Power standing wave null? discussed the “Power Standing Wave” concept unfolding on social media.
Already a correspondent has asked if the graphs given in Power standing wave null? can be replicated in SimNEC.
They can. The original Mathcad graphs were wrt displacement from the source along the line to the load, and the sign of displacement is -ve (consistent with the Telegraphers Equation). So, that requires a bit of manipulation in SimNEC, and because SimNEC does not allow us to sample a TL element at an arbitrary displacement, the following model uses two TL elements of overall length 30m, and by adjusting the length of each we can move the observation point (T1 input).
The calculations of lengths and power are visible in the popups.
In a passive system, real power (ie watts) must decrease monotonically from source to load. Online postings showing otherwise attest to the knowledge of AC fundamentals of the modeller / poster.
Why (T1.V*Conj(T1.I)).R ?
This relies on basic principles of Alternating Current. The power at a point excited by a sine wave is \(P=V I^*\) (meaning V times conjugate of I) where V and I are RMS values, it is a complex quantity comprising the real power (watts) and reactive power (VAr). We are interested in the real power, so we multiply V times conjugate of I at T1 input, and take the real component of it for real power.
Homework
Interested readers can construct the other plots.