Ground Parameters (GN)

Purpose: To specify the relative dielectric constant and conductivity of ground in the vicinity of the antenna. In addition, a second set of ground parameters for a second medium can be specified, or a radial wire ground screen can be modeled using a reflection coefficient approximation.

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Parameters:

Integers
IPERF (I1) - Ground-type flag. The options are:
-1 - nullifies ground parameters previously used and sets free-space condition. The remainder of the card is left blank in this case.
O - finite ground, reflection coefficient approximation.
1 - perfectly conducting ground.
2 - finite ground, Sommerfeld/Norton method.

NRADL (I2) - Number of radial wires in the ground screen approximation, blank or O implies no ground screen.
(I3) & (I4) - Blank.
Floating Point:
EPSE (F1) - Relative dielectric constant for ground in the vicinity of the antenna. Leave blank in case of a perfect ground.
SIG (F2) - Conductivity in mhos/meter of the ground in the vicinity of the antenna. Leave blank in the case of a perfect ground. If SIG is input as a negative number, the complex dielectric constant Ec = Er -j sigma/omaga epslon is set to EPSR - |SIG|.
Options for Remaining Floating Point Fields (F3-F6)
a. For the case of an infinite ground plane, F3 through F6 are blank.
b. Radial wire ground screen approximation (NRADL # O). The ground screen is always centered at the origin, l.e., (0,0,0) and lies in the XY plane.
(F3) - The radius of the screen in meters.
(F4) - Radium of the wires used in the screen in meters.
(F5) & (F6) - Blank.

c. Second medium parameters (NRADL = O) for medium outside the region of the first medium (cliff problem). These parameters alter the far field patterns but to not affect the antenna impedance or current distribution.
(F3) - Relative dielectric constant of medium 2.
(F4) - Conductivity of medium 2 in mhos/meter.
(F5) - Distance in meters from the origin of the coordinate system to join between medium 1 and 2. This distance is either the radius of the circle where the two media join or the distance out the positive X axis to where the two media join in a line parallel to the Y axis. Specification of the circular or linear option is on the RP card. see figure 16.
(F6) - Distance in meters (positive or zero) by which the surface of medium 2 is below medium 1.

Notes:

When the Sommerfeld/Norton method is used, NEC requires an input-data file (TAPE21) that is generated by the program SOMNEC for the specific ground parameters and frequency (see section III-4). The file generated by SOMNEC depends only on the complex dielectric constant, Ec = Er - j sigma/omega Eo. NEC compares Ec from the file with that determined by the GN card parameters and frequency. If the relative difference exceeds 10^-3 an error message is printed. Once TAPE21 has been read for the first use of the Sommerfeld/Norton method the data is retained until the end of the run. Subsequent data, including new data sets following FIX cards, may use the TAPE21 data if the ground parameters and frequency (Ec) remain unchanged. Other ground options may be intermixed with the Sommerfeld/Norton option.
The parameters of the second medium can also be specified on another data card whose mnemonic is GD. With the GD card, the parameters of the second medium can be varied and only the radiated fields need to be recalculated. Furthermore, if a radial wire ground screen has been specified on the GN cart, the GD card is the only way to include a second medium. See the write-up of the GD cart for details.
GN cards may not be grouped together. If they are, only the information on the last card will be retained.
Use of a GN card after any form of execute dictates structure matrix regeneration.
Only the parameters of the first medium are used when the antenna currents are calculated; the parameters associated with the second medium are not used until the calculation of the far fields. It is possible then to calculate the currents over one set of ground parameters (medium one), but to calculate the far fields over another set (medium two) by setting the distance to the start of medium two to zero. Medium one can even be a perfectly conducting ground specified by IPERF=1.
When a radial wire ground screen or a second medium is specified, it is necessary to indicate their presence by the first parameter on the RP cart in order to generate the proper radiation patterns.
When a ground plane is specified, this fact should also be indicated on the GE card. Refer to the GE card for details.
When a model includes surface patches, the fields due to the patches will be calculated by using only the primary ground parameters. Hence, a second ground medium should not be used with patches. The radial wire ground screen approximation also is not implemented for patches.

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