HOMEBREW HEXBEAM PAGE
by W1GQL

 

 

 

 

 

 

Modelling a Hexbeam with EZNEC 3.0

     I have been using W7EL's EZNEC antenna modelling software for a number of years and love it. I use version 3.0. My comments here are aimed at that version. Using it I can "play antennas" all though the winter when it is too cold to be in the back yard here in Maine cutting and stringing wires. I was very pleased several years ago when Dr. Cebik, W4RNL, the formost ham radio antenna modelling person in the country included information on my "Gull Wing Extended Double Zepp" antenna on his webpage, saying that I had solved one of the problems inherent in the usual Extended Double Zepp setup. I was also very pleased when a quite involved antenna that I "invented (?)" actually worked exactly as my model had predicted. See my reversible double slot antenna.

      I am just an amateur at this. I have had no formal training in either electronics or computer modelling. But I have spent hundreds if not thousands of hours modelling. If a reader can correct any errors I have made, I am very open to any comments and help. I want to learn more.

      In order to model an antenna like the Hexbeam, one has to first face the difficult task of establishing the coordinates of the various wires. Luckily I discovered that DL7IO had written a Excel/Lotus 123 spreadsheet that did the hard part of that geometry. You enter the frequency you want the dimensions for in cell ______ and read the wire end coordinates in cells _____ through _____. Here are the wires as I put them in EZNEC for the 20 Meter Hexbeam.

     The first thing I do when I have entered new wire coordinates is check the view of the antenna. If you have wires placed wrong it usually is imediately evident looking at the view.

     A Hexbeam model will usually use a "Split Current" source. This is used to place the power source at a point where two wires at an angle to oneanother meet. Your source screen should look something like this. Notice the SI indicating the Split Source. You place the source on one of the wires and EZNEC will automatically split the source and put half on each of the two wires that meet near your source point.

     There are a couple other things that you should always do when you are working on a new model. You should click on the "Outputs" menu option on the main EZNEC screen and then choose "Guidelines Check". This will tell you if your segment lengths are not appropriate. You may have to add or subtract segments in some lines. You should always look at EZNEC's "Average Gain" computation. This routine basically adds up all the gain in all directions that the model puts out and compares it with the power put into the antenna. They should be the same or 1.0. If you have a source misplaced you can get a model that shows a lot more or a lot less gain than is possible. To see the average gain figure you must run a FarField Plot in 3D mode. That is the only time the Average gain figure shows up. Look at the bottom of the main EZNEC screen. To check for problems with the model you should set the Ground Type to Free Space and the Wire Loss to zero before running the FF Plot. I initally had a very low Average Gain and had to reduce the segments in the wires where the source is placed. If you cannot get an Average Gain of close to 1.0 you should read the Help files about mathematical adjustments you must make in the results the program shows.

 


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