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Re: Time Domain Reflectometer (was Re: internet outages)




On Monday 24 December 2018 14:10:44 rhkramer@xxxxxxxxx wrote:

> Hmm, intended to send this to the list, sent to John Hasler, only, who
> did respond -- I hope he will copy his reply to the ist (or tell me it
> is ok to do so).
>
> On Monday, December 24, 2018 08:21:57 AM John Hasler wrote:
> > As to TDRs, if you can get by without actually seeing pictures of
> > all the impedence bumps
> >
> >
> > you can get by with a fast counter, a high risetime
> > pulse generator, and a couple of fast comparators.  Maybe $20 at
> > Digikey.
>
> Can you elaborate a little on how that would work?
I can imagine the first fast comparator would enable the counter, 
previously held in reset and the second comparator is then enabled, and 
its threshhold adjusted for a stable stop of the counter, giving the 
number of input cycles between the start and the stop. Decent calculator 
math would then give you the distance to the major impedance disturbance 
that caused the echo.

You need a GHz (at least) signal source to count, the higher, the more 
accurate. The delay of course is 2 way, out to the fault and back, so 
convert that to distance one way with a /2.000 after using the usual 
hambooks 984/frequency derivation to get the wavelength in feet IIRC. 
Ideally the frequency should be a wavelength short enough to give decent 
accuracy because this method will only give you the number of cycles as 
an integer. You send the pulse and wait for the echo to come back and 
stop the counter. Multiply the wavelength by the counter to get how far 
away the fault is, multiply that by the PV of the line being tested and 
divide by 2 to get the one way distance.

Clear as mud, right? 2000 feet of 6.125" inch line can be fun due to 
losses weakening the return signal. Specially a fast one like a tunnel 
diode might make. And no bets at all for a broadcast antenna because the 
pulse is not frequency shaped to match the antenna's operating 
frequency, so from the fine matcher on into the element array, its a 
broadband mess that splatters all over the TDR screen. Seeing that on a 
real TDR with trained eyeballs seems to be the only way to tell that, 
from a cracked and burned up elbow 50 feet below it at the tower top to 
antenna feed connections.  And that 50 feet might be a weeks work for a 
tower crew with some antenna styles. And of course it always happens in 
bad weather, making the high steel work dangerous because there may be a 
6" thick layer of ice on everything 2000 feet up. 

That broken elbow might be the starting point, but the fire then moves 
down the line towards the power source and may burn up 600 feet of line 
before the transmitters VSWR protection circuits can shut it down. Been 
there, done that, several times at what was once NETV's KXNE transmitter 
on UHF channel 19. Rosemounts rime ice detectors buried on the antenna 
structure had an average lifetime of a year. They cost quite a bit, but 
their failure mode was always safe mode, so you didn't know a thing 
until the main beam power breaker opened in response to the rising VSWR. 
By then you were out 10 to 50 grand and several days to get a crew 
rounded up to work on it, to get it back on the air. More than once in 
that decade we had to let them sit around, or go do another job while 
the ice went away. One time litterally tons of it had formed a flag 
about 8 feet long on the downwind side of that storm.  Cleaning 600 feet 
of that line and rebuilding it took time, rags by the pickup load of 
bags, alcohol in several 5 gallon buckets, and teflon parts aren't free 
either. I'm glad I'm retired now.

Now, back to the asinine arguments about boobs. Is this our regularly 
scheduled programming for this week? :)

Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>