Re: Joining co-axial cable outdoors

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Edwin Spector wrote:

Footnote: It's probably worth spending a couple of minutes making the joint as 'nice' as possible. A discontinuity in a TV feed could produce a reflection which could give a ghost on the telly image.
Edwin.
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Edwin Spector wrote:

Unlikly given
(a the small refelction that has been demonstrated = 20dB (b teh fact that teh distances involved are small. you need a fair few meters to give a discernile ghost.
a line is 60Us long, roughly, so for 600 dots per line rough image quality, 100ns or less is indiscernible.
That equates to 3meters of cable. Quite a bit

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The Natural Philosopher wrote:

Good points. However, just to clarify, the 20dB return loss was for the reasonably good joint. The bad joint had a return loss of about 11.6 dB. That's about 7% of the power being reflected. (20dB reflection corresponds to 1%). I'm not sure how noticable that ghost would be.
Also, don't forget that signals travel more slowly down cable than in air, by a factor of (say) 0.5 to 0.7 (.) So, it would only take 2m of cable to give a discernable ghost (by your test).
I added the note about reflections after a colleague's account of a bad ghost which was traced to a cable joint made using a 30A mains junction box. Who ya' gonna call?
Edwin.
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In fact there a very well written article on a subject called pre echo where there is a possibility of direct pick up off poor cables directly from the transmitter and the delayed signal via a distribution system. Very good site this is if you're into TV etc...
http://www.wrightsaerials.tv/Resources/Pre-Echo.pdf
--
Tony Sayer


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Edwin Spector wrote:

3db is about 50% reflection, so probably less than 1db

I don't think its that much reduction.

I never advocated a bad joint in a 30A mains junction box :-)
Its been a long time, but certainly one TV installation I saw, had lost the entire connection to the aerial earth lead...it worked, but certain stations were vastly better than others - big drop - 10 meters plus.
No ghosts tho - or very few. Possibly because the discontinutity was at the antenna.

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Except that he's mentioned several times that it's not for TV
--
geoff

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