Optimized wideband Yagi-Uda antenna for HDTV

In continuation of the antennas for digital TV topic now, dear anonymous, we will take a closer look at the very popular Yagi-Uda antenna. The antenna is quite capricious to the making, as we have already mentioned, but it is so popular with antenna DIYers that we simply cannot ignore this case. On the other hand, unlike the microwave range, in the UHF range, it is quite possible for a DIYer to make an antenna with good gain without tuning tools. Under the cut, two Yagi-Uda designs for HDTV from available materials are presented...

However, before we consider practical designs, an anonymous far from antenna theory needs to clarify several important points for himself:

• Foremost, Yagi-Uda is not just an antenna design, but it is a very large antenna class including a great number of subclasses and countless practical designs. The Yagi-Uda can be both narrowband or broadband, have completely different input impedance and at the same time it contains a different number of elements. Generally, other things being equal, a longer antenna with more elements has more gain.
• Some confuse the Yagi-Uda and the LPDA. Well, what's the difference? Here and there one sees such a "bottle brush on a stick". In fact, the difference is great. A LPDA belongs to a completely different ultra-wideband antenna class, and we have a separate article devoted to it.
• The Yagi-Uda calculator of the DL6WU design presented on our website calculates a narrowband antenna of the Long-Yagi subclass with an input impedance of 200 ohms, which was specially designed for amateur VHF radio communications. Obviously, such an antenna is completely unsuitable for HDTV.
• To receive digital TV we need a broadband Yagi-Uda, which, according to the SWR < 2 criterion, covers as much of the UHF range as possible and has an input impedance of about 300 Ohms. That will allow it to be united with a regnant plate amplifier or wideband 300:75 Ohm balun, as it has done in the BiQuad antenna design for HDTV.
• Old books on television antennas usually reprint dimensions for Yagi-Uda one-channel non-broadband designs. Since it is now difficult to find a region in which only one multiplex translates, such "old-fashioned, time-tested" designs are already of little use for HDTV.

A great number of practical designs can meet the selection criteria given above. Therefore, questions like “Why are there and there sizes different from yours?” will be incorrect, guys. For the same reason, there is no perfect Yagi-Uda design. Each design has its pros and cons compared to other similar ones. Common techniques for expanding the bandwidth of the Yagi-Uda are the use of a double or corner reflector instead of a single one, a double first (or several) directors, a closer location of the first director to the driven element, complicating the design of the dipole, the rejection of linear elements and the transition to more broadband "bowties". It is rather difficult to select the optimal dimensions using a computer, since the more elements an antenna has, the more complex they are structurally, the more degrees of freedom it has. However, the designs proposed below, authored by yurik82, are optimized using N.Mladenov's script for 4NEC2 with subsequent “finishing” in the ANSYS HFSS program (as well as the BiQuad antenna). In addition, in this case, you need to understand that since the antennas are designed for almost the entire UHF range, no “calculator” is needed to calculate, let alone a “formula”, it is enough to make the antenna as close as possible to the drawing with an accuracy of no worse than ±1 mm.

Yagi-Uda for HDTV from galvanized plates on a dielectric boom.

The first design is the cheapest option. It is made of galvanized strips 15 mm wide and 0.5 to 1 mm thick. The strips are placed on a dielectric boom, for example, a wooden one, made of pine bar, treated with an antiseptic and varnish. The design consists of a folded dipole, a dual reflector and ten directors. The antenna is not able to cover the entire UHF range according to the SWR < 2 criterion, so two sizes are offered. The first is designed for the band 470-690 MHz , the second - 500-800 MHz. You need to choose one of the options depending on what frequencies the multiplexes in your area operate on. Below is a drawing of the antenna. The dimensions are summarized in the table.

The input impedance of the antenna is 300 ohms. Gain - from 8.5 dBi in the lower part of the operating range to 14.5 dBi in the upper one. Suppression of the rear lobe of the radiation pattern is not worse than 17 dB. SWR within the operating range does not exceed two. With such an SWR, the antenna is able to work stably in conjunction with a modern low-noise antenna amplifier. In more detail with graphs, the characteristics of the antenna can be viewed at the second link at the end of the article, and at the fifth link a seven-element design with a bandwidth of 470...690 MHz and a gain of 8..12 dBi is presented.

Yagi-Uda for HDTV from tubes on a metal boom.

The second design with a corner reflector is similar to the one shown on the image at the beginning of the article, only with 12 directors. It is somewhat more difficult to make and can be made by more advanced DIYers. The input impedance of the antenna is also 300 ohms and it also works stably in conjunction with the antenna amplifier.

 The passive elements of the antenna are made of duralumin tubes with a diameter of 6 mm, a tube with a diameter of 8 mm is used to make the driven element. Boom and corner reflector are made of duralumin profile 15x15 mm. The antenna elements are not isolated from the boom, while the boom is mounted on a metal grounded mast, which helps to protect against atmospheric static. This is the advantage of this design over the previous one. The dipole is attached to the lower edge of the boom, the directors and reflector elements are pressed inside the boom. The axes of the directors are shifted upward relative to the axis of the boom by 3.5 mm (or 4 mm from the upper edge of the boom to the axis of the director), as a result of which they are actually located inside the boom directly along its upper edge. This is if the antenna is located as in the photo and drawing, of course it can be turned 180° relative to the axis of the boom. The structural dimensions of the antenna can be seen in the following image:

The operating range of the antenna according to the SWR < 1.7 criterion is from 470 MHz to 710 MHz. Antenna gain from 9.5 dBi in the lower part of the operating range to 14.5 dBi in the upper one. Suppression of the rear lobe of the radiation pattern is not worse than 14 dB. Frequency suppression of the VHF range is not worse than 19 dB, FM band suppression is not worse than 38 dB in comparison with an isotropic radiator. For more details, with graphs, antenna characteristics, see the third link at the end of the article.

For those who find it easier to attach the strips than to press in the tubes, a model of this antenna has been developed on a square metal boom 15x15 mm with elements in the form of strips 15x2 mm: https://ypylypenko.livejournal.com/91602.html Dimensions W1..W12 - lengths of directors, X1..X12 - distance from the center of the dipole to the center of the corresponding director. The constructive dimensions of the reflector and the dipole are shown in separate images. The distance between the ends of the folded dipole 24 mm can be safely changed within 16..24 mm, depending on the existing balun or SWA/PAE/ALN amplifier. It is only necessary to maintain the shape of the dipole ends. This is fundamentally important for the antenna matching.

Of course, these are just two examples of wideband Yagi-Uda for UHF. Depending on the reception conditions, you may need both a short three-element Yagi-Uda and a four-element one, etc. The variety of this class of antennas is endless. Links to similar optimized Yagi-Uda antenna designs for digital TV will help you choose the best option.

Полезные ссылки:

1. Yagi Antennas - a large set of optimized yagis with ready-made models for the 4NEC2 simulator. Primary source for this article;
2. 2R10D Uda-Yagi UHF TV @ 470-690 MHz @ 300 Ω - the first antenna made of galvanized strips on a dielectric boom (detailed specifications);
3. 6R12D Beta-19H v3 / Corner Uda-Yagi UHF TV (470-710 MHz) - second antenna made of tubes on a metal boom (detailed characteristics);
4. 1R1D (3-EL) Uda-Yagi UHF TV - short broadband three and four element yagis optimized for the UHF range, with a gain of about 6 and 8 dBi, respectively;
5. 1R5D Uda-Yagi UHF TV 470-690 MHz @ 300 Ohm - a seven-element Yagi-Uda for HDTV made of galvanized strips on a dielectric boom with a gain of 8..12 dBi;
6. Sofware YagiCAD from VK3DIP - good program for generating Yagi-Uda models with model export for 4NEC2;