In Figure 1, a coaxial cable is connected to a dipole antenna. For a dipole antenna to operate properly, the currents on both arms of the dipole should be equal in magnitude. When a coaxial cable is connected directly to a dipole antenna however, the currents will not neccessarily be equal. To see this, note that the current along a transmission line should be of equal magnitude on the inner and outer conductors, as is typically the case.
Observe what happens when the coax is connected to the dipole. However, the current that travels along the inner side of the outer conductor IB has two options: it can travel down the dipole antenna, or down the reverse outer side of the outer conductor of the coaxial cable labeled IC in Figure 1.
Ideally, the current IC should be zero. Over the insulator there is another conductor made of fine wires. At outer cover is made of plastic which protects the fine wire braid shield. Coaxial cable is unbalanced line because current in center conductor is referenced to braid.
The braid conductor is connected to the ground. Types and basics of transmission line Types and basics of microstrip line Types and basics of coaxial line. There are two types of wire transmission line viz. Balanced Line The figure-1 depicts balanced type of transmission line. RG is sometimes used to extend radio mike receiver antennas, but its loss is often more than the inverse-square loss of the radio signal traveling the same distance through the air.
For this application, RG-8 would be a better choice if more than five to 10 feet is needed. To use a cable, it must be terminated with some kind of connector. The most commonly encountered balanced microphone connectors are 3-pin XLR originally a model designation in the Cannon Brand, but now used generically. These connectors have a metal shell and three insulated contacts.
If you use these, solder a length of bare busbar wire to Pin 1 and run it out the back, between the rubber strain relief and the U-clamp. This standard is based on Pin 2 of the microphone going positive with respect to Pin 3 when pressure on the front of the mike is increasing.
In the s,the first Nagra recorders used male mike-input connectors, necessitating the use of mike cables with female connectors on both ends. Extension cables were female-male. Some European equipment manufacturers followed suit, with male in and female out.
Other Euro devices have female inputs and male outputs. There also are places in the eastern U. But the panel mounted female TA connectors were so much larger than the male ones that radio mike manufacturers were forced to use male TAs for their microphone inputs, and put the females on the mike cable. WARNING: Since some brands also use male panel connectors for receiver outputs, the possibility exists for accidently plugging in an electret lavaliere microphone to a line-level output and destroying the mike.
Unbalanced microphones normally use. They often are high impedance, and are not usually encountered in professional work, although you may have the occasion to tie into them when they are used as props, or if you have to make a field recording of a local small-town musical group or public speaker. There is no industry-wide standard, and some of the microphones may be quite high impedance. Impedance-matching transformers are available, and may include a housing with a. Unbalanced line-level signals may also use.
There are some high-quality semi-pro phono plugs, but even they become unreliable after repeated insertions. You can make up simple wired adapters to interconnect unbalanced and balanced devices, but using a balun transformer see below will allow longer runs of cable and block common mode interference. You may also get an intermittent connection. The mono plug is slightly larger in diameter than the stereo plug, especially the tip portion.
Depending on the particular manufacturer, a mono plug may not enter a stereo jack, of if it does, it may bend the contacts so a stereo plug will no longer work properly. The jacks on Comtek receivers are designed to accept either type. Both these sizes of stereo plugs are used with some cell phone headsets, and some of them use a double ring plug. Again, you will need an appropriate adapter to patch in. Coaxial cables are usually terminated with BNC Bayonet-style connectors.
Transformers have many uses, but here we are concerned with only four of them: changing impedance, converting between balanced and unbalanced circuits, blocking some kinds of noise, and splitting signals.
A particular transformer may be designed to perform one, two, three, or all four of these functions. A basic transformer consists of two coils of insulated wire wound around the same usually iron alloy core. Laminated iron sheets are used for low e. Air cores wound on a plastic bobbin if the wire is not stiff enough to keep its shape are used for even higher radio frequencies. If both coils have the same number of turns, a signal fed into one coil the primary will appear at the terminals of the other coil the secondary relatively unchanged.
The second signal will, however, be electrically isolated from the original circuit. This removes most C-M noise. An isolation transformer is usually , and will have an additional layer of non-magnetic metallic shielding over the secondary winding to block capacitive coupling of the electrical field produced by the noise on the primary winding. The entire transformer may be mounted in a shielded enclosure, with input and output connectors.
In this case, the shell of the input connector must be electrically isolated from the shell of the output connector to block transmission of the C-M noise by this route, because XLR and many other type connectors often have their metal shells connected to the cable shielding, and thus ground loop current could bypass the electrical isolation of the transformer by flowing through its metal housing.
A transformer designed to change impedance will have a differing number of turns on the primary and secondary. The actual number of turns required is determined by the impedance, frequency, core characteristics, power level, and other factors, again beyond the scope of this article.
NOTE: Theturns-ratio is always defined as primary input turns divided by secondary output turns. A balun BALanced-UNbalanced transformer is used to convert a balanced circuit to an unbalanced one, or vice versa.
At the same time, it can also change impedance if required. It may also provide the functions of isolation and blocking C-M noise. Changing a circuit from unbalanced to balanced will not remove T-M noise that is already there, but may prevent more from entering. A simple balun will have the two ends of one winding connected to the two conductors of the balanced circuit, and the two ends of the other winding connected to the center conductor and shield of the unbalanced circuit.
An even simpler balun has only a single winding, with a center tap. The balanced circuit is connected to the two ends of the winding and the unbalanced circuit has the shield connected to a center tap of the winding and the inner conductor also connected to one of the winding ends. Obviously, this type of balun does not provide any isolation or blocking of C-M interference, and is mainly used in antenna circuits. A splitter transformer has a single primary winding and two identical secondaries.
Since a splitter transformer is a passive device, each output will be -3 dB down from the input. Many commercial units do not have this feature, but it is possible to remove the connectors, enlarge the hole if it contacts the protruding back part of the connector, place an insulating plastic film between the back of the mounting flange of the connector and the splitter case, and reattach it with plastic screws.
Unless you have cables with the connector shells floating, or insulate the splitter connectors as just described, the ground-lift switch will be ineffective.
All types of transformers have certain parameters that must match their intended application. Only the ones relevant to this article are discussed here. Level: Mike or line. Mike-level transformers will overload and distort if used with line-level signals because the core will be completely saturated with magnetic flux lines well before the input signal reaches its maximum voltage. Line-level transformers can be used with mike-level signals, but the higher winding impedance might cause loss of high-frequency response when connected to certain types of output circuits.
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