Practical_Antenna_Handbook_0071639586

C h a p t e r 4 : T r a n s m i s s i o n L i n e s a n d I m p e d a n c e M a t c h i n g 147
where
984 × vF
λ =
f
(4.63)
where v F = velocity factor of each specific cable
f = frequency in megahertz
l = expressed in feet
Note that l is not the free-space wavelength but the (shorter) wavelength of the wave
in the transmission line. Make sure to use the velocity factor for the correct cable in each
of the two calculations!
Although the sign of B can be selected as either – or +, the use of + is preferred because
a shorter matching section results. If the term inside the square root sign is negative,
B is an imaginary number and the section cannot be constructed with the two line
impedances selected. Typically that will happen if the two impedances are too similar
and the required transformation is too great. The sign of A cannot be chosen by the user,
but if it turns out to be negative, add 180 degrees to the result.
Series-section transformers can also be designed graphically with the help of the
Smith charts of Chap. 26.