubled.
• Although some techniques can slightly extend antenna area beyond its physical size, and although a badly designed antenna can have a much smaller area than its physical size, capture area is pretty much proportional to the size you see. This means that little antennas cannot be as effective as large properly designed antennas.

Size of Waves and Antenna Parts.

• If the frequency is 1 Hertz per second, the wave length is 300,000 km (186,000 miles) (982,080,000 feet).
• If the frequency is 10 Hertz per second, the wave length is 30,000 km (18,000 miles) (98,208,000 feet).
• If the frequency is 100 MegaHertz, the wave length is 3 Meters (9.8 feet).
• Channel 2 TV North American) starts at 54 MHz (6 meters), Channel 13 is 216 MHz or 1.4 Meters.(see the table of TV wavelengths under Rabbit Ear.)
• Remember, a dipole is 1/2 wavelength.
• A 1/2 wave dipole has a donut shaped sensitivity pattern.
• A 1/2 wave dipole captures more signal when the wire is perpendicular to a line from the transmitting antenna.

Arranging Antenna Parts.

• A 1/2 wave dipole captures more signal when the wire is perpendicular to a line from the transmitting antenna.
  
• All radio antennas are comprised of parts and pieces of dipoles. In order to garther more energy, the parts, made from wires or reflecting surfaces, must funnel or direct the energy in phase into a transmission line , the line that connects an antenna to equipment. The line is usually co-axial, meaning one inside wire covered with insulating material, that covered in turn with copper braid, or aluminum.
• Rabbit Ears set horizontal to 1/2 wave length tip to tip are a dipole, and this is usually the best configuration for Rabbit Ear antennas.
  
• Rabbit Ears set horizontal to 1/2 wave length tip to tip are a dipole, and this is usually the best configuration for Rabbit Ear antennas.

Antenna Physics links.

1. Look here for some math with graphics.
2. Terminology
3. Tutorial
4. More stuff, for Hams.
5. Twin Lead Dipole.