Quote:
I apologize in advance for copying your statements and posting them outside the quote box. I have changed your text to red to keep track. I've not mastered this forum's editing features..
The reason it doesn't have a defined impedance is that it doesn't have a stable, defined geometry. If you twist it (easily done) the impedance changes, unlike ladder line (which mustn't be twisted), coaxial or twisted pair (which has a specific TPI). I don't know where you get the 'whacky geometries' idea from but the assertion that that they 'seem incapable of' maintaining an impedance which is uniform over length is inaccurate. The impedance is maintained as long as the cable geometry is not abused, i.e. it is used as specified.
What he was speaking of
was the lack of geometric control of the wires. Many times people will twist the wires using a drill or the like, and when you release the cable, it will untwist. When it does this, the spacing between the conductors will open up, and you've lost control of the parameters of the cable. To twist two or more wires together and maintain the characteristic impedance, you have to not twist the individual conductors as the drill does. I made such a contraption to twist up a quad #12awg speaker cable run, it had 4 spools of wire each on it's own lazy susan, and the overall spool assembly was on a larger lazy susan. (I could post a pic of it, but it seems I've made a mess of my default picture album). What I really need is a ten year old kid to show me how to work this forum)... As for "wacky geometries, some of the vendors out there do indeed have wacky geometries, and some do not control the geometry sufficiently to maintain any impedance along the length.
ps..twisting cables does not change the cable's impedance in general. Only if the twist brings the conductors into intimate contact where they previously were not will the impedance change.
So which is it? Is it 'whenever a switchbox is used' or just sometimes, as in 'sometimes just swapping or switching doesn't preserve the differences being tested for'?
The introduction of any additional entity to make switching easier always requires consideration of the change. If you are trying to test a cable which has an extremely low inductance per foot against one with a normal inductance per foot, it is very important to consider the inductance of any additional switching mechanism. If you are testing power cords, the grounding resistance and the hot/neutral coupling to external grounding loops is important.
EMI is a problem only when suitable measures to prevent it are not taken. I have taken numerous items through CISPR tests. Some failed at first, but it has always been possible to engineer a solution.
You are attempting to shoehorn RF testing where it is not being discussed. As someone once said to me, you have a hammer so consider everything to be a thumb..
RF susceptibility and radiation are indeed important, but that's not what's being discussed. EMC is wider in scope. As I said, I can either answer your questions, or point you to content.
Your introduction of EMC to invalidate comparison testing is a red herring. EMI can be engineered down to whatever level is required.
As I stated, it is important for you to learn the distinction between full EMC and RF.
If necessary testing could be carried out inside a Faraday cage.
This also assumes only RF, not EMC. It is not about far field incursion of planar waves, it is about nearfield coupling.
Please tell me how the inclusion of a switch for comparison purposes automatically invalidates ABX testing for reasons other than problems with EMI.
I already have, so do not understand why you repeat this question.
Why would 'swapping or switching' not preserve the differences being tested for?
Already answered.
What is it about 'even putting the box in' that 'renders comparisons meaningless'? It sounds close to superstition to me. Please explain the scientific basis for this assertion.
Already have.
edit:I've removed my statement formerly here referencing the post of yours that was deleted, this is only fair..
I will admit that much of this has not been taught in venues of higher education, so I cannot blame you nor others for not understanding it. Continued refusal to learn it, that is a different story. Heck, the knowledge doesn't even have to come from me, as that is an obvious problem for you.
As I've stated, I can answer your questions, or I can refer you to links. Once understood, it's not rocket science.
Cheers, jn