16
5.2.5 Making 1/4-
λ
, 1/2-
λ
and other coaxial stubs
Pieces of cable of certain electrical length are often used as components of baluns
(balancing units), transmission line transformers or delay lines.
To make a stub of the predetermined electrical length,
1.
Calculate the physical length. Divide the electromagnetic constant by the
required frequency (in Hz). Multiply the result by the velocity factor of the
cable, then multiply by the desired ratio (in respect to
λ
).
Example:
1/4-
λ
stub for 28.2 MHz, cable is RG-58 (velocity factor is 0.66)
299792458 / 28200000 * 0.66 * (1/4) = 1.75 meters
- or -
983571056 / 28200000 * 0.66 * (1/4) = 5.75 feet
2.
Cut a piece of cable slightly longer than this value. Connect it to the analyzer.
The cable must be open-circuited at the far end for 1/4-
λ
, 3/4-
λ
, etc. stubs, and
short-circuited for 1/2-
λ
,
λ
, 3/2-
λ
, etc. ones.
Example:
A piece of 1.85 m (6.07 ft) was cut. The margin is 10 cm (0.33 ft). The
cable is open-circuited at the far end.
3.
Switch the analyzer to the
Show all
measurement mode. Set the frequency the
stub is designed for.
Example:
28200 kHz was set.
4.
Cut little pieces (1/10 to 1/5 of the margin) from the far end of the cable until the
X value falls to zero (or changes its sign). Do not forget to restore the open-
circuit, if needed.
Example:
11 cm (0.36 ft) were cut off.
