Unbalanced Transformer: Transmission Line Equalizer (balun) Principle, Design, Fabrication And Testing

A balancer (Barron) origin

Baron Balancing Device balancer, its main function is accomplished by the single ended transmission (such as coaxial line and microstrip line etc.) is transformed into differential transmission (such as: half wave dipole antenna, push-pull circuit etc.) between the transformation, also known as the balun is Balance-Unbalance, English merge abbreviated as a word Balun, transliterated as baron. Balancer, balance - hereinafter referred to in the unbalanced converter, balun, is refers to this kind of device.

Balun balancer (balun) is widely used in radio, due to its principle of structural diversity, and can be combined with each other, make many friends in the self-made balun balancer (balun) when there is no start feeling for what? How to choose the material? How to calculate the production parameters? How to measure the performance of the Baron? For our amateur, mainly used in the antenna feed and high frequency power amplifier, the completion of the role of unbalanced and impedance transformation, work in the short wave 1.8MHZ~30MHZ, and the requirements of materials and production is easy. According to our understanding of the balun, that the structure of transmission line balun, more suitable for shortwave communication, its performance is good, convenient, easy to manufacture, but the theory is not easy to understand, it will cause a lot of friends in a magnetic coupling transformer structure appears narrow frequency band, small power capacity, standing wave uneven of course, the results are not up to the effect of transmission line transformer.

Two, the transmission line equalizer (Barron simple principle)

Balun balancer (balun) has many kinds, according to the balance conditions can be divided into four categories: choke type (curb unbalance current) and symmetric (on the impedance, inverted (balanced) voltage inverter) and magnetic coupling (current conjugate). Here I mainly describe the transmission line transformer balun transforms based on impedance transformation function with balun, has a characteristic of choke type and magnetic coupling type.

The structure of transmission line transformer winding as above, it is a group or several groups of transmission line in high frequency magnetic ring, using different connection method to complete the impedance transformation and balun transformation. From the beginning of the energy converter to the terminal is transmitted through the distribution of transmission line capacitance, inductance and electromagnetic energy exchange, this is usually around the transformer, it overcomes the coils in high frequency transformer due to the adverse effects caused by the distributed capacitance coil, improve frequency characteristic. In addition, the direction of two equal amplitude of each conductor current transmission line on the contrary, they produced in the core flux offset each other, so the core loss is very small, even if the core sectional area is very small, but also has high power capacity. Therefore, it has the advantages of wide frequency band, small size, large power capacity and so on,

Transmission line transformer in low frequency can be equivalent to the traditional low frequency converter, deterioration of the frequency response is due to transmission line between two parallel electrical conductors due to magnetizing inductance caused by Na, it determines the minimum working frequency converter.

In the end it is the high frequency transmission line has a characteristic impedance, in order to avoid any resonance phenomenon, especially for complex load, it will cause fluctuations increase substantially, the length of the transmission line does not exceed the upper limit frequency of 1/8 wavelengths, short low-frequency characteristics become worse.

In order to select the magnetic core for the RF transform, it is required to know the saturation magnetic flux and its nonlinear characteristics, to calculate the maximum magnetic flux density according to the maximum power of the minimum operating frequency.

Since it is using the transmission line converter balun transforms, so in the end with unbalanced weight balance. The number of important indicators is balun transform, can be used in similar circuit common mode signal and differential mode signal to describe the unbalanced signal and balanced signal, it can inhibit to describe this I think the index with the common mode, can also be called unbalanced inhibition. It is affected by the ratio of the impedance transformation, but also by the impact of the common mode inductance.

Three, the transmission line balancer (Ba Lun) using the calculation software

Based on the above principle, some related calculation formula is given, but the calculation is a trial - and then count the adjustment parameters to adjust, find the most suitable results back and forth several times, at the same time, the parameters of the magnetic core are familiar with, therefore, I combined with the characteristics of transmission line transformer understanding and balun and the understanding of the core, try one for the calculation of the transmission line balun wrote a small software, hoping to solve the trouble of manual calculation.

After running the software such as window, maximum working frequency, transmission line balun design please input the lowest working frequency, the maximum power and the choice of a ring and turns around the ring, number in the first two digits is two ring diameter represents the different properties of the material, can be calculated for the first time select a test, turns of the coils can not write an integer, such as 10, and then click the "calculate" button, the lower right corner will need to adjust the parameters according to the calculation results, adjusted to calculate, until a satisfactory result, if cannot obtain meaningful results, it may be frequency coverage the range is too wide, or too much power, appropriate to reduce some try.

Four, the transmission line balancer (Barron test)

1, the Bobbi, the common mode rejection ratio, insertion loss:

A. Bobbi - the balanced output of the Ba Lun is then matched to a false load, which is used to measure the standing of the coaxial terminal in the frequency band.