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#11
July 8th 15, 07:07 PM
posted to rec.radio.amateur.antenna
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An antenna question--43 ft vertical
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 |
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