Use can be made of standing waves on sections of line to provide filters and RF transformers. When a line one-quarter wavelength long (aλ/4 stub) is open circuit at the load end, i.e. high impedance, an effective short-circuit is presented to the source at the resonant frequency of the section of line, producing an effective band stop filter.
The same effect would be produced by a short-circuited λ/2 section. Unbalanced co-axial cables with an impedance of 50 ohm are commonly used to connect VHF and UHF base stations to their antennas although the antennas are often of a different impedance and balanced about ground.
To match the antenna to the feeder and to provide a balance to unbalance transformation (known as a balun), sections of co-axial cable are built into the antenna support boom to act as both a balun and an RF transformer.
Balun
The sleeve balun consists of an outer conducting sleeve, one quarter wavelength long at the operating frequency of the antenna, and connected to the outer conductor of the co-axial cable as in Figure 3.5.
When viewed from point Y, the outer conductor of the feeder cable and the sleeve form a short circuited quarter-wavelength stub at the operating frequency and the impedance between the two is very high.
This effectively removes the connection to ground for RF, but not for DC, of the outer conductor of the feeder cable permitting the connection of the balanced antenna to the unbalanced cable without short-circuiting one element of the antenna to ground.
RF transformer
If a transmission line is mismatched to the load variations of voltage and current, and therefore impedance, occur along its length (standing waves). If the line is of the correct length an inversion of the load impedance appears at the input end.
When a λ/4 line is terminated in other than its characteristic impedance an impedance transformation takes place. The impedance at the source is given by:
Zs = Z0^2/ ZL
where
Zs = impedance at input to line
Z0 = characteristic impedance of line
ZL = impedance of load
By inserting a quarter-wavelength section of cable having the correct characteristic impedance in a transmission line an antenna of any impedance can be matched to a standard feeder cable for a particular design frequency.
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