The Nashua Area Radio Society has been using a 3-element wire beam antenna for Field Day for the last several years. The antenna uses three guyed 50 ft. fiberglass masts from Max-Gain Systems. The antenna uses three inverted-V style elements separated by a little over 50 ft. Since we are in the northeastern United States, we can point the antenna on a fixed, 260° heading and it covers the entire U.S. well.
Our 40m V-Beam antenna was initially designed in EZNEC 5.0. It was manually optimized for decent gain and front to back performance and it worked quite well. Recently, we decided to try automatic optimization software on the antenna as part of a tune-up on the design for Field Day 2018. After looking around on the Internet a bit, we discovered a software package called AutoEZ which looked ideal for my project.
- Ability to use formulas and variables to construct and modify EZNEC wire models
- A Test Case generator and execution environment to vary model parameters and graph the results calculations run by EZNEC
- An automatic optimizer that can adjust the parameters of an antenna model in EZNEC to optimally meet a defined set of performance goals
- Harnesses the full power of Microsoft Excel formulas for trigonometry, logical, and other mathematical capabilities for use in EZNEC modeling
The first step in the project was to rebuild the EZNEC model that I already had for our 40m V-Beam antenna in AutoEZ. I began by defining several AutoEZ Variables and Excel Formulas in the AutoEZ Variables Tab that enabled me to easily modify the design of the antenna and to optimize it. Some of the basic variables included the target design frequency for the antenna, the height and separation of the antenna elements, the distance to the element anchor points, and the length of the element wires.
The image above shows the model variables in “Formula View”. You can see some of the math and trig functions that were used to compute values for some of the variables. AutoEZ can only optimize variables that do not contain formulas so I was careful to ensure that the base separation between the elements and the length of the element wires were constants as these are the parameters that I wanted to optimize later.
Excel Trig formulas and the Variables were used on the Wires Tab to determine the coordinates of the wires in the antennas. There are a total of 7 wires in the model. Six are the two ends of the three inverted-V elements. The Seventh wire is a short 4″ section in the middle of the Driven Element to allow a current source to be inserted to drive the antenna there. I was careful to create an accurate model of the wire gauge, insulation, and loss that we are using for our V-beam
With the model built, it was back to the Variables Tab to select the parameters to be optimized. Optimization is best done as a multi-pass process and I did this in two steps. The first set of runs included optimization of both the element spacings and their lengths. This led me to conclude that the mast spacing of 27 ft (Driven to Director) and 28 ft (Driven to Reflector) were the best choices. I then set these as fixed values in the variables tab and ran the optimizer a second time with some starting element lengths to optimize the element lengths by themselves.
One must create a set of frequencies and objectives for the optimizer before running it. This is done in the AutoEZ Optimize Tab. The antenna is being used for SSB on 40m so I choose a range of frequencies that covered the SSB sub-band on 40m. Note that I weighted the center frequency heavier than the edges by including it more times in the optimizer’s list. The use of the Optimization Objectives and their associated weights and values are well covered in the AutoEZ documentation so I won’t cover them in detail here. The parameters above were chosen to create a reasonable balance between SWR values across the 40m SSB sub-band, good Front/Back and Front/Side performance from azimuth values ranging from 60º to 300º, and a reasonable amount of forward gain for a 3 element antenna of this type.
I expected that the final impedance of the antenna would be a typical value for a yagi in the 20 to 30 ohm range. Thus, I set the SWR calculations based upon a 25 ohm target impedance. More on the matching of the resulting design later…
It took several runs of the optimizer with different sets of Optimizer Objectives to get the final results I was looking for. The Optimizer tried 130 combinations of element lengths to arrive at the final lengths shown above. Note the improvements in SWR (1.6 -> 1.04), Forward Gain (+1 dB), Front/Back (+4.2 dB) and Front/Rear (actually Front/Side) performance that the Optimizer was able to achieve over my manual, trial and error optimization.
Next, I rounded the optimized element lengths and plugged them into the Variables Tab.
The image above shows the optimized Azimuth pattern for the antenna as generated by AutoEZ and EZNEC. A very clean result!
And here’s the optimized Elevation pattern near the center of the SSB sub-band. This antenna is a little low for 40m but the resulting maximum gain at a 35º angle should work well for US contacts during Field Day.
The final step in the optimization process was to calculate a full set of performance calculations for the antenna using the Calculate Tab. AutoEZ makes it very easy to generate a set of Test Cases for incremental frequencies in the SSB sub-band on 40m. Note the setting of the Elevation angle of 35º to match the maximum gain angle for the optimized antenna. Also, note the parameter settings for Ground Type and Characteristics. I set these to model the less than ideal soil conditions that we have here in New England.
AutoEZ provides several nice graphical capabilities via the Patterns, Triple, Smith, Custom and Currents Tabs. I used some of them to plot the data from the performance calculations. These graphs help to visualize the results of the optimization to verify that the design objectives for the antenna have been met.
I am using a 1:2 matching Balun from Balun Designs at the feed point of the antenna to transform the antennas final 25 ohm feed point impedance to 50 ohm to match our coax feed line.
We are looking forward to using the optimized version of our 40m V-Beam at Field Day 2018. It took me a couple of days of time to read all of the AutoEZ documentation and learn to use the excellent tools it provides. I don’t think I will build another EZNEC antenna model without using AutoEZ. Even without the optimization features, AutoEZ makes the construction and modification of an antenna model in EZNEC far easier than it would be using EZNEC alone. I hope that you’ll give AutoEZ a try for your next antenna design project.