Jeff wrote:
On 07/07/2015 19:44, wrote:
Ian Jackson wrote:
In message , Jerry Stuckle
writes
Sure, there is ALWAYS VSWR. It may be 1:1, but it's always there.
If there's no reflection, there can be no standing wave. So, being
pedantic, there's no such thing as an SWR of 1:1!
Despite the name, VSWR is defined in terms of complex impedances
and wavelengths, not "waves" of any kind.
VSWR is defined as Vmax/Vmin on the transmission line and is independent
of phase or wavelength.
It can also be expressed in terms of the *magnitude* of the reflection
co-efficient. 1+|p|/1-|p|
The reflection coefficient can also be expresses as (Zl - Zo)/(Zl + Zo)
where Zl is the complex load impedance and Zo is the complex source
impedance.
The complex impedances are functions of wavelengths, i.e. frequency.
All complex numbers have a frequency dependant part.
In the real world of transmission lines and antennas, the source
impedance is usually 50 + j0 and thus the second part is ignored.
or in terms of forward and reflected power
1+sqtr(Pr/Pf)/1-sqrt(Pr/Pf))
VSWR is the same regardless, of phase, when measured at any point on a
lossless line. The phase of the reflection co-efficient will change but
not its magnitude.
Jeff
--
Jim Pennino