BALUN or not BALUN? Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Theory

Comments on the article

The purpose of the device is to prevent the flow of high-frequency currents along the outer surface of the braid to reduce the antenna-feeder effect [2]. The device captivates with its simplicity and ease of manufacture, but does it meet the requirements well?

Let's consider them. The balun should have as high a resistance as possible to the HF currents on the braid without breaking the DC contact, i.e. be a choke. Inductors used as chokes are made according to well-known rules: the desire to obtain the maximum inductive resistance with a minimum self-capacitance forces the use of a sectioned winding and / or a cylindrical one with a certain step.

Often, broadband chokes do this: from the beginning ("hot" output), winding is carried out with a large step, then with a smaller one, then turn to turn, and, sometimes, the last section is wound in the "universal" way.

The intrinsic capacitance of the inductor C0 with the inductance of its winding L forms a parallel oscillatory circuit (Fig. 1), the resonant frequency of which f0 is the higher, the smaller the capacitance. At frequencies above f0, the inductor has a capacitance that drops rapidly with increasing frequency, i.e., ceases to perform its functions.

The solid line in the graph (Fig. 1) shows the dependence of the inductor reactance on frequency for an ideal coil with an infinite quality factor. The losses in the coil reduce the quality factor, the branches of the curve no longer go to infinity (dashed line on the graph), and the active component R appears in the total resistance. It is maximum at the resonant frequency and is equal to pQ, where р = (L/C0)1/2 is the characteristic resistance. From this it is clear that in order to increase the impedance of the inductor, it is necessary to increase its inductance in every possible way and reduce its own capacitance.

But back to our Valuns.

A coiled cable must have a noticeable intrinsic capacitance (up to several tens of pF/m!). This means that the cable bay will not become a choke, but an oscillatory circuit with a certain resonant frequency. The natural desire to wind more turns in the bay (to increase the inductance) can lead to the opposite result: the resonant frequency will be lower than the operating frequency, and the balun will behave like a capacitance, and with an increase in the number of turns, the capacitance will fall.

To test this assumption, a simple measuring setup was assembled (Fig. 2), consisting of a standard signal generator (SGS) and an oscilloscope. Balun was located directly on a wooden desktop and was connected with one output of the cable braid (the core was not used) to the GSS case, a VD1 detector diode and an input cable of a low-frequency oscilloscope were connected to the other output. The AM signal from the GSS was fed to the balun through a very small coupling capacitance formed by a segment of an insulated conductor about 10 cm long. Thus, the installation added practically nothing to the cable coil's own capacitance (diode capacitance - fractions of a picofarad).

The resonance was detected immediately by a sharp increase in both the constant component and the amplitude of the modulation signal at the oscilloscope input. The Q-factor of the circuit (cable bay) turned out to be not at all small - from 30 ("Shirpo-Trebovsky" TV cable) to 60 (cable with rigid polyethylene outer insulation). The resonant frequency f0, as expected, depends on the number of turns N and the diameter of the coil D. The data of several measurements for the widely used cable PK-75-4-11 (outer diameter over insulation 7,3 mm, over braid 5 mm, core 0,72, XNUMX mm) are tabulated.

Of course, these data are indicative, since the resonant frequency depends on the density of the coils, the proximity of surrounding objects, and other factors. According to the table, graphs of the dependence of the resonant frequency on the number of turns were plotted (Fig. 3). They will tell you the maximum number of turns at which the balun is still a throttle.

For comparison, in one of the experiments, instead of a coil (D = 20 cm, N = 11), the same cable 7 m long was wound on a plastic tube 10 cm in diameter. A cylindrical coil was obtained containing 20 turns with a winding length of 15 cm. The resonant frequency increased from 4 to 7 MHz, and the quality factor - from 30 to 65. The advantage of the traditional coil design is obvious!

So what to do? The easiest way is to make a balun from a cable coil for a single-band antenna - it should be tuned to resonance at the operating frequency, choosing the diameter and number of turns. Then its impedance will be the maximum possible, and consequently, the effect of weakening the currents on the braid will also be maximum.

For broadband baluns, the resonant frequency must be chosen such that it is near the upper edge of the operating range. For frequencies below the resonant inductive reactance balun'a can be found by knowing the inductance L: Xl = 27πfL, or by more precise formulas for the impedance of a parallel resonant circuit given in [3]. With a decrease in frequency, the balun will stop working approximately at the frequency where its inductive reactance will be of the same order as the wave resistance of the cable, considered as a wire with a diameter equal to the diameter of the braid, in free space (400 ... 600 Ohm).

In conclusion, we present several useful techniques and formulas from [3]. They can be useful to those who will experiment or calculate such devices. The length of the cable in the bay is easy to determine by the formula πDN. The inductance can be calculated as follows: L = 2πN2D[lp(8D/d) -2]. The diameters of the coil D and the outer sheath of the cable d are taken in centimeters, and the inductance is obtained in nanohenries. The quality factor is measured by the width of the resonance curve 2Δf at a level of 0,7 from the maximum: Q = f0/2Δf. The intrinsic capacitance C0 of a balun is difficult to calculate, but it can be found experimentally. If you connect an additional capacitor of known capacitance C1 to the terminals, then the resonant frequency will decrease and become equal to f1. Then C0 = C1/[(f0/f1)2-1].

Using this technique and formulas, it was found, for example, that the coil inductance D = 10 cm, N = 4 is 3,2 μH, and its own capacitance is 10 pF, which gives a resonant frequency of 28 MHz, coinciding with the measured one.

Literature

1. Balun from the feeder. - Radio, 2001, No. 10, p. 65.
2. Grechikhin A., Proskuryakov D. Feeder Antenna Effect. - Radio, 2000, No. 12, p. 56 -58; 2001, no. 1, p. 64 - 66; No. 3, p. 67.
3. Ginkin G. G. Handbook of radio engineering.-M., L.: GEI, 1948.

Author: V.Polyakov

See other articles Section Antennas. Theory.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Artificial leather for touch emulation 15.04.2024

In a modern technology world where distance is becoming increasingly commonplace, maintaining connection and a sense of closeness is important. Recent developments in artificial skin by German scientists from Saarland University represent a new era in virtual interactions. German researchers from Saarland University have developed ultra-thin films that can transmit the sensation of touch over a distance. This cutting-edge technology provides new opportunities for virtual communication, especially for those who find themselves far from their loved ones. The ultra-thin films developed by the researchers, just 50 micrometers thick, can be integrated into textiles and worn like a second skin. These films act as sensors that recognize tactile signals from mom or dad, and as actuators that transmit these movements to the baby. Parents' touch to the fabric activates sensors that react to pressure and deform the ultra-thin film. This ... >>

Petgugu Global cat litter 15.04.2024

Taking care of pets can often be a challenge, especially when it comes to keeping your home clean. A new interesting solution from the Petgugu Global startup has been presented, which will make life easier for cat owners and help them keep their home perfectly clean and tidy. Startup Petgugu Global has unveiled a unique cat toilet that can automatically flush feces, keeping your home clean and fresh. This innovative device is equipped with various smart sensors that monitor your pet's toilet activity and activate to automatically clean after use. The device connects to the sewer system and ensures efficient waste removal without the need for intervention from the owner. Additionally, the toilet has a large flushable storage capacity, making it ideal for multi-cat households. The Petgugu cat litter bowl is designed for use with water-soluble litters and offers a range of additional ... >>

The attractiveness of caring men 14.04.2024

The stereotype that women prefer "bad boys" has long been widespread. However, recent research conducted by British scientists from Monash University offers a new perspective on this issue. They looked at how women responded to men's emotional responsibility and willingness to help others. The study's findings could change our understanding of what makes men attractive to women. A study conducted by scientists from Monash University leads to new findings about men's attractiveness to women. In the experiment, women were shown photographs of men with brief stories about their behavior in various situations, including their reaction to an encounter with a homeless person. Some of the men ignored the homeless man, while others helped him, such as buying him food. A study found that men who showed empathy and kindness were more attractive to women compared to men who showed empathy and kindness. ... >>

Random news from the Archive Free encrypted iPhone calls 17.08.2014 An analogue of the RedPhone application for Android has become available to iPhone owners. It allows you to talk with other subscribers over an encrypted communication channel. Connection is possible not only between iPhone, but also between iPhone and Android. In the near future, the authors promise to add support for encrypted text messages to the program. In order to make encrypted calls, it is necessary that the second subscriber also has the Signal application or a compatible RedPhone equivalent, which was previously released for the Android platform. Thus, secure voice communication is possible both between iPhone owners who have installed Signal, and between an iPhone owner and an Android owner, provided that the latter has the RedPhone application. Android device owners can also communicate with each other. Calls are made using a data network or Wi-Fi connection. The applications themselves are free, the user only needs to pay for traffic. Signal and RedPhone are based on ZRTP, an open source technology co-authored by Philip Zimmermann, creator of the popular PGP email encryption technology.

Other interesting news:

▪ Invented fabric that mosquitoes won't bite through

▪ Shift work is harmful to health

▪ Refrigerator Sharp JH-DT55B

▪ Nokia 41 808MP camera phone

▪ EVGA GQ Series 80Plus Gold Power Supplies

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ site section Computer devices. Article selection

▪ article Economic geography. Crib

▪ article Which of the states in the UN has the largest territory and which is the smallest? Detailed answer

▪ article by Ferul Lehmann. Legends, cultivation, methods of application

▪ article A simple three-channel color and music console. Encyclopedia of radio electronics and electrical engineering

▪ article Generator to power the LED. 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