Antenna Maximum Linear Dimension

Value to Enter:

Enter the length of the longest linear dimension of the antenna itself. For a dipole or yagi it's simply the length of the antenna. For a parabolic dish the dimension is the diameter of the dish. For any other shape, use the longest single dimension (which could be a diagonal across a rectangular element as in a grid sectional parabolic antenna.)

Significance of This Value:

Near Field Radiation and the Near Field / Far Field Boundary

Metal objects that are very close to the radiating element of an antenna distort the transmission pattern. This distortion is used in a controlled way by antenna designers in Yagi and Grid antennas where reflecting and directing metal elements are positioned in a way that shapes the transmitted signal making the signal stronger in some directions than in others (a "directional antenna").

When you enter a value for Max Linear Dimension, and check the corresponding radio button, the Connect802 Path Link Budget and Antenna Calculator uses that value, in conjunction with the transmission frequency, to calculate boundary between a region surrounding the antenna called the Near Field and the region further away, called the Far Field . This calculation, along with the calculation for the wavelength of the transmitted signal help to design a system that is not degraded by metal objects too close to the antenna.

Metal objects in the Near Field region will significantly distort the antenna's transmit and receive characteristics. Metal objects that might accidentally end up in the Near Field or within 1/2 wavelength of the antenna include aluminum wall studs or plumbing pipe concealed behind drywall, ceiling supports in retail or warehouse buildings where the ceiling is exposed and an antenna is mounted on the support gridwork, and antenna towers where improper mounting places a side-mounted antenna too close to the tower. Keep metal objects a minimum of 1/2 wavelength away from any antenna and avoid metal objects in the Near Field as much as is practical.

Background and Technical Perspective:

The electric current impressed on the radiating element of an antenna alternates in strength at the frequency of the transmitter (2.4 GHz, 5.8Hz, etc.) During the first 1/4 of each wavelength the current is increasing, creating an expanding magnetic field around the conductor. During the next 1/4 cycle the current is decreasing to zero, and the magnetic field collapses. A portion of the energy stored in the magnetic field is returned to the radiating element. This is called the induction energy. It is the energy stored in the magnetic field during one 1/4 cycle, and returned to the antenna in the next. Induction energy exists only in very close proximity to the radiating element. This region is called the Near Field. Induction energy is greatest within 1/2 wavelength of the radiating element.

The expanding and collapsing magnetic field sets in motion an electromagnetic field that continues to expand away from the antenna. This is called the radiating energy and this is what travels through space between the transmitter and receiver. It's the radiating energy that we normally think about when considering a wireless network design. Radiating energy exists in the region called the Far Field. The Far Field is also referred to as the Fraunhofer Region after Joseph Fraunhofer who is best known for his work in the early 1800's with spectral absorption lines, the dark lines in a solar spectrum that help scientists determine the chemical makeup of stars.

Because the radiating characteristics of an antenna are closely related to the interaction of the collapsing Near Field and the radiating element it can be useful to know the distance to the boundary between the Near Field and the Far Field. A metal object in the Near Field will absorb some of the induction energy created by the antenna. Since there is no transmitter circuitry attached to this metal object there is no place for the induced energy to go. The energy is re-radiated from the metal object just as if the object were another antenna. When these interactions are carefully calculated the elements of a Yagi antenna can be designed so as to create a directional beam. When the interactions are the result of unintended metal objects in the environment they can cause unexpected antenna behaviors.

A detailed discussion of the Near Field and Far Field must take place in a context that is well outside the scope of the present overview. Keep unintended metal objects out of the Near Field as much as is practical and definitely avoid metal objects within 1/2 wavelength of the radiating element.