- Materials & Industrial Technology
Common Antenna Terminology and Its Translation
Front to Back Ratio
The ratio of power gain between the front and back of a directional antenna is the front to back ratio. High performance antennas have a higher front to back ratio. This ratio is usually given in dB.
You can determine the front to back ratio by turning around a directional antenna and measuring the gain. This metric is only relevant with directional antennas and can’t be used with wheel antennas.
Directivity refers to the ability of antenna to focus energy it sends or receives in a particular direction. Omnidirectional antennas like wheel antennas have 360° coverage.
Directional coverage is focused in one direction from a very narrow 15° to broader 180°.
Antenna gain is a relative measure of antennas ability to concentrate RF energy in a particular direction. It may be measured in dBi, (dB compared to isotropic radiator) or dBd (dB compared to a dipole radiator). Antenna gain can be calculated by multiplying the antenna efficiency by the directivity.
If you know which direction the antenna will be coming from and you can point the antenna in that direction, then you want a high gain antenna. If you don’t know which direction the antenna will be coming from, you want a low gain antenna.
An antenna beamwidth is the angle between the half power of an antenna power over the relative or peak power. Thus the antenna beam width is also called its half power. Beam width is usually expressed in degrees.
Coaxial cable, or coax for short, that has a central conductor held in place by insulators. Coax cables are used to carry various types of signals.
The length of coax to use with an antenna depends on frequency and the diameter of your coax.
If you are working with frequencies around 5.7 GHz and small coaxial cables (2-4 mm), then you can only use 1 to 2 meters or 3 to 6 feet of coax. If you are working with 5-10 mm coaxial cables and frequencies from 800 to 900 MHz, then you could use 5 to 10 meters or 15 to 30 feet of coax with the antenna without affecting its reception.
Coax can affect the performance of your antenna. For example, small log periodic antennas like the one shown have .085 size coax, but the smaller diameter flexible Teflon coax can be used.
The 141 size coax should not be used with a small antenna like the one shown here, since it will change the performance to the antenna drastically above 5 GHz.
SMA is short for surface mount assembly. SMA connectors are mounted to a circuit board, typically through pin holes in the board, and soldered into place.
A BNC or Bayonet Neill–Concelman connector is a type of small, quick disconnect connector used to attach and detach coaxial cables. The BNC connector is mated to the coax when the connector is turned a quarter of a turn versus multiple full turns.
A filter blocks or reduces signals at certain frequencies while letting others pass through to the receiver or radio. The need for filters depends on your receiver. Good receivers usually do not need filters.
A dipole antenna is simply a conductive wire made of two sections separated by an insulator. A dipole antenna has the length of one half of the wavelength of the frequency it is supposed to receive. A monopole antenna is made of a single length of wire, but it lacks the separation by an insulator that gives the dipole two pieces.
A yagi antenna, sometimes called a patch antenna, has a main dipole element with a reflector element behind it.The main element can be considered a type of folded dipole antenna.Yagi antennas are always directional antennas.
The signal it can send or receive depends on the size of the elements and spacing of the main elements and any parasitic elements. Yagi antennas tend to receive only a narrow frequency range.
Wheel antennas are simply antennas that resemble wheels. They are relatively thin but wide disks. They may be made from a literal disk or be made from three or four loops of wire that form a wheel shape.
Patch Array Antennas
A patch array antenna provides high gain across a narrow frequency range. A quad patch antenna simply has four “patches” in the array. An antenna with two “patches” is called a dual patch antenna.
Log Periodic Antennas
A log periodic antenna always has the longest element in the array at the rear. The driven elements in the log periodic then get smaller as they move closer to the front of the antenna, and none of them are of the same length. The elements are half the length of the wavelength they can receive, so the highest frequency the antenna can receive is determined by the length of the smallest element in front while the longest element determines the lowest frequency it can receive.
Vivaldi antennas are a type of planar antenna. They are ultra wideband or UWB antennas. The tapered slot design of the Vivaldi antenna gives it a high gain.
Log Periodic antennas are limited to a maximum frequency of10 GHz. Ultrawideband antennas like Vivaldi's work better above 10 GHz.