Log-periodic antenna (LPDA) for On Air TV

Regarding the topic of digital television reception, we cannot ignore the popular design—a log-periodic antenna. Many anonyms often confuse the log-periodic antenna and the Yagi-Uda antenna. Outwardly, they are quite similar, especially from afar. Such a “cleaning brush on a stick.” However, if you look closer, it turns out that the brush on the log-periodic antenna is located on two sticks, and in Yagi-Uda - on one. It's not just some external difference. The log-periodic antenna, according to the principle of operation, is fundamentally different from the Yagi-Uda and belongs to a completely different class of ultra-wideband antennas.

The antenna was invented and patented in 1952 by American engineer John Dunlavy. In connection with the spread at that time in the United States of color television with taking of the UHF range, the log-periodic antenna quickly occupied a solid market share in the sales of television antennas and still occupies it. It should be noted that the well-known log-periodic dipole array (LPDA), which we view in this article, is only one of the possible options for a log-periodic antenna along with a tooth, zigzag, etc. The fact is that the log-periodic structure is a special case of a large class of ultra-wideband antennas, and can take on completely different forms. This class of antennas was most fully investigated and described by the Japanese scientist Yasuto Mushiake. He showed that such a structure must comply with the principle of self-complementation and have a characteristic impedance Z = 0.5 * Zo, where Zo = 376.73Ω is the free space characteristic impedance—a concept that is a fundamental physical constant that reflects the properties of a photon but not of the vacuum and not of the ether! The LPDA we are viewing here, unlike Yagi-Uda, does not contain passive elements—a reflector and directors. All dipoles on the “cleaning brush” are active elements within the operating frequency band.

Of course, the real design is not equivalent to the ideal theoretical model, but we will not continue to bore you with history and theory, but will immediately get down to business. You can calculate the LPDA antenna using our online calculator and for this you do not need knowledge of MathCAD and C ++ at all, in the way as some "creative" copywriters intimidate an anonymous. But still, with such a calculation, the anonym may be in a dead end when choosing the values of τ and σ. Therefore, in this article we lay out a ready-made optimized design from available materials that can be found in hardware stores. This is a 15-element log-periodic UHF antenna for On Air TV. The input impedance of the antenna is 75 ohms, the gain over the entire UHF range is about 11 dBi, the SWR is not more than 1.25, the suppression of the rear lobe of the pattern is not worse than 14 dB. A duralumin square profile 15x15 mm is used as halves of boom, the elements are made of aluminum strips 15x2 mm. Antenna scheme (click to enlarge):

Elements consist of two halves. Each half is attached to "its own" part of the boom, upper or lower, with alternation. The drawing shows the dimensions of the dipoles “from end to end”, and in brackets the size of the halves, taking into account the overlap on the traverse (“under the cut”). Every DIY-er knows that if you measure the distances from one to the next element in turn, then the errors are summed up. To avoid this, measurements should be made along the boom from one control point. It is marked as “0” on the diagram, and distances to each element go from it. In parentheses for control are the distances to the previous antenna element. The boom halves are spaced 9 mm apart by three to four plastic spacers. Specifically, this design does not have a short-circuited jumper behind the first element at a distance of λmax / 8, as in the online calculator design. However, a DC short circuit between boom halves (as well as grounding the entire antenna through a metal mast) is essential to protect against static. This can be done by taking the antenna attachment point to the mast with a metal clamp at a distance of 144 mm (λmax / 4). In this case, this short circuit is transformed to the first element into infinite impedance. To match, the feeder is laid inside one of the traverses and connected by soldering through the petals near the last, shortest element, the braid to the traverse through which the feeder is laid, the central conductor to the opposite one. It is desirable to isolate this place from atmospheric precipitation in a plastic box, as in the photo in the header of the article. A barrel-shaped feed-through antenna amplifier can be used with the antenna, which should be located on the mast as close as possible to the feeder inlet to the antenna traverse.

Calculations show that a simple replacement of strip elements with tubes has practically no effect on the characteristics of the antenna. Therefore, in this design, you can safely replace the strips with tubes with a diameter of up to 8 mm, pressed into the center of the corresponding boom half. You only need to save the positions of the elements and the total length of each element. LPDA ultra-wideband antenna allows such liberties.

The antenna was optimized using N. Mladenov's script and recalculated in the HFSS program. The characteristics of the antenna are summarized below (click on the image to enlarge):

Input impedance	VSWR	Gain
Front/back ratio	Radiation Pattern	Aperture efficient

In conclusion, friends, let's compare the log-periodic antenna and Yagi-Uda. They are similar in appearance, as we noted at the beginning of the article. So, which one to choose for a DIY-er to make with your own hands? Log-periodic dipole array antenna has the following advantages:

1. Ultra-wideband. Covering the entire range of UHF, and with a margin, is not difficult for LPDA. Moreover, given its shortcomings, which will be discussed below, it does not make sense to use a log-periodic antenna in a narrower band. Google for queries like ”log-periodic antenna at a frequency of 544 MHz”, as some do, or to calculate the antenna for a narrow range, it does not make sense.
2. The constancy of characteristics in the working band. The input impedance, gain, and shape of the radiation pattern change very little in the operating frequency band. Therefore, a log-periodic antenna can also be used as an exemplary measuring one.
3. The high repeatability of the design is based on the previous advantages. A carefully crafted log-periodic antenna has a 99% chance of working as it should. Yagi-Uda is more capricious in this respect.

But there are also disadvantages, as without them:

1. A log-periodic antenna with the same boom length tends to have lower gain than the Yagi-Uda. And given that the boom is double, the “iron consumption per one decibel of gain” for a log-periodic antenna is much higher.
2. The real efficiency of a log-periodic antenna decreases with increasing frequency. This is due to the fact that the working active area of the log-periodic antenna is not constant, as in the Yagi-Uda, but shifts with increasing frequency towards shorter dipoles. As a result, the aperture efficiency decreases with increasing frequency. The aperture efficiency characterizes the amount of energy received by the antenna. Obviously, with a decrease in this area, this energy decreases and, accordingly, the signal level at the output of the log-periodic antenna decreases with increasing frequency.
3. More complex design. It is necessary to accurately maintain the spacing and isolation between the boom halves and at the same time make the antenna strong enough, which is quite a challenge.
4. A small gap between the boom halves can collect dust, dirt and be affected by atmospheric precipitation. This can lead to poor performance and even complete failure of the antenna in heavy rain and sleet.

As you can see, the log-periodic antenna has even more disadvantages than advantages, and it loses the competition with Yagi-Uda on points. Making an antenna with an input impedance close to the theoretical optimum of 0.5*Zo = 188.5 Ohm can only help eliminate its last drawback. However, the resulting difficulties in matching with the feeder and the disadvantages inherent in such matching schemes can negate all the advantages of this antenna. As a result, we recommend a log-periodic antenna only if your HDTV multiplexes are located at different ends of the UHF band, or if you doubt your DIY skills and need a reliable result. Otherwise, Yagi-Uda is preferred. In the case of buying a finished industrial antenna on the market, not everything is so simple. The capricious Yagi requires not only careful calculation and optimization, but in mass production it often needs measurements and fine-tuning already in hardware. This is a rather costly procedure that many manufacturers neglect. As a result, often-dense market Yagis have no advantages over similar log-periodic ones.

Related links:

• EU 15-Element FLAT LPDA OPT - prototype of the antenna described in this article;
• Log-periodic antenna - Wikipedia;
• Antenna Yagi-Uda for digital television;