Baluns and ununs are fun construction projects.They are electrically simple, and easy to fabricate with hand tools. I have enjoyed building many baluns; here are pictures of some I have built recently.
Pdf Understanding Building And Using Baluns
The most common use for baluns is when a single-ended power amplifier is used to drive a balanced load. Examples include dipole antennas or single-ended antennas such as whips, which are needed to feed an additional front-end amplifier (see figure 1). In the past, baluns were also widely used in the CATV industry, for example when matching between a 300Ω dipole antenna for broadcast TV with a 75Ω coaxial cable. With the development of RF Integrated Circuits (RFICs), baluns are now also widely used to improve noise immunity and common mode rejection. The growth in 5G applications has also led to tremendous demand for small, wideband baluns to interface with highly integrated radio transceivers using differential inputs and outputs.
Understanding the difference between common mode signals and differential signals is crucial for understanding the importance of baluns. In a balanced circuit, common mode signals are those that are equal in magnitude and polarity between the two lines. Differential signals, on the other hand, are equal in magnitude, but opposite in polarity. In general, differential signals are much more robust. This is due to the fact that differential signaling has inherent noise immunity. In a typical system, external noise is equally present on both lines of a balanced configuration and appear as a common mode signal. A differential signal is represented by the difference in voltage between the two lines. Since the common mode signal is equal on both lines, it is cancelled out.
Broadband balun transformers are also constructed with various impedance transformations using several transmission lines in series and parallel arrangements. In this case, the impedance transformation is 1:n2, where n is the number of series-parallel transmission lines. Quarter-wave and half-wave transmission line balun transformers are also possible, although these types of baluns are best suited to applications with a narrow operating frequency range.
Balun production can also involve the use of planar metal structures with technologies such as LTCC and MMIC. This choice of design and fabrication is often proprietary. Generally, the main benefit of these designs is the small footprint that can be readily integrated into a microwave assembly. Unlike other baluns, LTCC- and MMIC-based baluns are produced using high-precision assembly machines and seminconductor manfacturing methods that yield much higher repeatability.
For use with RF antennas, baluns are needed where a balanced antenna such as a dipole is used. The most convenient form of feeder to use is coaxial cable as this can be routed far more conveniently along antenna towers, or poles, along the ground and through buildings, etc without the need to worry about affecting the coax cable performance.
Prevent radiation / pickup from coax outer conductor: If a balanced antenna is used and fed directly using coaxial feeder, then it is found that the outer or shield of the coax can radiate and pick-up radio frequency signals. For receiving, this can mean that interference from electrical apparatus, etc within the home, building, or just close to the feeder can be picked up. For transmitting, it can mean that the feeder will radiate signal which could cause interference and result in feeder loss.
Figure 9-12 shows the frequency responses of these two baluns with various floating terminations, which demonstrates not only the superiority of the Guanella balun with a ferrite core but also the danger of using a powdered iron balun in an antenna tuner. It also shows that the powdered iron balun starts falling off, at the low end, around 7 MHz. This means there is insufficient reactance in the windings to prevent a sizable shunting effect. Below 7 MHz, the transformer becomes inductive. This condition allows for flux in the core. Further, this inductance can become part of the tuned L-C network, resulting in very high currents and flux densities. The Guanella balun in the figure demonstrates an exceptional response from 1.7 to 60 MHz. This balun, which uses the popular 2.4 in OD, no. 61 toroid (μ = 125), should be investigated for possible use in antenna tuners.
A more subtle type results when the transformer type (magnetic coupling) is combined with the transmission line type (electro-magnetic coupling). Most typically the same kind of transmission line wires are used for the windings as carry the signal from the radio to the antenna, although these baluns can be made using any type of wire. The resulting devices have very wideband operation.[3] Transmission line transformers commonly use small ferrite cores in toroidal rings or two-hole, binocular, shapes.
Baseband video uses frequencies up to several megahertz. A balun can be used to couple video signals to twisted-pair cables instead of using coaxial cable. Many security cameras now have both a balanced unshielded twisted pair (UTP) output and an unbalanced coaxial one via an internal balun. A balun is also used on the video recorder end to convert back from the 100 Ω balanced to 75 Ω unbalanced. A balun of this type has a BNC connector with two screw terminals.VGA/DVI baluns are baluns with electronic circuitry used to connect VGA/DVI sources (laptop, DVD, etc.) to VGA/DVI display devices over long runs of CAT-5/CAT-6 cable. Runs over 130 m (400 ft) may lose quality because of attenuation and variations in the arrival time of each signal. A skew control and special low skew or skew free cable is used for runs over 130 m (400 ft).[citation needed]
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