SHARES Radio and Electronics Club: November 2009

Thanks to all who could make it to our meeting on the 11th. Here are the notes and a link to the software for any who would like to try modelling an antenna.

As we discussed, its best to use one of the existing models supplied in the library and modify it to look like the antenna you are testing.

73 Rex - G8UBJ

MMANA-GAL – Shares presentation 11/11/2009 - G8UBJ

MMANA-GAL is an antenna-analyzing tool based on the moment method, which was introduced in MININEC.

Once upon a time when you dreamed about an antenna you had to actually build it and hang it in the sky to find out anything about it.
With MMANA you can construct antennas on your home computer and see precisely how they perform. What can MMANA tell you about your antenna?

Exact resonant frequency.
Radiation resistance at the antenna feed point.
SWR for a range of frequencies across the band.
Precise length and thickness of wire or aluminium tubing required.
What the radiation pattern looks like; the vertical take off angle.
Effect of height above ground and the quality of ground.
Gain and front to back ratio.
The inductance or capacitance needed to match it.
The diameter and number of turns of wire for a loading coil or trap.
Optimise! Eg. Adjust Yagi element length and spacing automatically.

You can test many interesting things with MMANA, like bending your dipole to fit it into your yard, check the effect of height above ground of the ends of your inverted V, or calculate the gain and performance of your favourite Uda Yagi.

MMANA helps eliminate the uncertainties that plague antenna construction and lets you play antennas inside on a rainy day without upsetting the XYL.

ANTENNA ANALYSIS BY COMPUTER

With the proliferation of personal computers since the early 1980s, significant strides in computerized

antenna system analysis have been made. It is now possible for the amateur with a relatively inexpensive computer to evaluate even complicated antenna systems. Amateurs can obtain a greater grasp of the operation of antenna systems—a subject that has been a great mystery to many in the past.

The most commonly encountered programs for antenna analysis are those derived from a program developed at US government laboratories called NEC, short for “Numerical Electromagnetics Code.” NEC uses a “Method of Moments” algorithm. The mathematics behind this algorithm are pretty formidable to most hams, but the basic principle is simple. In essence, an antenna is broken down into a number of straight-line wire “segments,” and the field resulting from the RF current in each segment is evaluated by itself and also with respect to other mutually coupled segments. Finally, the field from each contributing segment is vector-summed together to yield the total field, which can be computed for any elevation or azimuth angle desired.

The effects of flat-earth

ground reflections, including the effect of ground conductivity and dielectric constant, may be evaluated as well. In the early 1980s, MININEC was written in BASIC for use on personal computers. Because of limitations in memory and speed typical of personal computers of the time, several simplifying assumptions were necessary in MININEC, which limited potential accuracy. Perhaps the most significant limitation was that “perfect ground” was assumed to be directly under the antenna, even though the radiation pattern in the far field did take into account real ground parameters. This

meant that antennas modelled closer than approximately 0.2 l over ground sometimes gave erroneous impedances and inflated gains, especially for horizontal polarization.

Despite some limitations, MININEC represented a remarkable leap forward in analytical capability. See Roy Lewallen’s “MININEC— the Other Edge of the Sword” in Feb 1991 QST for an excellent treatment on pitfalls when using MININEC.

Because source code was made available when MININEC was released to the public, a number of programmers have produced some very capable versions for the amateur market, many incorporating exciting graphics showing antenna patterns in 2D or 3D. These programs also simplify the creation of models for popular antenna types, and several come with libraries of sample antennas.

By the end of the 1980s, the speed and capabilities of personal computers had advanced to the point where PC versions of NEC became practical, and several versions are now available to amateurs. Like MININEC, NEC is a general-purpose modelling package, and it can be difficult to use and relatively slow in operation for certain specialized antenna forms. Thus, custom software has been created for quick and accurate analysis of specific antenna varieties, mainly Yagi arrays.

The most difficult part of using a NEC-type of modelling program is setting up the antenna’s geometry—you

must condition yourself to think in three-dimensional coordinates. Each end point of a wire is represented by three numbers: an x, y and z coordinate.

Websites -

Hamsoft Website - http://mmhamsoft.amateur-radio.ca/

Here is the antenna we modelled - G5RV JR