With the Numerical Green's Function (NGF) option, a fixed structure and its environment may be modeled and the factored interaction matrix saved on a file. New parts may then be added to the model in subsequent computer runs and the complete solution obtained without repeating calculation for the data on the file. The main purpose of the NGF is to avoid the unnecessary repetition of calculations when a part of a model, such as a single antenna in a complex environment, will be modified one or more times while the environment remains fixed. For example, when modeling antennas on ships, several antenna designs or locations may be considered on an otherwise unchanged ship. With the NGF, the self-interaction matrix for the fixed environment may be computed, factored for solution, and saved on a tape or disk file. Solution for a new antenna then requires only the evaluation of the self-interaction matrix for the antenna, the mutual antenna-to-environment interactions, and matrix manipulations for a partitioned-matrix solution. When the previously written NGF file is used, the free space Green's function in the NEC formulation is, in effect, replaced by the Green's function for the environment.
Another reason for using the NGF option is to exploit partial symmetry in a structure. In a single run, a structure must be perfectly symmetric for NEC to use symmetry in the solution. Any unsymmetric segments or patches, or ones that lie in a symmetry plane or on the axis of rotation, will destroy the symmetry. Such partial symmetry may be exploited to reduce solution time by running the symmetric part of the model first and writing a NGF file. The unsymmetric parts may then be added in a second run.
Use of the NGF option may also be warranted for large, time-consuming models to save an expensive result for further use. Without adding new antennas, it may be used with a new excitation or to compute new radiation, near-field, or coupling data not computed in the original run.
To write a NGF file for the structure, the data deck is constructed as for a normal run. After the GE card, the frequency, ground parameters, and loading may be set by FR, GN, and LD cards. EK or KH may also be used. Other cards, such as EX or NT that do not change the matrix, will not affect the NGF and will not be saved on the file. After the model has been defined, a WG card is used to fill and factor the matrix and cause the NGF data to be written to the file TAPE20. TAPE20 should be saved after the run terminates. Other cards may follow the WG card to define an excitation and request field calculations as in a normal run. WG should be the first card to request filling and factoring of the matrix, however, since it reserves array space for the matrix in subsequent runs when the NGF is used. Hence, WG should come before XQ, RP, NE, or NH. The FR card must not specify multiple frequencies when a NGF is written.
To use a previously generated NGF file, the file is made available to the program as TAPE20. The first structure-geometry data card, following the CE card, must be a GF card to cause the program to read TAPE20. Subsequent structure data cards define the new structure to be added to the NGF structure. All types of structure geometry data cards may be used although GM, GR, GX, and GS will affect new structure but not that from the NGF file. GR and GX will have their usual effect on the new structure but will not result in use of symmetry in the solution. Symmetry may be used in writing the NGF file but not for new structures used with the NGF.
For connections between the new structure and NGF structure, the new segment ends or patch centers are made to coincide with the NGF segment ends or patch centers as in a normal run. The rules still apply that only a single segment may connect to a given patch and a segment may have a patch connection on only one of its ends. Also, a wire may never connect to a patch formed by subdividing another patch for a previous connection.
Following the GE card the program control cards may be used as usual, with the exception that FR and GN cards may not be used. The parameters from these cards are taken from the NGF file and cannot be changed. LD cards may be used to load new segments but not segments in the NGF. If integers I3 and I4 on a LD card are blank, the card will load all new segments (new segments with tag LDTAG if I2 is not zero) but not NGF segments. If I2, I3 and I4 select a specific NGF segment, the run will terminate with an error message. The effect of loading on NGF segments may be obtained with an NT card, since NT (and TL) may connect to either new or NGF segments.
Computation time for a run using a NGF file may be estimated from the formulas in section V by evaluating the time to run the complete structure and subtracting time to fill and factor the matrix for the NGF part of the structure alone (T1 and T2). If the new structure connects to the NGF structure, new unknowns - in addition to those for the new segments and patches - are produced and should be included in the time estimate for the complete structure. If a new segment or patch connects to a NGF segment, the current expansion function for the NGF segment is modified. One new unknown is then added to the matrix equation to represent the modified expansion function and suppress the old expansion function. If a new segment connects to a NGF patch, 10 new unknowns are produced in addition to that for the new segment. Four new patches are automatically generated at the connection point accounting for eight unknowns. The remaining two new unknowns are needed to suppress the current on the old patch that has been replaced.
Although connection to a NGF segment modifies the old basis function, the current on the segment will be printed in its normal location in the table of segment currents. When a new wire connects to a NGF patch, the patch is divided into four new patches that will appear after the user-defined patches in the patch data. The original patch will be listed in the tables but with nearly zero current. Also, the Z coordinates of the original patch will be set to 9999.