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Understanding, designing, constructing and using radio antennas
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25 Apr 07 Directory of Ham and Amateur Radio Blogs

I’ve just found a directory you might find useful, especially if you have an amateur radio blog of your own. It’s Ham Radio Blogs.

It doesn’t have a lot of entries yet, so you can get in early.

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22 Mar 07 Modelling an Antenna with EZNEC

EZNEC is a powerful, low-cost, antenna modelling program for Windows developed Roy Lewallyn – W7EL. It is based on the powerful NEC-2 (which stands for Numerical Electromagnetics Code) software for modelling antennas and other electromagnetic structures using the Method of Moments. NEC was developed at Lawrence Livermore Laboratories.

EZNEC is easy to use, and is particularly effective for investigating the characteristics and performance of a design before construction, and for especially for trying out new ideas. The version I am using here is EZNEC+ version 4.0. The models will also work fine in EZNEC version 3.0 which I have used for many years.

The operations centre for EZNEC is the following window:

Main EZNEC Window

Main EZNEC Window

From here you can carry out everything needed for a simulation. The model used here as an example is for a simple 20m 3 element yagi.

The first step is to set the frequency you want the similation to run at by clicking on ‘Frequency’. It has been set here for 14.175 MHz, the centre of the 20m band.

The next step is to insert descriptions of the elements. This is done in the Wires window, accessed by clicking on ‘Wires’ on the main window. Here is the data for the yagi:

Wires for a 20m 3 element yagi

Wires for a 20m 3 element yagi

Click on the above image for a larger, more readable version in another window. We will look at the significance of some of the numbers in this table later.

Once you have the antenna data entered you can view the antenna by clicking ‘View Ant’. You will see something like this:

View of the 20m 3 element yagi

View of the 20m 3 element yagi

Here you can see the ‘wire’ elements (tubes in reality, but modelled as thick wires here), and the current in each wire with its phase. Depending on the ‘View’ settings you might see other information, such as the wire numbers:

Antenna view with wire numbers

Antenna view with wire numbers

The antenna will need feeding with power from a feedline, and this can be simulated by placing a source in the centre of the driven element, using the ‘Sources’ window:

Feed source for the antenna

Feed source for the antenna

Once you are satisfied that the data has been entered correctly and the antenna looks something like it should, you can begin checking its performance. For example, one of the things of great interest will be its match to the impedance of the feedline across the frequency range to be used. For this you can click on SWR. A window will come up allowing you to enter the lower and upper frequency of the band of interest, and the step size across the band. Then when you run the simulation you will obtain something like:

SWR of the antenna

SWR of the antenna

You can see here that this antenna presents a pretty fair match to 50 ohms right across the 20m band.

The other features of interest are the directivity charactersitics of the antenna, such as gain, front-to-back ratio, and beam width. These can be seen by running an ‘FF Plot” (Far Field Plot).  With the ‘Ground Type’ set to ‘Real/High Accuracy’ and the ‘Plot Type’ set to ‘Elevation’ the following plot is obtained:

Elevation plot of the antenna

Elevation plot of the antenna

You can see here that maximum gain is obtained with an elevation angle of 27 degrees. So, with  the Plot Type’ set to ‘Azimuth’ and the elevation angle set to ‘27 Degrees’ we obtain:

Azimuth plot of the antenna

Azimuth plot of the antenna

The gain turns out to be 11.05 dBi at an azimuth angle of 27 degrees. Front/Back Ratio is 19.79 dB and Beam Width is 72 degrees. This is quite a respectible performance for such an antenna.

It is possible to get a 3D plot to help us visualise the antenna’s performance:

3D plot for the antenna

3D plot for the antenna

Once we have tested the antenna we can begin to experiment. For example, we can easily change its height and check the effect on gain and radiation angle. Or we can vary the element lengths and spacings to adjust gain, front/back ratio, srw and swr bandwidth. We can put in alternative source impedances to see if this gives us a better match. We could even add another element to see what effect this has, and so on.

And all of this is possible, to a surprising degree of reliability, without once ruining a length of aluminium tube or risking the tower falling onto the neighbour’s house.

In future posts we will explore further aspects of simulating antennas, investigate some of the standard antenna designs, and then look at producing some new ideas of our own.

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