But open-wire line is still the best way to move RF signals, since the lowest-loss dielectric available is still air. The cable can't be wet or covered with snow or ice, nor can it rest on or be placed near metal objects, such as gutters, flashing, windows, ducting, screens, and pipes.
Finally, open-wire line doesn't like to be bent at severe angles - gentle, long-radius curves are all it can tolerate. So it is obvious why coaxial cable came into existence. We run cables alongside plenty of metal objects, often burying them in hostile environments. We put all kinds of twists and turns into cable and not forgetting connections all of which diminish signal quality that kind of path is a nightmare to negotiate with ladder line.
But there's a problem. The AC voltages in coaxial cable want to travel as if they are along open-wire transmission lines. Electrically, that's not a big problem one leg flows in the center conductor, and the other flows in the outer shield. The problem occurs when these AC signals arrive at their "load", which could be a speaker, antenna, resistor, splitter, or an amplifier, and one side is grounded. Most AC loads are designed to function as balanced components - most antennas use a dipole radiator as the driven element.
Connecting that unbalanced, grounded transmission line coaxial cable directly to a balanced load can result in all kinds of problems, such as currents flowing back down the shield to the source. These stray currents create standing-wave problems, resulting in a mismatch from the cable to the load. The result can be ringing or ghosting, not to mention unwanted radiation of the signal from the coaxial cable which is now working as part of the load. The solution to these issues is to use a small transformer to convert from an unbalanced to a balanced signal, and vice-versa.
This transformer is known as a balun, and they are in operation in anything from telephone lines to transmitters. Baluns are used both to sort out the flow of AC signals and make the necessary impedance transformation between coaxial cable, which has a low impedance, and balanced loads, which have higher impedances.
With a balun, I can now send AC signals of any kind over a pair of wires and make the necessary transformations at either end to effect a clean transfer of AC signals. One of the best-performing varieties of transmission line balun is the Marchand balun Figure 5. Isolation balun transformers are RF transformers with the unbalanced side tied to ground and the balanced side connected to the load.
Similarly to standard RF transformers, the ratio of windings can also be used to create an impedance transformation. Figure 4 illustrates an autotransformer balun with balanced inputs at either end of the winding, a center tap to ground, and one end of the winding creating the unbalanced port.
One major benefit of this type of transformer is that the input and output are electrically separated, allowing a degree of protection for systems that are prone to ground loops in their ground-level voltages. Autotransformers have a different configuration from that of typical RF transformers, since this topology has only a single conductive path. Autotransformer baluns can be fabricated by winding a single wire around a ferrite core, or by cross-wiring the primary and secondary windings.
A tap point between the two ends of the winding is used to access different voltage potentials respective to the transformer input voltage. This configuration incorporates a DC current path to ground for each terminal, dissipating any static build-up. Transmission line balun transformers are typically constructed of a transmission line such as a coaxial cable wrapped around a ferrite core, and in some cases merely air.
This type of balun transformer creates a high choking reactance on the outer conductor of the coaxial cable, effectively reducing common-mode signals while allowing the internal currents of the coaxial transmission line to pass unimpeded i. Additionally, configurations include a bifilar capacitively coupled balun using two wires wrapped around each other as well as capacitively and magnetically coupled transmission lines wrapped around each other and then wrapped around a common core.
The purpose of using a magnetic core with broadband transmission line couplers is to enable low frequency operation. 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:n 2 , 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.
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. Baluns share most of the same performance parameters as RF transformers, but the unique construction and use of baluns introduces additional considerations. These parameters include:.
The amplitude balance is the absolute value of the output power on each balanced line. For these parameters, each balanced line should be as close to equal as possible.
The determining factors for phase and amplitude balance in real baluns include material properties, fabrication methods, and match between the output lines.
Modern high-performance baluns are generally specified for no more than a few degrees of phase unbalance and a few dB of amplitude unbalance.
This ratio is dependent on amplitude and phase unbalance. A balun with better amplitude and phase balance will also exhibit enhanced CMRR. A generally accepted guideline is that a 0. Balanced port isolation is a measure of the ratio of input signal strength to output signal strength from one balanced port to another insertion loss.
More often than not, this parameter is not very high in most balun designs. DC isolation is a measure of the DC conductivity between the unbalanced port and balanced ports while ground isolation is a measure of the isolation between the unbalanced port ground and the ground or pseudo ground of the balanced ports.
Although the essence of many baluns is a transformer, there are several different types of balun that can be used. Some employ different circuit techniques, while others use different concepts. However all form the basic function of changing from a balanced to unbalanced system. Classical transformer balun: The traditional approach for a balun is to use a transformer. This consists of two electrically separate windings, normally wound on a core - typically a toroidal ferrite core.
Having two separate windings in this fashion means that the two circuits are electrically isolated which can have advantages for some applications. The core on which the transformer is wound can either be an air core, generally with a former which may be porcelain to keep the two coils in place, or it can be made from a magnetic conductor like ferrite.
The ferrite needs to be chosen so that it can handle the frequencies and power levels involved. The ratio of the turns between the primary and secondary, determines the ratio of the input and output impedance levels. It is therefore possible to not only use the balun to convert between balanced and unbalanced, but to also act as a matching transformer if needed. In some instances it may be necessary to step the impedance up for the coaxial feeder. It should also be remembered that there will be losses in the balun.
Although these are normally small, they still need to be considered in some applications. One issue with this form of balun is that the two halves are totally electrically isolated. It is found that external antennas can pick up static and a charge can build up. To overcome this, a balun with a centre tap on the balanced winding can be used. The centre tap is taken to earth to dissipate the charge. Auto-transformer balun: As the name implies, this form of balun uses an auto-transformer, i.
One of the advantages of the auto-transformer style of balun is that it inherently provides a DC path to ground for all connections, both at the unbalanced coaxial input, and also for the balanced input as well. As described above, this enables static to be discharged.
RF choke as a balun: One of the main issues with feeding a balanced antenna with an unbalanced feeder like coax, is that RF travels along the outer of the coax and this can result in unwanted radiation and pickup. The concept of the choke balun is to increase the inductance on the outer conductor of the unbalanced conductor to prevent signals travelling only along this conductor.
One implementation of a choke balun is to coil a few turns of feeder up at the feed-point of the antenna to act as a feed-line choke. Although a very easy implementation, it does not always work well. The common mode rejection ratio of a balun depends on the impedance of the system at the point that the balun is used. Also, the inductance of a few turns of coax may not be sufficient to provide adequate rejection at frequencies below 10 MHz or so.
Another implementation is to wind the coax around a toroidal former. Obviously the bend radius of the coax needs to be considered and this means that larger toroids are generally required.
The properties of the toroid ferrite also need to be considered to provide the best attenuation at the frequencies of use. Although baluns take many forms, they are often used with external antennas and require the mechanical arrangements to be addressed as well. Weather protection: As many baluns are used externally they should be made in a fashion that will be able to withstand the rigours of the weather.
0コメント