What Is a Balun?

Balanced lines and unbalanced lines have different electrical characteristics, so that they cannot simply be connected to each other. Balancing-unbalancing transformers are matched by providing impedance conversion for two different lines. Unbalanced transformer for twisted pair.

Barron

(Theoretical characteristics)

Balanced lines and unbalanced lines have different electrical characteristics, so that they cannot simply be connected to each other. Balancing-unbalancing transformers are matched by providing impedance conversion for two different lines. Used for

Balanced and unbalanced effect: According to the antenna theory, dipole antennas are balanced antennas (dipole antennas, dipoles, double cones, and Yagi), and the antenna's feed line is generally a coaxial cable, which is an unbalanced transmission line. If you connect it directly, high frequency current will flow through the outer sheath of the coaxial cable (known as Iz in Claus's book). High frequency current flowing through the outer sheath will inevitably have radiation components, and even affect the polarization direction of the antenna ( For a horizontal dipole antenna, there is a current in the outer layer of the vertical RF line, which will affect the purity of horizontal polarization). Therefore, it is necessary to add a balanced and unbalanced converter between the antenna and the cable to suppress the current Iz flowing into the outer layer of the cable shield, that is, to cut off the high-frequency current flowing from the vibrator through the outer layer of the cable shield.

Impedance transformation effect: The essence of the balun is a transmission line transformer. According to the input impedance of the antenna, a winding method is designed to achieve n ^ 2: m ^ 2 impedance transformation. Common are 1: 4, 1: 9 and so on.

Typically used to connect a 10BaseT network with a coaxial cable or shielded double conductor

For

Used to connect the Token Ring network to a high-speed ATM hub in the campus backbone.

Balun is an English transliteration of balanced and unbalanced converter. The principle is that according to the antenna theory, the dipole antenna is a balanced antenna, and the coaxial cable is an unbalanced transmission line. If it is directly connected, the outer sheath of the coaxial cable will be high. High frequency current flows (according to the principle of coaxial cable transmission, high frequency current should flow inside the cable, the outer sheath is the shielding layer, there is no current), so it will affect the radiation of the antenna (think of it as the shielding layer of the cable) Also involved in the radiation of radio waves). Therefore, a balanced-to-unbalanced converter must be added between the antenna and the cable to suppress the current flowing into the outer layer of the cable shield, that is, to cut off the high-frequency current flowing from the vibrator through the outer layer of the cable shield. There are many ways to achieve this purpose. One is the high-frequency open circuit method. A quarter-wavelength sleeve is connected to the quarter-wavelength of the cable shield's sheath (equivalent to the quarter-wavelength open circuit). ), Because a quarter-wavelength open line treats the frequency as an open circuit, and achieves the function of cutting off high-frequency currents. This method has a narrow working bandwidth, and the quarter-wavelength sleeve appears very long at low frequencies, which is suitable for large frequencies. Power is used at high frequencies. The other is the cancellation method, which finds ways to make the currents flowing in equal directions and cancel each other in the opposite direction. This is the case for balanced unbalanced converters that use more three-wire magnetic loops. This type of band has a wide bandwidth and is used but large. The power is limited by the magnetic saturation of the magnetic ring, which is suitable for low frequency and low power use. The other is the transformer method, which realizes balanced and unbalanced conversion through a high-frequency transformer. The principle is like a push-pull output transformer, which converts a bidirectional balanced current into a unidirectional unbalanced current. The transformer can be made of magnetic core or hollow winding, suitable for high power use. The other method is the suppression method. The vibrator is connected to the outer sheath of the cable shield through a high-frequency choke coil to prevent high-frequency current from flowing to the outer sheath of the cable shield. This method is relatively simple. Well, it doesn't matter if it's hollow. Generally, it is a few times around the low frequency and a few times around the high frequency. However, the suppression effect is not as good as the aforementioned ones, so the former ones are mostly used for professional applications, and there are many such amateur applications. Keep in mind that we are only cutting off the high-frequency current of the shielding layer, not all the high-frequency current flowing to the shielding layer (in this case, it is necessary to disconnect the oscillator from the cable sheath). The high-frequency current is in the shielding layer. It's flowing inside. In the image, the cable can be imagined as a water pipe. Originally, water should flow in the water pipe. Without the balun, the water not only flows in the water pipe, but also part of it flows to the outer skin of the pipe. The role of Balun is to prevent running, running, dripping, and leaking, forcing water to flow in the water pipe. The production of the inverted V antenna requires that it be erected as high as possible, and that the ground ^[1] should be as wide as possible, and that it should be installed as far away as possible from interference sources.

The antenna element HF can be a general power line (commonly known as a flower line). It has little effect on the insulation or bare copper wires. Selecting a thicker wire can improve the mechanical strength and radiation efficiency (the effect is not very obvious, in theory) To match the antenna to the cable by trimming the length of the vibrator (the effect of this step is obvious, it is worth doing carefully). VHF can be aluminum tube or copper tube, the size of the tube depends on the mechanical strength, of course, a bit thicker is advantageous.