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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Underwater tv



 
 
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  #11  
Old January 5th 17, 01:31 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,818
Default Underwater tv

On 05/01/2017 07:42, Brian Gaff wrote:
Well using your logic, then any mountain that got in the way of a tv signal
should not matter.


I haven't expressed any logic. I've expressed what appears to be a
paradox, in the hope that someone will be able to resolve it.

Bill
  #12  
Old January 5th 17, 01:41 PM posted to uk.tech.digital-tv
Phi
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"Bob Latham" wrote in message
...
In article ,
R. Mark Clayton wrote:
On Wednesday, 4 January 2017 15:07:13 UTC, Brian Gaff wrote:
I was just thinking. If you wanted to transmit tv to an underwater
habitat,
how could you do it?


Wire or fibre, possible light.


Presumably light can only go so far under water and
radio even less far.
Also I wonder what the speed of light is under water.


About two thirds that in a vacuum / free air. Ditto copper.


Slower in diamond.


I don't understand. I thought Einstein had taught us that the speed of
light was a constant, always the same for all observers everywhere?

Bob.


Depends how fast you are moving

  #13  
Old January 5th 17, 06:01 PM posted to uk.tech.digital-tv
Tim+[_4_]
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wrote:
On Thu, 5 Jan 2017 07:42:39 -0000, "Brian Gaff"
wrote:


Does anyone recall that miracle stuff on Tomorros world where they showed
you could run a normal electric drill under water if you sprayed it in?
I never expected that to be saleable as you would probably have hod to keep
on doing it over and over to remain even semi safe. Of course the scceptics
amongst us suspected the water was distilled and non conductive in any
case.


I remember that demo, ISTR that the manufacturers representative held
the drill in his hand as it was plunged into what was probably a large
aquarium. I have always wondered under what name the product was sold
and if it is still around.

G.Harman


WD40?

Tim

--
Please don't feed the trolls
  #14  
Old January 5th 17, 06:25 PM posted to uk.tech.digital-tv
Benderthe.evilrobot
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Posts: 148
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"Brian Gaff" wrote in message
news
Not recommended. tjhe little chinaman in the manual with the frown an
umbrella and a tv should point you this way I feel.


It was a production line style servicing operation on a vanload of TVs that
had just been procured from the local dump.

The colour sets were left to dry out - the portable was something to do
while I wait.

The prevailing theory was that the portables wouldn't self destruct so
expensively.

  #15  
Old January 5th 17, 06:59 PM posted to uk.tech.digital-tv
Chris Youlden[_5_]
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Default Underwater tv

On 05/01/2017 14:31, Bill Wright wrote:
On 05/01/2017 07:42, Brian Gaff wrote:
Well using your logic, then any mountain that got in the way of a tv
signal
should not matter.


I haven't expressed any logic. I've expressed what appears to be a
paradox, in the hope that someone will be able to resolve it.

Bill


From memory the attenuation of sea water is because salt water is a
damn good conductor. Fresh water presents less of a problem.

Whilst looking for the frequency/attenuation chart in my old copy of BBC
Communications Data Sheets, I found the article below which provides
some light reading on the subject, and also contains similar charts.

https://goo.gl/DTVR5u

Regarding ELF communications between subs and base, the U.K. also uses
that mode. In fact communications between the UK government and HMS
Conqueror during the Falklands War giving the order to attack the
Belgrano was via ELF.

The base site for this was a certain small island alongside the premier
naval base on the South Coast.

When BBC South first had an SNG, clearance had to be obtained to uplink
from the MOD for each location. If within a short distance and pointing
in the general direction of a military site, permission would be
withheld, with no reason given. This occurred regularly in the
Portsmouth area and by triangulation over a period of time, one
determined where the "site" was.


--

Chris
  #17  
Old January 5th 17, 07:29 PM posted to uk.tech.digital-tv
Benderthe.evilrobot
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"Brian Gaff" wrote in message
news
Oh really, well, so what do you do with all these old tvs?


It was what I did for a living.

  #18  
Old January 5th 17, 09:33 PM posted to uk.tech.digital-tv
David Woolley[_2_]
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On 04/01/17 17:49, R. Mark Clayton wrote:
About two thirds that in a vacuum / free air. Ditto copper


Actually, the phase velocity in copper is over two times that in vacuo
http://refractiveindex.info/?shelf=3d&book=metals&page=copper. It is
the group velocity that is limited to to c. The group velocity in
copper is slightly more than 1/3 of c.

I think you are being confused by the velocity in a cable, which is
mainly the result of propagation in the insulation, not the wire. the
extreme refractive index tend to keep the wave out of the metal.
  #19  
Old January 5th 17, 09:34 PM posted to uk.tech.digital-tv
David Woolley[_2_]
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On 04/01/17 23:04, Bill Wright wrote:
Since EM waves/particles travel in a vacuum it appears that don't need a
medium. Why then would they be slower in a medium, and stopped by some
media?


Because they are hopping from electron to electron.
  #20  
Old January 5th 17, 11:00 PM posted to uk.tech.digital-tv
David Woolley[_2_]
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Posts: 518
Default Underwater tv

On 05/01/17 22:33, David Woolley wrote:
Actually, the phase velocity in copper is over two times that in vacuo
http://refractiveindex.info/?shelf=3d&book=metals&page=copper. It is
the group velocity that is limited to to c. The group velocity in
copper is slightly more than 1/3 of c.



These figures are for optical frequencies. Extrapolating from the
graph, the refractive index at radio frequencies is going to be in the
millions. If I interpret
https://en.wikipedia.org/wiki/Relative_permittivity#Metals correctly,
the refractive index is very large and very imaginary (in the maths
sense) at RF.
 




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