Thanks guys for the further graphs and explanations - appreciated, but some further questions
- Despite the new graphs, I still cannot see that early narrow 50 tone wave reflecting across the whole spectrum of tones.
- Would you not expect this narrow wave to make an appearance in the smooth lines of the wider wave. There are several places where the main wave is smooth enough and wide enough that I would expect it to make an appearance
- From other graphs, namely SNR, bit load & QLN we can very clearly see that something is causing noise ingress up to about tone 500. Does this have any bearing on why the wave changed shape after tone 500.
I've never seen a bridge tap wave distinctly change width shape before. It is my understanding that a bridge tap produces a fairly even wave that will continue across the whole spectrum. It is the frequency at which the first dip occurs which is used to calculate the length of the bridge tap.
Even though we may primarily use hlog for detection of bridge tap, it does also show up capacitive problems that can cause noise and increased attenuation.
I have seen certain types of severe noise make an appearance in hlog as one off narrow dips on many occasions before now. iirc even some of the sample graphs in other learning documentation can show slight noise on hlog of perfect lines or outside of the fault area.
I'm afraid you've lost me a bit with the more complex calculations and what you are trying to do with the table. It was my assumption that wombat was doing further investigations to be able to calculate the appx location of the bridge tap. I may have not gotten what he was trying to achieve, but I have at least once seen him be able to estimate the distance of the fault from the NTE. I distinctly remember BS congratulating him upon diagnosis of being within 1m. I never really got much involved in those conversations primarily because I don't have tools to analyse the raw data and was content with wombat saying he would later write up his findings, I just waited for that to appear and only read what was said in public.
I'm not quite sure what the additional table was meant to bring other than I know wombat was doing further investigations into it, but conversely I also pretty sure I do remember him concluding that the first dip was the most important. Since b*cat was actively involved in what wombat was doing with that table, then he will know far more about it than me.
The formula I use is the standard set out by the broadband forum TR-197 namely
160 ÷ Frequency in MHz = Tap Length in feet using the frequency in MHz at the point of the first null, or center frequency of the dip.
- 4) Going back to the table and how it mentions subdips. Is that not for graphs that show 2 (or more) recurring dips within a larger wave. Hence the reason for the term "sub-dips". I dont have any descriptive documentation other than what is shown, but to me it suggests further notches that show up within what would otherwise be a smooth wave formation. Chrys's graph shows no sub-dips that I can see, if we exclude that noisy mess up to tone ~500.
To explain what I mean about sub-dips, I attach awfully crude diagram where "0" defines the main wave and "1" shows the sub-dips. Apols for my non-leet paint skills, but hopefully it sufficiently shows what I mean about subdips.
IMHO because of all the noise in tones <500, I'm really not sure if we can draw anything conclusive about those first early dips. I really cannot see anything where they repeat outside of that noise zone. The first regular and continual wave I see starts at around tone 500. I deliberately didn't mention a distance in my first post because I was unsure about those early tones and to the naked eye the first visible continual wave only started after that. However when pushed I think I said about 22m... which uses the TR-197 formula as such
160 ÷ (Tone 500 = 2.16MHz) = 74 ft = 22.5m
If we start theorising, based on we know chrys has had no water in his direct area and there was no electricity in a totally different part of the city:
What if a burst mains caused the collapsed duct and damaged the telephone cable or joint. The cable and joints are underground and under water. Moisture in the cable is known to cause resistance changes and capacitance faults, which can and do show on the hlog. What I'm not certain about is if this sort of damage can then also trigger off a bridge tap effect. It doesn't seem impossible that damage could cause something elsewhere not to terminate properly.
Other theories seem to ignore everything that is happening in those tones up to 500 - which are abundantly clear in all the other graphs.
I am interested in what you are trying to do, but it's just that I cannot understand what is going on with the 2x50 tone wide waves that appear to vanish after the noise stops. If there wasn't all that noise in several other graphs telling us something else was going on, then perhaps I could. I don't like making assumptions without knowing full facts but with the mention of water problems, its water ingress and damaged cable/joint more making my whiskers tingle - especially from what all 4 of the graphs (QLN,bit-load,SNR & hlog) are showing us for tones 0-500. I'm not trying to dismiss anything you say, its just that those early tones are telling me something different is going on.