Exploitation of NEC’s GM card #2

NEC-2 and NEC-4 support a GM card to translate and rotate a structure in three dimensions.

At Exploitation of NEC’s GM card I gave a small example of using the GM card.

Antennas often have some degree of symmetry and it is the key to simplification through use of the GM card.

This article gives a simple but practical example of exploitation of this often overlooked facility to a Lazy H antenna.

Modelling a LazyH

A simple horizontal LazyH is just a pair of horizontal dipoles spaced vertically and fed in phase by typically an open wire feed line between the two dipole centres, and the centre of that line being the feed point so that the dipoles are fed in-phase.

The first question is whether to model that feed line or phasing line as it is often known using NEC TL cards (for Transmission Line) or as GW elements.

Contrary to online opinions like the vertical part should be modeled as a transmission line, not wires offered without explanation (you do need to be wary of accepting such advice… because the giver may not understand what he is saying and whether it is appropriate), either can be used and which one is better suited to your own application depends on exactly what you are constructing.

If you are constructing with sections of commercial feed line of known Zo and VF, then model them as TL elements. If you are using wires that are uniformly spaced and essentially air space (so VF is very close to 1), then use GW elements using the dimensions employed.

I will show you the two methods.

Here are the 4NEC2 example model files: LazyhModels.

Transmission line for the phasing sections (TL)

The LazyH is symmetric vertically about the feed point, so lets exploit that by defining one dipole as a single GW element, and copying it. Since the array is symmetric about the feed point, lets build it with the feed point at the origin initially, then move the entire array to the correct height above ground.

I am going to demonstrate this using 4NEC2 / NEC-4, and firstly I will exploit the symbol substitution of 4NEC2.

Screenshot - 11_02_2015 , 08_25_26

Above are a bunch of calculations and symbol definitions for the key dimensions of the system. l1 is the half length of a dipole. In this case the dipoles will be 1.25λ long, and spaced vertically by 1.25λ.

Screenshot - 11_02_2015 , 18_10_42

Above, the geometry definitions.

First, the upper dipole GW card, then a GM card to create the lower dipole by rotation about the X axis. Then a GW feed point segment is added, and the whole thing raised above ground with the final GM card.

Screenshot - 11_02_2015 , 18_38_53

Above, we wire it up. The transmission line cards define transmission lines of length l1 divided by the velocity factor (the specified length must be the electrical length).

Screenshot - 11_02_2015 , 18_39_05

And we run it. Above the main screen result.

Clip 107

The current / phase distribution shows the two dipoles excited, the blue lines represent the transmission line.

The actual NEC deck follows.

CM
CE
GW 1 101 -13.19509 0 13.195091 13.195091 0 13.195091 0.001
GM 1 1 180 0 0 0 0 0 1
GW 20 1 -0.05 0 0 0.05 0 0 0.001
GM 0 0 0 0 -20 0 0 20 1
GE 1
LD 5 0 0 0 58000000
GN 2 0 0 0 13 0.005
EK
EX 0 20 1 0 1 0 0
TL 20 1 1 51 400 14.661212 0 0 0 0
TL 20 1 2 51 400 14.661212 0 0 0 0
FR 0 0 0 0 14.2 0
RP 0 46 181 1003 -90 0 2 2

Wires for the phasing sections (GW)

Using the same symbol definitions, the geometry is defined.

Screenshot - 11_02_2015 , 08_25_38This time, the right half of the  upper dipole is defined with a GW card. It is then copied to the left to make the other half of the dipole with a small gap where the vertical phasing wires attach. The next GM card makes the lower dipole by rotation about the X axis.

There are other and simpler ways to define this, but this method gives two dipoles where each of the four halves are define left to right so that current phase plots dont have reversals at their ends.

Now one vertical wire is defined and two more GM cards to create the additional wires, this time always defined from bottom to top for the same reasons as the dipole wire orientation.

Then a GW feed point segment is added, and the whole thing raised above ground with the final GM card.

Screenshot - 11_02_2015 , 08_25_49

Above, the source is wired up. It is simpler as there is only the one feed point.

Screenshot - 11_02_2015 , 08_25_19

And we run it. Above the main screen result. There will be some small differences, notably in the impedance. Once includes loss in the phasing section, the other doesn’t.

Clip 105

The current / phase distribution shows the two dipoles excited and the wires that form the phasing sections.

The actual NEC deck follows.

CM
CE
GW 1 50 0.05 0 13.195091 13.195091 0 13.195091 0.001
GM 1 1 0 0 0 -13.24509 0 0 1
GM 2 1 180 0 0 0 0 0 1
GW 10 50 0.05 0 0 0.05 0 13.195091 0.001
GM 1 1 0 0 0 0 0 -13.19509 10
GM 2 1 0 0 180 0 0 0 10
GW 20 1 -0.05 0 0 0.05 0 0 0.001
GM 0 0 0 0 -20 0 0 20 1
GE 1
LD 5 0 0 0 58000000
GN 2 0 0 0 13 0.005
EK
EX 0 20 1 0 1 0 0 0
FR 0 0 0 0 14.2 0
XQ
EN

Links / References

  • Burke and Poggio 1977a. Numerical Electromagnetic Code (NEC-1) Part I: NEC Program: Lawrence Livermore Laboratory.
  • Burke and Poggio 1977b. Numerical Electromagnetic Code (NEC-1) Part II: NEC Program Description – Code: Lawrence Livermore Laboratory.
  • Burke and Poggio 1977c. Numerical Electromagnetic Code (NEC-1) Part III: NEC User’s guide: Lawrence Livermore Laboratory.