Electrically shortened loop antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

Comments on the article

In a number of practical cases, the dimensions of the antenna, commensurate with the wavelength, are unacceptably large. Therefore, electrically shortened (small) antennas were developed. It is also important that the antenna is the only element that emits or receives electromagnetic energy. One of the variants of such antennas, an electrically shortened loop antenna, is discussed in the article. The experiments were carried out at frequencies of 14, 27, and 430 MHz.

A loop antenna has the best properties when it covers the largest area for a given perimeter, i.e., it is a round frame. But for the convenience of consideration, let us turn to the square frame and the vertical polarization of the emitted (received) waves (Fig. 1a).

The directivity coefficient (DAC) of a square frame with a perimeter e = λ (λ is the wavelength) in free space compared to a half-wave dipole is 1,35 dB with a resistance of rΣ = 100 Ohm [1].

A vertically polarized square loop can be thought of as consisting of two half-wave vertical dipoles loaded at the ends (capacitive loading) and separated by a quarter wavelength. There is no up and down radiation, since the currents in the horizontal conductors of the frame cancel each other out. The distribution of current I and voltage U along the frame conductor is shown in fig. 1b. Points A and C, current antinodes, are points of zero potential. When the antenna is connected to the gap at point A, one dipole is supplied with current, and the other - at points B and D - with antiphase voltages. The radiation patterns of the frame located in free space, in the vertical and horizontal planes, are shown in fig. 1 c, d and are close in shape to the radiation pattern of a half-wave dipole [2].

The electrical shortening of the loop antenna can be carried out by increasing the capacitive load of the dipoles by turning on the voltage antinodes, i.e., at points B and D, capacitances, which additionally changes the phase of the voltage at these points by 180 ° (Fig. 2, a). Let's call this antenna a shortened loop antenna of two half-frames.

The current in the frame circuit now flows in only one direction (Fig. 2, b), i.e., the currents in the opposite conductors of the frame turn out to be oppositely directed relative to each other. This means that there is no radiation in the direction perpendicular to the plane of the frame, i.e., the radiation pattern is approximately a circle in the vertical plane and a "figure eight" in the horizontal (Fig. 2, c, d). If we consider two half-wave dipoles with oppositely directed equal currents as an analogue of this antenna, then the gain in the horizontal plane relative to the circular radiation pattern is 3,8 dB [1].

Thus, in its properties, this antenna approaches a magnetic antenna - an antenna with a constant current distribution along the loop contour, and radiating mainly the magnetic component of the electromagnetic field in the near zone.

The antenna under consideration has four points of zero potential: A, C - antinodes of the current and B, D - midpoints of the distances between the plates of the capacitors - voltage nodes. Accordingly, the antenna can be powered near these points. The easiest way is to use a T-shaped matching when powered in the current antinode (Fig. 3, a) [2] or a capacitive voltage divider when connected to a voltage node (Fig. 3, b) [1].

Approximate parameters of the connecting circuit can be determined from the following considerations.

• Initially, the frame of the selected perimeter e < λ is tuned to resonance at the operating frequency f using two identical capacitors with a capacitance of 2C, and the quality factor of the unloaded frame Q is measured.
• The wave impedance of the loop antenna as an LC circuit is PA=1/(2πfC).
• The coefficient of inclusion of a power line with a wave impedance rl into the antenna circuit, based on the condition that, upon matching, the quality factor of the circuit is halved, k = (PAQ / Рl) 1/2.
• The distance between the connection points of the T-shaped matching et= e/k.
• Elements of the capacitive divider Cn = kC, C' = 4C·k / (k -2).
• If you connect the power line to the gap in the frame at point A, then the input resistance rin = Pa / Q.
• Radiation resistance of the frame [3] rΣ = 80π2(2πSp/λ2)2, where Sp is the area of ​​the frame, or for a round frame rΣ=20π2(e/λ)4.

Another version of an electrically shortened loop antenna is known - a half-wave dipole with a capacitive load, folded into a frame (Fig. 4, a). To implement this antenna, the perimeter of the loop must be less than half the wavelength. Points of zero potential - point A and the middle of the distance between the plates of the capacitor C (Fig. 4, b). The radiation pattern approximately coincides with the radiation pattern of a loop antenna of two half-frames. The power line is connected near zero potential points, as is done in a loop antenna with two half-frames.

A shortened loop antenna of two half-frames is characterized by a more uniform distribution of the current amplitude along the loop conductor compared to a loop antenna with a continuous loop, therefore, with the same perimeters, it has a large radiation resistance rΣ and, as a result, a higher efficiency ηA (efficiency). The gain according to the measurement data reaches 3 dB at e/λ = 0,2.

The study of a shortened loop antenna of two half-frames was carried out at frequencies of 14, 27 and 430 MHz. For a frequency of 14 MHz, a square frame made of copper wire with a diameter of 2 mm had a perimeter e = 0λ, capacitance 2C = 22 pF, quality factor of an unloaded frame Q = 100, radiation resistance rΣ = 2 Ω, input resistance in the current antinode rin = 10 Ω, efficiency ηA - 0,1 and bandwidth 0,3 MHz. To connect a cable with a wave impedance Pl = 50 Ohm, the capacitive voltage divider consisted of capacitors C = 47 pF and Cn = 510 pF, and the length of the frame conductor section for T-shaped matching eτ = 160 mm. Balancing was carried out using a ferrite ring and several turns of a coaxial cable [2]. The antenna was used as a room receiver for monitoring the work of amateur radio stations. In some cases, it was possible to reduce the level of interfering signals due to the symmetry of the antenna, which attenuates common-mode interference, and the presence of "nulls" in the radiation pattern.

For a frequency of 27 MHz, several antennas were made from two half-frames with perimeters e = (0,1 ... 0,5)%. Their study gave the following. For the loop antenna under consideration, the radiation resistance drops very strongly with a decrease in size. It depends on the square of the area (or the fourth power of the perimeter of the circular frame). Therefore, for loop antennas with a perimeter e < 0,1 A, the radiation resistance is much less than the loss resistance, and the antenna bandwidth is determined only by the parameters of the antenna LC circuit. For a loop antenna with a perimeter e > 0.2A. the radiation resistance becomes commensurate with the loss resistance, the quality factor begins to fall, and the antenna bandwidth and efficiency increase. In addition, when feeding the antenna, it is desirable to strive to ensure that the central parts of the vibrators, i.e., the antinodes of the current, are free from connecting elements. Therefore, a capacitive voltage divider is preferable to a T-shaped matching.

The 430 MHz loop antenna measured 36x22 mm and was made of silver-plated copper wire with a diameter of 1,5 mm. Capacitances 2C consisted of trimmer capacitors 1...5 pF. Asymmetric T-shaped matching was used. The cable braid with Pl = 50 Ω was connected to the zero potential point - current antinode, and the central core - at a distance of 10 mm from it. The bandwidth of the loaded antenna is 4,5 MHz, the efficiency is 0,05 ... 0,1.

The considered loop antenna is symmetrical, does not require a counterweight, has zero potential points (which allows the use of conventional power supply methods), can be adjusted by variable capacitors, is not very sensitive to the presence of dielectric and weakly conductive objects in its immediate vicinity, and does not contain inductive matching elements. For a given perimeter, the shortest round loop antenna, consisting of two half-frames, powered through a capacitive voltage divider with the lowest possible losses in the frame material and capacitors, has the highest efficiency.

Literature

1. Devoidere John. Low band DXing. American Radio Relay League, Inc. - 1987, p. 266.
2. Grigorov I. Practical design of antennas. - M.: DMK, 2000, 352 p. silt,
3. Grechikhin L. Electrically small antennas: possibilities and delusions. - Radio, 1992, No. 11, p. 8-10.

Author: N. Turkin, St. Petersburg

See other articles Section VHF antennas.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

A New Way to Control and Manipulate Optical Signals 05.05.2024

The modern world of science and technology is developing rapidly, and every day new methods and technologies appear that open up new prospects for us in various fields. One such innovation is the development by German scientists of a new way to control optical signals, which could lead to significant progress in the field of photonics. Recent research has allowed German scientists to create a tunable waveplate inside a fused silica waveguide. This method, based on the use of a liquid crystal layer, allows one to effectively change the polarization of light passing through a waveguide. This technological breakthrough opens up new prospects for the development of compact and efficient photonic devices capable of processing large volumes of data. The electro-optical control of polarization provided by the new method could provide the basis for a new class of integrated photonic devices. This opens up great opportunities for ... >>

Primium Seneca keyboard 05.05.2024

Keyboards are an integral part of our daily computer work. However, one of the main problems that users face is noise, especially in the case of premium models. But with the new Seneca keyboard from Norbauer & Co, that may change. Seneca is not just a keyboard, it is the result of five years of development work to create the ideal device. Every aspect of this keyboard, from acoustic properties to mechanical characteristics, has been carefully considered and balanced. One of the key features of Seneca is its silent stabilizers, which solve the noise problem common to many keyboards. In addition, the keyboard supports various key widths, making it convenient for any user. Although Seneca is not yet available for purchase, it is scheduled for release in late summer. Norbauer & Co's Seneca represents new standards in keyboard design. Her ... >>

The world's tallest astronomical observatory opened 04.05.2024

Exploring space and its mysteries is a task that attracts the attention of astronomers from all over the world. In the fresh air of the high mountains, far from city light pollution, the stars and planets reveal their secrets with greater clarity. A new page is opening in the history of astronomy with the opening of the world's highest astronomical observatory - the Atacama Observatory of the University of Tokyo. The Atacama Observatory, located at an altitude of 5640 meters above sea level, opens up new opportunities for astronomers in the study of space. This site has become the highest location for a ground-based telescope, providing researchers with a unique tool for studying infrared waves in the Universe. Although the high altitude location provides clearer skies and less interference from the atmosphere, building an observatory on a high mountain poses enormous difficulties and challenges. However, despite the difficulties, the new observatory opens up broad research prospects for astronomers. ... >>

Random news from the Archive High-precision optical probe for studying the human brain 08.01.2024 Scientists at the University of Massachusetts Amherst have pioneered a new era in optogenetics with the unveiling of a high-precision optical probe specifically designed to monitor brain activity. This innovative class of probes incorporates microscopic light-emitting diodes (LEDs) at their tip to modulate neuronal activity by inhibiting or excitating signals in nerve tissue in the brain. A high-precision optical probe from the University of Massachusetts represents a significant advance in brain research, opening up new possibilities for treating neurological disorders. This innovative tool promises to improve our methods and approaches to understanding and treating various brain-related diseases. Optogenetics, based on modification of neuronal genes, creates an optical channel to regulate excitation and inhibition. By using light of a specific wavelength, scientists can manipulate the activity of neurons. The new probe, unlike those existing in optogenetics, emits light in two colors - red and blue, which makes it possible to both suppress and excite the activity of neurons. Designed to measure 0,2 mm wide and 0,05 mm thick, this microprobe promises to be an extremely powerful tool for studying the function of specific groups of neurons. The application of this technology in the medical field, especially in the study and treatment of neurological disorders including epilepsy, is clear. The ability to inhibit and excite neurons using this probe provides new prospects for combating epileptic seizures. Research conducted in mice, which have been studied using new probes, promises to deepen our understanding of neurological mechanisms and contribute to the development of more effective treatments for various disorders.

Other interesting news:

▪ Laser broom sweeps clean

▪ The brain has a built-in noise reduction system

▪ Western Digital introduced the device 3 in 1

▪ TV from SEIKO EPSON with a built-in photo printer

▪ Fast Charger for Smartphones

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ site section Indicators, sensors, detectors. Article selection

▪ hang glider article. History of invention and production

▪ article Whose gravestone says he was the husband of another man's widow? Detailed answer

▪ article Pile cone. Legends, cultivation, methods of application

▪ article Marking of fluorescent lamps. Encyclopedia of radio electronics and electrical engineering

▪ article Two simple analog stabilizers. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages ​​of this page

Arabic	Hebrew	Polish
Bengali	Hindi	Portuguese
Chinese	Italian	Spanish
English	Japanese	Turkish
French	Korean	Ukrainian
German	Malay	Vietnamese

Home page | Library | Articles | Website map | Site Reviews

www.diagram.com.ua
2000-2024