4. Specifying Grounds and Ground Radials in NC

If you were to go to the Summary tab view of the Output Window, you will find that NC has been generating antennas model for the case where the relative dielectric constant of the ground is 20 and the ground conductivity is 0.0303 mhos per meter. This is the so called "good ground."

You can select a different set of ground parameters to use by using the function ground, with two arguments, the first is the relative dielectric constant and the second argument is the ground conductivity. E.g.,

ground( 20.0, 0.0303 ) ;

NC also has some functions you can call to get other preset ground parameters:

freespace() ;
poorGround() ;
averageGround() ;
goodGround() ;
perfectGround() ;
saltWaterGround() ;
freshWaterGround() ;

freespace and perfectGround are special ground conditions in NEC-2 and are not associated with a relative dielectric constant or ground conductivity.

By calling the function poorGround, NC will use (3.0, 0.001) as the pair of numbers for the relative dielectric constant and ground conductivity, averageGround will set the pair of numbers to (13.0, 0.005), goodGround uses (20.0, 0.0303), saltWaterGround uses (81.0, 5.0) and freshWaterGround uses (80.0, 0.001).

You can also turn on or off the use of the Sommerfeld/Norton ground approximation. The useSommerfeld function takes a single integer argument. Calling the function with a 0 will turn the Sommerfeld/Norton approximation off, and calling the function with a 1 will turn the Sommerfeld/Norton approximation on, e.g.

useSommerfeldGround( 1 ) ;

NC defaults to turning the Sommerfeld approximation off. The Sommerfeld ground approximation is more accurate than the standard NEC-2 ground if there are wires close to ground, but the approximation also uses more computational cycles.


Radials

NEC-2 has a provision for generating a ground screen (radials) for ground mounted antennas. The only caveat is that there can only be a single one, and the coordinates of the center of the radials must be at (x,y,z) = (0,0,0). The NEC-2 ground radials can be generated by using the following function call:

necRadials( length, wireRadius, elements ) ;

length is the length of each radial wire (measured from the center) and elements is the number of radials. Since they are not real wires, these radials do not show up in the Output geometry view.

The following example is how you can specify the use of NEC-2 radials:




Notice that the ground mounted vertical antenna is fed by a short piece of wire (6") at the base. One end of it is attached to the ground screen and the other end is attached to the rest of the wire that forms the rest of the vertical antenna.

NC has a shortcut for generating the wires to create sets of symmetrical above-ground radials, . Radial wires that are generated this way are specially tagged so they do not appear in the Output Window's Geometry view unless you turn on the Draw Radials option.

The function for generating above ground radials is

radials( x, y, z, length, wireRadius, elements ) ;

x, y, z specify the center of the radial system. Each radial wire has a length of each spoke, together with a wireRadius. elements is an integer specifying how many spokes there are in the radial system.

Because of limitations of NEC-2, these above ground radials cannot be placed close to the ground itself unless you have specified free space or perfect grounds. Typically, you have to raise the radials to at least 12" at a frequency of 14 MHz to get reasonable feed point impedances.

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