Our HAM Station

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A 40m High Performance Antenna for Field Day

Posted: 25 May 2018 04:28 PM PDT
https://stationproject.blog/2018/05/...for-field-day/

40m V-Beam Wire Antenna at Field Day

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.Â*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.
40m V-Beam EZNEC Antenna View

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.
AutoEZ Antenna Modeling and Optimization Software Wires Tab

AutoEZ is a collection of Visual Basic code and Microsoft Excel macros that
act as a front-end to EZNEC 5.0 and later. AutoEZ provides a number of very
useful enhancements to EZNEC including:

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

AutoEZ Antenna Model Variables

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 FormulasÂ*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.
AutoEZ Antenna Model Variables (Formula View)

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.
AutoEZ Antenna Modeling and Optimization Software Wires Tab

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
Model Variables to be Optimized

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.
Optimizer Objectives

One must create a set of frequencies and objectives for the optimizer
before running it. This is done in the AutoEZÂ*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 optimizers list. The use
of the Optimization Objectives and their associated weights and values are
well covered in the AutoEZ documentation so I wont 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 ofÂ*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
Element Optimization Results

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.
Post-Optimization Model Variables

Next, I rounded the optimized element lengths and plugged them into the
Variables Tab.
Post Optimization Azimuth Pattern

The image above shows the optimized Azimuth pattern for the antenna as
generated by AutoEZ and EZNEC. A very clean result!
Post-Optimization Elevation Pattern

And heres 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.
Calculated Performance for the Optimized Antenna

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.
Performance Plots for the Optimized Antenna

AutoEZ provides several nice graphical capabilities via the Patterns,
Triple, Smith, Custom and Currents Tabs.Â* 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.
1:2 Matching Balun (25 ohm to 50 ohm)

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.
40m V-Beam Antenna

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
dont 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 youll give AutoEZ a try for your next antenna
design project.

Fred, AB1OC