From time to time I see online discussion about determining matched line loss (MLL) from Return Loss.
Something like 150 years ago a self educated genius thought about the problem of ‘smearing’ of telegraph signals in submarine cables and developed a model for the behavior of transmission lines that remains a very good model today. (The guy’s day job was as a telegraphist.)
He needed to develop new mathematics and new concepts like reactance, and he did this in the face of recognised engineers of the day dismissing the ideas.
The smearing problem was different propagation speed of the fundamental and harmonic components of the telegraphy waveform, he referred to it as distortion and in terms of his RLGC transmission line model he defined the requirements for a “Distortionless Line”. Zo for a Distortionless line is a purely real number, and a Lossless Line is a special case of a Distortionless Line. Continue reading RF transmission lines – quite old art
I was asked whether the nanoVNA can display VSWR in terms of 400Ω or some other arbitrary impedance.
Some antenna analysers and VNAs support display of results in terms of some specified impedance other than 50Ω, sometimes only a limited fixed set.
The direct answer to the question is “probably no, not directly on the ‘original’ nanoVNA today”, there are many firmware forks and many PC clients for nanovna, and now many significantly hardware versions appearing. Things may change.
However, if you can save a set of impedance measurements, they can be converted to VSWR relative to some other impedance reference.
A similar set of measurements could be made with a standalone nanoVNA and saved, but given that it is such a clumsy device and its USB socket has become so unreliable, I will not repeat the measurement but use the data used for the article above.
So we have a table of frequency and (R,X) measured looking into a ‘real’ 450Ω feed line, so we will calculate wrt 450Ω but we could use any reference.
(Purdum 2020) describes a small transmitting loop (STL) which is a little novel in that it uses an arrangement of four circular conductor loops, two in parallel in series with the other two in parallel.
There is some evidence that the common 1.024mm (#18) single core CCS windowed ladder line advertised as 30% IACS conductivity supplied recently may be closer to 21%. This is based solely on comparison of measured DC resistance with specification, but that is a strong hint that the copper cladding is less than specification.
Above is a diagram of the so-called “cable balun”.
My evaluation essentially showed that it was not effective in an example practical scenario where one might want to use a balun, and that of itself, it was not likely to significantly reduce common mode current in most scenarios.
Radcom Mar 2020 published a letter in “The last word” from the author defending the device citing a NEC model of one scenario, curiously though without explanation, a different topology to the diagram above from the original article. Note also that it is a structure in free space with no discussion of how that is relevant to real world antennas near ground. Continue reading Radcom Feb 2019 “cable balun” – comment on Radcom “The last word” letter