4nec2\exe” folder, in your Word or Wordpad text editor. Other recommended sources for NEC related informations are the Nec-2 and Nec-4 user-manuals and . 4NEC2 is a freeware antenna simulation software program. The program can be used to model an antenna, and then determine through. Antenna Modeling With 4NEC2. Presented to HOTARC. By Craig Weixelbaum. May 22, Basic Modeling Approach using 4Nec2. The goals of this
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Daily evolution of the ionosphere. Geometry and operative aspects of the simple modes of ionospheric propagation. Study and Experiences ESP. Emergency Communications glossary ESP. This article summarizes some useful advices for efficient antenna simulation with programs based on the NEC-2 algorithm, developed by the Lawrence Livermore Laboratory.
On the other hand, some guidelines to simulate multiband and wideband antennas are provided. Those notes are a collection of advices and rules of design to perform simulations of antennas with programs based on the NEC-2 algorithm. The notes are not oriented to be a guide to work with the different available programs based on this algorithm, but they pretend to be a quick reference for users already initiated on its use.
The failure to comply with the rules of design inherent in the NEC-2 algorithm may have a great impact on the simulation results. For example, the resulting calculations of antenna impedance will very often be erroneus and the simulated radiation patterns unreal. Note that some of the rules are not applicable to later versions of the algorithm NEC3 and NEC-4which are more advanced and whose code is not available to the public.
On the other hand, take into account that not all the most known antenna simulation programs among all the available as shareware are based on this algorithm. In order to define a wire of length Lh in NEC-2, it is necessar to specify the coordinates of the two points delimiting its ends, the conductive material employed, the wire diameter Dh and the number of segments which will form the wire.
Let Zs be the height of the wire over the ground. If the wire is not paralell to the ground, the height of the end nearest to the ground will be considered. We will name Ns the number of segments making up a wire. The segments will have a length Lswhich may be specified by the user or computed automatically by some of the programs, a process named autosegmentation. The method of the moments in which NEC-2 is based on, computes the interaction between segmentss. Note that in the case of multiband or wideband antennas, the segment density will be different for each working frequency.
Terminology used in NEC Folded dipole with resistive load model. The understanding of the coordinate system employed in a simulation program is fundamental both for the design of the models and to correctly evaluate the results of the simulations. The coordinate systems may be different among the different programs based on NEC Coordinate system used in 4Nec2.
The ground is simulated in the XY plane and the Z axis represents the height over the ground.
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An elevation of 90 degrees will correspond to the perpendicular to the ground plane. The azimuth angles are measured clockwise from the Y axis towards the X axis. The Y axis has an azimuth of zero degrees. In order to make possible the comparison of the radiation patterns os different antennas in an easy way, it is advisable that all the models are represented geometrically agreeing with a set of general conventions.
In this section, an example of conventions gathered from [Ref. All the linear elements of the antenna will be represented in the Y axis or in other axes parallel to the Y axis. The X axis will be used for the front-to-back direction. If the model has symmetry, as far as possible the axes of the coordinate system will be used as axes of symmetry.
Before defining the structure of a model in NEC-2, it is necessary to consider in which way the current flows are managed in each simulation program. For example, in 4Nec2 the positive current always flows from the end 1 to the end 2 of each wire. On the other hand, it is necessary to observe the geometric rules presented in the next sections. In order to avoid errors, it is of interest to observe the following general rules while designing simulation models:.
In a model with several wires, it is advisable not to connect short segments with long segments [Ref. Some programs may impose conditions in this regard.
Two parallel or near parallel wires very close of each other must have their segments aligned [Ref. It is advisable for the distance between them to be manuaal least several wire diameters as well [Ref. If two wires of different diameters are close of each other, despite the fact that their segments are aligned, errors in the computation of the 4nce2 gain and its input impedance may be obtained, especially in the part of the reactance [Ref.
In models of those characteristics it is advisable to perform the AGT test see section 4ne2c. If two wires of different diameters join in a point, the results of the simulations may not be precise [Ref.
EA4FSIT1 :: Notes on antenna simulation using NEC-2
This problem can be minimized making the diameter of the wires to change in a series of steps through several additional intermediate wires [Ref. NEC-2 allows to join up to around 30 angularly equispaced wires in the same point [Ref. It is not advisable to employ a higher number because the angle majual each pair of wires would be small and the results of NEC-2 would lose precision [Ref.
Regarding the minimum length of the segments Lsminin the different references checked the following restrictions related to the diameter of the wire and the wavelength have been found:. In regards of the maximum length of the segments Lsmaxthe following restrictions related to the wavelength are specified. Restrictions related to the minimum and maximum radius of the wiresas a function of the length of the segments, may be imposed in some programs. Under normal circumstances, the higher the segment densitythe better results will be obtained in the simulations, with the cost of a higher computing time.
Nevertheless, if a model is already precise enough, an increase of the segment density will not provide further improvements. On the other hand, it is necessary to take into account that if the total number of segments in a model is very high, the maximum limit which some programs may impose can be reached [Ref. One wire must have at least segments per each semi-wavelength [Ref.
If a wire is a quarter-wavelength long, 5 segments will be a good minimum [Ref. In the case of multiband or wideband antennasthe segment density will be different for each working frequency. Therefore, the number of segments 4mec2 must be changed with the frequency, or must be calculated in order to be above the recommended minimum corresponding to the highest working frequency lower wavelength [Ref.
In order to determine the optimum segment densityit is interesting to perform a convergency test see section 7. In this section, the characteristics of the different types of ground available in most programs implementing the NEC-2 algorithm are described.
For each case, specifications are provided regarding whether the wires may touch ground or not and in the negative case at which height over ground must be placed.
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In every case apart from the free space, it is necessary to take into account the diameter of the wires: In the simulations in free space, logically there is no ground. Normally, the results will be similar to those one may find in the textbooks. The perfect ground consists of a perfect electrical conductor ground plane, without losses. It is a good option to make simulations omitting the losses of the ground, a task which may allow to evaluate those losses comparing the results with further simulations using more realistic ground types.
In the case of the horizontal wires, it is necessary to observe a height over the ground of at least one tenth of the working wavelength.
The MININEC ground model assumes a perfect ground for the computation of the currents and changes to a dielectric ground for the computation of the far field patterns ground without losses. The horizontal wires cannot touch the ground. They must be placed at a height over the ground of at least one fifth of the working wavelength:. If this rule is not observed, the simulations with NEC-2 will mqnual as a result erroneus impedances and anomalous high gains, especially for horizontal polarization.
The Sommerfeld-Norton S-N is the most precise 4ec2 model, with the highest 4nrc2 cost and is the best method to simulate horizontal wires very close to the ground. Using the S-N ground it is important to take into account that both the vertical and the horizontal wires cannot touch ground. One way to simulate a connection of a vertical wire to the ground is by means of a virtual plane of at least eight radials [Ref.
In the references checked, the following rules related to the height of the horizontal wires have been found:. This section shows some advices and rules of design about the characteristics of the generators, loads and transmission lines to be used in the simulations.
The loads and transmission lines used in NEC-2 are mathematical models instead of physical models. Therefore, they do not contribute to the radiation of the antenna [Ref. If it is considered that the transmission line may have influence on the radiation of the real antenna which is being simulated, it is possible to try to create a model of the line in some particular cases, such as with the parallel transmission line.
In the simulation of arrays, it may be necessary to use several generators phase shifted. Take into account that it is not allowed to use generators or loads in the ends of wires in open circuit [Ref. The generator would see load in one of its poles and open manuzl in the other one, so theoretically there would not be current mznual in any way.
In the simulations, the input impedance of the antenna would be erroneous, reaching too high values especially manyal the reactive part and a too low level current would be generated through the mutual coupling between the part of the generator connected to the antenna and the piece of segment connected to the other pole of the generator, which remains in open circuit.
If the generator is to be placed in the center of a wireit is recommended to model this wire with an odd number of segments [Ref. If a generator is to be placed in the apex formed by two wires joining with an angle different from degrees, such as in the case of inverted-vee antennas, several techniques can be used [Ref. The distribution of currents may be asymmetric and the values of the input impedance of the antenna may be inaccurate. It is interesting to configure a high segment density in order for those generators to be very close to the apex.
This is the most precise technique from the point of view of the simulation of the impedance of the antennas, provided that the length of the three segments of the additional wire is similar to the length of the segments of the two main wires.
The method of the moments [Ref. NEC-2 lacks the capacity to model dielectrics, such as underground radials or antennas with electrically isolated wires twin-lead J-pole, twin-lead folded dipole [Ref. Take into account that NEC-2 computes the total current in the antenna, without making differences between common mode currents and differential mode currents [Ref.
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NEC-2 is not adequate to model small size loop antennas perimeter lower than 0. Once the model has been defined, some programs implementing the NEC-2 algorithm provide options for geometric validation in order to look for non-connected wires or incorrect crossings, as well as segment validation options to verify whether the rules described in the previous sections are observed.
It is interesting to perform both validations before continuing with the convergency and average gain tests. In the case of wideband or multiband antennas operating between a minimum frequency Fmin and a maximum frequency Fmax, I suggest the following strategy of design, for example with the help of a spreadsheet. Once the geometry of the antenna is defined, number the wires, identifying also their diameter Dh i 4ne2c their length Lh i. Get the optimum final value performing the convergency test shown if the next section.
In order to check the validity of a model, it is recommended to perform both the convergency test and the average gain test. Using just one of the tests does not ensure that the model will be adequate [Ref. The convergency manuual allows to determine the optimum number of segments to be used in a particular model.