Alan,
As I remember well the catv 75 Ohms cable needs the smallest amount of
copper for a certain attenuation...
Another statement from that time was that 60 Ohms would have been the
optimum impedance for that specific purpose.
Wil - pa0bwl
-----Original Message-----
From: Alan Melia [mailto:[email protected]]
Sent: Friday, May 09, 2003 1:33 AM
To: LF-Group
Subject: LF: Cantat submerged cable system
Hi Johan and Jim, as Johan will well remember (I have bored him with
personal e-mails before) I was involved peripherally in Cantat and in the
later transistor cable systems. I can conform the cable was a nominal 50
ohms. What you have to remember is that a whole load of this was under many
thousands of feet of water, which presumably has some effect on the
dimensions !! I must ask one of my old buddies if 54 ohm coax cable becomes
50 ohms if you squeeze it hard !! I can also confirm the + / - 7kV power
feed to the ends. One of the jobs I had was advising on protecting the
transistor amps in the later cables from the eventuallity of a disgruntled
fisherman taking his axe to a trawl-snagged cable. You can imagine what
happens when 7000m of 50ohm coax (300pF per metre) charged up to 7kV is
suddenly shorted by the axe blade. I understand from oft repeated anecdotes
that the result was quite spectacular, and there was not a lot of a
"lumberjack style" long-handle axe blade left afterwards. Also the axeman
had somwhat of a headache and tingling feeling, after he picked himself up.
We must have been reasonable successful as I dont believe we lost a single
repeater to that cause.
Geomag storms were a problem.....it is quite some antenna when you get a
signal of about 4kV at the terminals !! I think it operated like a
center-tapped single turn loop...more like a simple generator. It was usual
to lose an hour or so of traffic time I believe when conditions were bad. I
only think I am glad it was not me manning the power-feed shed on some
lonely beach under those conditions.
In Cantat the amps consisted of 3 valves in cascade (sons of the SP41 !!)
sat in the middle of a bridge of directional filters so that it amplified
sigs going both ways. The DC was stripped off by a LPF and the HT was
developed across zener I believe. I have a sample of the later
transistor-amp power-zeners.
Yes the path loss may have been more than the Satellite link, but it didnt
cost as much to lay and commission. I have not checked but I believe the
cable is still usable over 40 years after it was commissioned, but I suspect
a lot of the terminal gear has been stripped out. I bet there is not a lot
of life in Telstar !!
Back to topic ....would the fact that TV cable is normally semi air spaced
account for the use of 72 ohms for our (in UK anyway) TV feeder??
Cheers de Alan G3NYK
[email protected]
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