Software for Antenna Design

Marco Zennaro

ICTP, Trieste-Italy

Software for Antenna Design

Using software for antenna desing is useful because:

• Appropriate antennas contribute greatly to the communication abilities of transmitting and receiving stations.
• A directional antenna that concentrates radiated power at an optimum angle of radiation in a desired direction can provide many decibels of additional improvement, allowing use of less transmitter power or providing stronger received signals.

NEC

NEC stands for Numerical Electromagnetics Code

The original NEC has been developed at the Lawerence Livermore Laboratory, Livermore, California, under the sponsorship of the Naval Ocean Systems Center and the Air Force Weapons Laboratory. It is an advanced version of the Antenna Modeling Program (AMP) developed in the early 1970's.

The code has always been a "card image/batch run" operation initially designed for CDC and later for VAX computers. It has been ported to many other machines by myriad individuals.

From the original NEC, newer versions have been developed, NEC2, NEC3 and NEC4.

NEC

NEC is a large antenna analysis program written in FORTRAN and designed to run on main-frame computers.

Being a Simulation Software, it is important to evaluate the results given by the computer. It is important to understand how well a computer model matches the actual one. Incertainties arise associated with the numerical model.

NEC is a complex software that requires a lot of attention!

Websites about NEC

Nec2.org has links to the NEC2 documentation and user guides.

Ray Anderson's 'Unoffical NEC Archives' contains LINUX, Mac and UNIX versions of NEC2 software

Applied Computational Electromagnetics Society Home Page.

How NEC works

An antenna described to NEC is given in two parts, a structure and a sequence of controls.

The structure is simply a numerical description of where any part of the antenna is located, and how the wires are connected up. Thanks to the old fashioned structure card style input, it takes some time to understand how the syntax works!

How NEC works

The controls tell NEC where the RF source is connected.

An antenna is modelled when transmitting, but due to the famous reciprocity theorem the transmitting gain pattern is the same as the receiving one.

The range of frequencies of the RF signal must be specified, as can also loading.

The next important element is the character of the ground. The conductivity of the earth varies from place to place, but in many cases it plays a vital role in determining the antenna gain pattern.

A NEC example

As an example we will design a vertical monopole antenna above conducting ground and we will comment the code line by line

Comments

CM NEC Input File
CM Monopole radius 0.001m, length 17m above perfect ground
CM Excitation at base by a 1V source
CE

A NEC example

NEC structures are built up of elements such as straight wires, small surface patches, etc. In this case the model is very simple, a single vertical wire.

Structure

GW 1 8 0.00000 0.00000 0.00000 0.00000 0.00000 17.00000 0.00100
GE

GW 1 8 0.00000 0.00000 0.00000 0.00000 0.00000 17.00000 0.00100
GE

GW = Card Type
1 = Tag Number used to identify all parts of this wire
8 = how many segments this wire should be divided up into
0.00000 0.00000 0.00000 = x1, y1, z1, coordinates of the first end of the wire
0.00000 0.00000 17.00000 = x2, y2, z2, coordinates of the second end of the wire
0.00100 = radius of the wire in meters
GE 1= Geometry End. The one is to indicate that a Ground Plane is going to be used.

A NEC example

Ground modelling is a difficult problem, since in most places it isn't a particularly good conductor (at sea is an exception to this of course!). There are various degrees of sophistication possible with NEC. In our example we will use a perfectly conducting ground.

Control Cards

GN 1 0 0 0 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00

The first number, here `1' means use a perfectly conducting ground. The remaining numbers are all zero for a perfect ground.

A NEC example

Frequency Range

FR 0 1 0 0 1.81E+01 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00

This is a simple single frequency analysis, so the card tells the computer to energise the source at 18.1MHz.

A NEC example

Excitation

We've described the antenna, and the frequency to energise it at, but not how the RF energy is applied to the antenna. This is to be done with the EXcitation card.

EX 0 1 1 0 1.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00

This means energise by a voltage source, at the segment with tag number one, at the first segment. The eighth segment would of course be the top end. The other numbers are only used for incident plane wave excitation, so they are left at zero.

A NEC example

What to plot

Now we must tell NEC where we want to know the antenna gain distribution. This is done via the Radiation Pattern card.

RP 0 37 72 1000 0.00E+00 0.00E+00 5.00E+00 5.0E+02 0.00E+00 0.00E+00
EN
This example asks for a Normal Mode plot, with 37 values of Theta, and 72 values of Phi, at which the field is to be computed. The `1000' isn't a number but a series of binary flags, telling the computer what information to print out (vert+horiz total gain). Now the initial values of theta and phi are given (both zero degrees), and the increment angles (both 5 degrees). EN stands for End of the input data.

A NEC example

CM NEC Input File
CM Monopole radius 0.001m, length 17m above perfect ground
CM Excitation at base by a 1V source
E GW 1 8 0.00000 0.00000 0.00000 0.00000 0.00000 17.00000 0.00100
GE 1
GN 1 0 0 0 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00
FR 0 1 0 0 1.81E+01 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00
EX 0 1 1 0 1.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00
RP 0 37 72 1000 0.00E+00 0.00E+00 5.00E+00 5.0E+02 0.00E+00 0.00E+00
EN