de NØBEL, Kevin

As part of the 160m sloper project, I needed to decide on the antenna modeling software that I wanted to use.   In the past I've used EZNEC, and ELNEC before that having purchased that software from W7EL, and have always been very happy with the results.  My interest in antenna modeling has always been casual, needing it when an upgrade to my antenna system was needed.

As its been 7 years since I've done so, I surveyed the available software again and found EZNEC 5.0 and 4NEC2 as the major players in terms of up to date software.

Finding that my previous version EZNEC 3.0 would not install on Windows 7, I downloaded the 5.0 demo, and started working with it.   It seemed to be pretty much the same software, as I remembered, with various plots and 3D visualizations available.    The spreadsheet-like model entry method was still there, which always seemed to me to simply be the "card" format that NEC2 (the underlying fortran based modeling engine, which underpins 99% of all modeling software).

There may be more features in this version than before, but since my use is fairly casual, I really didn't notice anything.   The version upgrade numbers may simply be to handle newer versions of windows.

Then I downloaded 4NEC2. http://www.qsl.net/4nec2/  I spent about 3 hours going through the learning curve with this software, understanding its UI, and its features, as well as trying some of their example antenna models.  Seeing the various outputs, and visualizations, I found nothing missing that I'd need. Soon into it I discovered you have your choice of 4 methods of model entry (Edit the file with notepad NEC2 "card" format, two different spreadsheet style entry methods, and a3D cad-like entry method)

The 3d cad-like entry method they call "Geometry Edit" is by far the easiest method.   It lets you draw your wires in a 3d perspective view, or the common 3 orthogonal views.  It gives the options to enter coordinate values as well.   And a particularly useful function is the ability to enter the length, of a wire after you've drawn it, and it will calculate the endpoints for you.   It allows for easy hand optimization.   After seeing this, I decided 4NEC2 was my new antenna modeling software of choice.

As can be seen in the picture, I modeled my current 80-10m off center fed dipole, just to prove I was familiar enough to move on to the new design.  This is an example of the 3d viewer that is included in the package.  Its not possible to edit in this viewer format, and the wire sizes are exaggerated so they are visible (this can be toggled off).  Antenna currents and patterns can be superimposed over this view as well.   It is a great way to get a final visualization of the antenna.

People will wonder why I put the tower in the model, and my rain gutter and satellite antennas.   This is mainly historical.   When I modeled my old multi-band trap dipole, 15 years ago or so, was closer to the rain gutters and therefore was influenced by it.  When dealing with multi-band wire antennas, every little thing can move the resonance on one band, to the detriment of another.   I found that by including the tower and the rain gutter, I could get my wire lengths and trap positions very close to real world values.  This lead to less need for physical optimization later.

Others may wonder why the antenna goes off at weird angles and slopes.   These are the constraints of where the evergreen tree is (the shorter length of the OCF) that supports it, and the fence at the corner of my yard (for the longer length) which supports that end.   The house (which is in the middle of this all) and these other items are not on the model, although it might be nice to figure out how to incorporate them for visualization purposes at least.

Many plotting options are available after "calculating" a model,  I'll show two of them here.

The SWR Plot seems to be fixed at 50ohm.  Since this antenna is designed to have a 4:1 balun at its feed point, it means that a 4:1 SWR is actually the target.   Various dips even down to 2:1 show up in the plot, corresponding the the bands its designed for.   Notably 15m has a poor showing.   And this is true of this design.   However, in practice shifting the center will slightly mismatch the other bands, and bring 15 into closer match.  When actually building the antenna and doing the final tuning with the antenna analyse, I did this.   It allowed the tuner in the radio to be able to tune 15m with no issues, other than it is possibly not too efficient.

The Impedance plot is quite informative too.   Z crosses the 200 ohm line corresponding to the various resonance points near where the phase angle is close to zero.   Phase is showing the inductive or capacititive reactance at that point in the plot.   Of course having the L or C near zero is what you'd want to get a nearly resistive load. 

This image shows the Geometry Edit part of the software, which as I said is the easiest method of entering the model.  Its a smile wire frame representation, but its very effective in creating the geometry of the wires in question.

4NEC2 is the obvious choice for the casual antenna designer.   The 3D entry method, combined with a flexible set of output formats and visualizations, really makes it easy to use.

Next I'll be moving on to create the new 160m antenna, which is the whole point of this exercise.