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How to Synthesize and Build a Custom HDTV Antenna

Parent Category: 2018 HFE

By Derek Linden

Overview: Antenna Design Process

The growing demand for wireless connectivity requires antenna solutions that are optimized for system performance, cost, and size. In addition, with the expected demand for design experience greatly exceeding the current supply of antenna engineers, an antenna synthesis approach that can create automated antenna designs in less time is very beneficial.

In the traditional antenna design process, the engineer usually starts with books or models, deciding which antennas to try or not to try, and then goes into parameter sweeping and tuning to optimize each chosen design type for the application of interest. This is a very time intensive and laborious process.

With sufficient expertise in optimization, electromagnetic simulation, and antennas, it is possible to combine that knowledge and simulation/optimization capability into a software solution to synthesize antennas. This capability would expedite antenna design exploration, especially for difficult specifications and/or novel antenna topologies, because there are many variables to adjust in pursuit of optimum antenna performance, size and configuration.

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Figure 1 • Evolution of antenna design process: traditional, semi-automated, and, finally, fully-automated design process.

NI AWR software offers AntSyn™antenna design and synthesis software that automates the process of designing and synthesizing antennas. Users do not have to know programming or optimization to use this tool and are able to make progress even with limited antenna knowledge. Figure 1 shows the evolution of antenna design processes from traditional, to limited synthesis, to the AntSyn system.

This application example demonstrates how to use AntSyn to design and build a customized high-definition TV (HDTV) antenna by generating a set of antenna specifications such as frequencies, bandwidth, and gain, for planar antennas such as Yagi and ultra-wideband dipoles.

HDTV Overview

Most antennas in electronics products are now fully integrated within the application and not user accessible. HDTV is one of the few applications where consumers still have access to antenna components because televisions still have antenna connections to receive over-the-air signals.

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Figure 2 • Highest gain and bandwidth, with this size covering the full UHF band (470-890 MHz) limits gain to 4-6 dB.

There are many kinds of HDTV antennas on the market. Worldwide channels include very-high frequency (VHF) (as low as 43 MHz) and ultra-high frequency (UHF). This particular application example is limited to the UHF band due to size limitations that were imposed for easy construction, but it is possible to expand this concept to VHF if one is willing to build larger antennas. Even with the size and shape limitations of this example it is still possible to cover the entire worldwide UHF band (470-890 MHz) with a single antenna that is smaller than a sheet of U.S. letter paper and has a gain of 4-6 dB, as shown in Figure 2 and described in more detail later.

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Figure 3 • The two basic choices for HDTV antennas are omni antennas for use near many stations and directional antennas for use where stations are more distant, and the antenna can be oriented.

There are two basic choices in HDTV antennas: omnidirectional antennas that are appropriate if there are many stations nearby such as in a city, and directional antennas with more gain that are potentially better if the stations are further apart and the antenna can be oriented to point at the origin of the signal (Figure 3). Frequencies covered would generally be the same for each type.

Setting Specifications

AntSyn enables the user to set a very wide range of specifications in a form called the “SpecSheet.” Users can set their own specifications or use a standard example provided by AntSyn as a starting point. If a new project is needed, click the button on the top left of the page labeled “new project” and a new folder will be added to the project tree. To start a new SpecSheet, simply click on the “Create SpeSheet” button that appears when a project folder is selected, as shown in Figure 4.

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Figure 4 • Buttons to creating a new project and specsheet.

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Figure 5 • Duplicating an example specsheet.

To use an example SpecSheet, right click on the specification of interest and select “duplicate.” Follow the prompts to put the spec into the right project folder (Figure 5).

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Figure 6 • Setting band and frequency specs.

The SpecSheet is designed so that the user can usually start at the top of the form and work down the sheet to the bottom. The first specs to be chosen are the bands and frequencies, which are fundamental to almost every antenna, shown in Figure 6.

In this case, a start frequency of 470 MHz and stop frequency of 890 MHz covers the complete range of UHF HDTV frequencies worldwide, including the U.S. (470-698 MHz), Canada (470-806 MHz), Korea/Taiwan/Japan (470-890 MHz), UK/Ireland/Hong Kong (470-862 MHz), Western Europe/most of Asia and Africa (470-861 MHz), France/Eastern Europe/Former Soviet Union (470-862 MHz), and Australia (527-820 MHz).

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Figure 7 • Setting impedance match.

The specification for the impedance match is next. Voltage standing-wave ratio (VSWR) or S11 can be set (among other things), along with the desired impedance. The impedance of a standard TV cable is 75 Ohms, so that is what has been set in Figure 7.

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Figure 8 • Setting gain spec for directional case – skipped for omni case.

The next antenna property to address is gain. For the omnidirectional case, the gain does not need to be set because the antennas to be selected are omni by nature and fairly small. By not having a gain specification, the antenna will run a bit faster. If this assumption turned out to be incorrect, or if some other error were made to the SpecSheet, the user would simply need to duplicate the specification from the right-click menu, add a gain spec, and re-run the antenna.

With the directional case there are some gain requirements built into the SpecSheet (Figure 8). None of the polarization or other advanced features for gain specification are being used this example and the gain is actually specified for the downward direction, this is explained in the results section.

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Figure 9 • Setting size limits.

One of the key areas is the shape or overall limit on the form factor. To make these antennas easy to construct, they were all limited to 8” (203 mm) in width and 10.5” (267 mm) in height, with a planar form factor. This limit was chosen because the design was printed to a standard U.S. letter size piece of paper to be used as a template to cut out the antenna on a single-sided foil-clad board.

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Figure 10 • Antennas used for this example: (a) standard UWB dipole, (b) UWB edge-fed dipole, (c) UWB coplanar monopole, (d) planar Yagi with variable-width strips.

Selecting Antenna Types

There are 240 antenna types currently available in the AntSyn library as a starting point for the synthesis/optimizer, and if the user does not select any, AntSyn will select antennas automatically based on what could work with the given specifications. In this example, four different types of antennas were selected: three versions of a planar ultra-wideband (UWB) omni antenna and a directional Yagi antenna (Figure 10).

These antennas are described in more detail in the Results section below.

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Figure 11 • SpecSheet run in progress.

Running AntSyn

Once the specs have been added as desired, the quality level and run are set. For this example, omni antennas run best at medium quality and should finish in less than five minutes. Directional antennas take much longer to run (low quality about 15 minutes, medium quality about 40 minutes, high quality 3-4.5 hours). Medium quality has been selected here because it only takes 1-10 minutes to complete. If it is desired to see the progress of a run while it is ongoing, keep clicking on the antenna name and the latest results will be shown (Figure 11).

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Figure 12 • Standard UWB dipole results.

Results

The results for the standard UWB omnidirectional antenna, which has a pattern that looks omnidirectional across the entire frequency range, are shown in Figure 12. This antenna would be used if the antenna is near strong TV stations and forward gain is not needed to pick up the signals.

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Figure 13 • UWB Edge-fed dipole results.

Figure 13 shows an antenna that is fed on its edge, which has the interesting characteristic of not requiring the feed cable to cross the antenna; it can just be put on the edge of the card to save space.

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Figure 14 • UWB coplanar monopole results.

Figure 14 shows a UWB coplanar monopole antenna with a small side and a large side. It is still an omnidirectional pattern, regardless of the frequency. The gain changes a bit with frequency, but it is still basically omnidirectional. This could be useful if RF equipment such as an amplifier would be integrated on the large side as it provides a larger surface for mounting.

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(a)

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Figure 15 • Directional Yagi antenna (a) AntSyn results, (b) photo of a prototype.

The result for the directional antenna optimized under these size constraints was shown in Figure 2 above, and is repeated in Figure 15 (a), along with a photo of a prototype in Figure 15 (b).

This antenna has gain that is pointed backwards compared to a typical Yagi. The antenna Z-axis is actually turned upside down and pointed toward the bottom of the diagrams for the antenna rendering and the patterns. It was noted during synthesis that higher gains were resulting toward the -Z direction, so the spec was changed to take advantage of this behavior. This shows the ability of AntSyn to discover new ways of configuring typical antennas to achieve unusual results.

A typical Yagi antenna, starting at the origin and working along the +Z axis, has a single reflector element, then the driven element, then a number of director elements that help the energy to flow from the driven element in the +Z direction. It is also usually narrowband and would not be able to cover the full UHF band. AntSyn’s result is quite unusual in how it solves the requested specifications. AntSyn turned the reflector into a so-called sleeve element for the fed dipole, which extended the frequency range to the full 470-900 MHz. It also turned the director elements into a reflector element to increase gain. The result is that more gain is pointed in one direction, which is what we wanted, and the antenna works across the full UHF band.

Build and Test

When the optimization run is finished, the new antenna design is ready to build. To do so, the user can download the drawing (DXF file) using the DXF button at the bottom of the results page and print it to scale using a program like AutoCAD’s DWG TrueView. With a printout of the design, the following steps can be followed to manufacture a prototype for testing:

  • Make a foil-covered board. Use spray adhesive to make the surface of a foam or coroplast board sticky and unroll aluminum foil across the board. Cut off the excess foil and smooth it to the board using a plastic paint or putty spreader.
  • Tape the printed page to a foil-covered board.
  • Use a sharp utility knife to cut out the pattern. Only cut the paper and foil, do not cut too deeply into the board which will hold the antenna together.
  • Carefully peel off the excess foil.
  • Bring the panel to use to drill the holes in the board for the cable.
  • Screw down the leads from a standard TV coaxial cable, with the center conductor on one side of the feed point and the outer jacket on the other side. Note that screws or conductive metal tape will be needed to attach the leads, as solder does not work on aluminum foil.
  • Cover with a clear plastic sheet to protect the foil, if desired, such as a transparency slide.
  • Write any information necessary on the antenna (such as frequency, gain).
  • Test using, for instance, a network analyzer, HDTV receiver, or spectrum analyzer.

Conclusion

The AntSyn antenna synthesis tool combines a proprietary genetic algorithm with electromagnetic (EM) simulation and antenna design expertise- defined performance requirements, size, and material constraints. In this simple HDTV antenna example, AntSyn synthesized a unique Yagi type antenna that was easily fabricated and shown to provide the required performance in the constrained form factor.

A related video demonstrating this design example can be found at: https://vimeo.com/230666087.

About the Author

Dr. Derek S. Linden is Director of Technology at AWR Group, NI. He earned his BS degree from the U.S. Air Force Academy and his MS and PhD degrees in electrical engineering from the Massachusetts Institute of Technology. He has performed groundbreaking research in the automated design of antennas using evolutionary optimization since 1995 and is co-inventor of the patented genetic antenna design process.

November 2018

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Insulated Wire Inc. is pleased to announce that following the retirement of a French delegate, IW Microwave Products Division President, John Morelli, has been appointed as the Secretary for IEC/SC46F, a subcommittee of IEC/TC46. IEC/SC46F is responsible for RF and Microwave Connectors and Components International Standardization.

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