Serial cable transformers

[Admin 2545]

One of the most simple and easy ways of coordination feeder impedance Z0 and the antenna ZA is a consistent inclusion in the feeder a small line segment with another wave the impedance ZS(on heterogeneity in a line). Length matching section (up to a quarter of wavelength) and the place of its inclusion in the feeder depending on the required the transformation ratio of the resistances and from the how its characteristic impedance differs from the wave impedance of the feeder. [1]

Wave the impedance matching section ZScan to be within Z0 > ZS > (Z0 x ZA)1/2 , if ZA < Z0 , or Z0 < ZS < (Z0 x ZA)1/2 if ZA > Z0.

Practically, when use feeder with Z0=75 Ohm, transformation of resistance of the antennas from 8.33 to 75 Ohm you can use a section of two parallel connected cables at 50 Ω (with the resulting ZS = 25 Ω). If Z isA not below the 33.3 Ohm matching section can have an impedance of 50 Ohms. Section two connected in parallel cuts 75-a lot of cable (ZS = 37,5 Ohm) provides the coordination 75-lot feeder with any of found in practice impedances of a multi-element Yagi antennas and Quad.

If ZS = (Z0 x ZA)1/2, the length of the section becomes equal to 1/4 wavelength, and the distance from the load where the partition should be included in the feeder zero. This is a limiting case of serial transformer, widely known as "quarter-wave the transformer".

Another private case - when ZS = ZA. Then the length of the section, and its distance from the load equal approximately 1/12 of the wavelength. [3] In another the limiting case, when ZA is close to  Z 0 , the length of insertion of the Z's is committed to the zero point of the feeder, located at a distance of 1/8 the wavelength of the load (this case is not practical interest, as means that the transformation resistance is not required).

For operational calculation of the matching antenna circuit that is configured to resonance (XAND= 0), you can use graphs 1-5. Sizes are in fractions of the average the operating wavelength, and physical length of the segments cables should be calculated based on the velocity factor, which is not always equal to 0.66, it is necessary to measure each cable individually.

 

 

This kind the transformer allows you to coordinate not only the resonance of the antenna. For example, charts 6 and 7 shows how to change length of 37.5 Ohm section and its place in the inclusion of 75 Ohm feeder in depending on the magnitude of the reactive component (XA) the impedance of the antenna at two constant values the active component RA = 22 Ω, and 45 Ohms.

 

 

by Yuri Baltin, YL2DX

[Admin 2545]

Another form of transformer 75:50 coax can be made by using the impedance transforming effects of transmission lines. Two sections of 1/12 wavelength feed line will transform one impedance to another.  Figure  shows how it is done.  For short lengths it might be easier to use RG-11 or similar coax instead of hard line for the 75 ohm segment.
Normally you would want to put one of these at each end of the 75 ohm run. Don’t forget to include the coax velocity factor when calculating the length of the 1/12 wave sections.

Where
L = Length in meters or feet depending on value used for S
S = speed of light in free space, use 300 if you want L in meters, 984 if you want L in feet
W = number of wavelengths desired, 1 = 1 wavelength, 0.5 for half wavelength, etc

[Admin 2545]

,,,and a wideband transformer on PCB...