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Old 29th Mar 2023, 10:16 am   #1
lesmw0sec
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Default History of co-ax.

Had to pause for thought a while back, when some research I was assisting with indicated that underground aerial feeders to an old Marconi VLW station (Waunfawr), was by what looked like lead-sheathed coax, but may have been simply screened wires.

This got me to wondering when proper coax (as a constant impedance feeder) was modelled in theory and put to use as such for the first time. I am aware of its wide use during WW2, and possibly it was employed for the first TV transmissions pre-war?
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Old 29th Mar 2023, 10:32 am   #2
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Default Re: History of co-ax.

Quote from https://en.wikipedia.org/wiki/Coaxial_cable

Quote:
Coaxial cable was used in the first (1858) and following transatlantic cable installations, but its theory was not described until 1880 by English physicist, engineer, and mathematician Oliver Heaviside, who patented the design in that year (British patent No. 1,407).
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Old 29th Mar 2023, 10:39 am   #3
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Default Re: History of co-ax.

Quote:
Originally Posted by Terry_VK5TM View Post
Quote from https://en.wikipedia.org/wiki/Coaxial_cable

Quote:
Coaxial cable was used in the first (1858) and following transatlantic cable installations, but its theory was not described until 1880 by English physicist, engineer, and mathematician Oliver Heaviside, who patented the design in that year (British patent No. 1,407).
Strewth - rather earlier than I imagined, if that is the case!
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Old 29th Mar 2023, 1:15 pm   #4
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Default Re: History of co-ax.

The history of treating long (Transatlantic) cables as constant-impedance transmission lines involved one William Thomson, later Lord Kelvin. Run with badly mismatched source and load even C keyed Morse was garbled. Out of this came the concept of modifying Zo with added series inductance to suit the terminal equipment. Oddly, I've not seen any reports of attempts to modify the equipment to match the cable. Out of this came the discrete loading coils added to long distance phone lines.

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Old 29th Mar 2023, 1:16 pm   #5
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Default Re: History of co-ax.

The first transatlantic cables were insulated with Gutta Percha. The company that came up with the idea became the Gutta Percha Company.

Fast forward - that company became Cable and Wireless.

Gutta Percha as a material from a tree is still alive and kicking. It is used as a tooth packing material after root canal work. So I have a tooth filled with the same material that was used on the first Atlantic cables

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Old 29th Mar 2023, 2:10 pm   #6
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Default Re: History of co-ax.

My copy of "Short Wave Wireless Communication" [published 1932] by Ladner and Stoner of the Marconi co. has some considerable discussion of what it describes as 'concentric feeders' both of the solid and flexible/semi-flexible varieties, intended for use as antenna feeders for transmission [powers up to 100Kw] and reception. They are of the semi-airspaced variety with insulators made of 'Trolitul' or Steatite every 3 inches apart to support the central conductor in the external copper or lead tube.

Joints are made by 'wiping' in the same way as the old lead water and gas pipes! Mention is also made of pressurising the tube with dry air or inert gas to prevent moisture ingress when the feeder is buried or immersed.

What we now know as Velocity-factor is described in said book as 'velocity slip'.
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Old 29th Mar 2023, 3:26 pm   #7
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Default Re: History of co-ax.

Attached is a shot (From a paper by John Rowlands) showing the remains of lead-sheathed feeder from the Waunfawr transmitter (near Caernarfon). Given that the very low frequencies in use at the time was practically audio, I have doubts as to whether or not this was designed with any characteristic Z in mind.
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Old 29th Mar 2023, 5:36 pm   #8
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Default Re: History of co-ax.

Crikey, this topic takes me back to my days at Pirelli Cable’s telecomms cable division about 50 years ago!

One of my projects was to investigate what the Wiki article calls ‘Signal leakage’ but I think we called it ‘transconductance’. This was the phenomena whereby a signal launched down the centre of a long coax could induce a signal in the outer shield. Another long coax cable running alongside it and bonded(earthed) at both ends to the first coax could have a signal induced into its outer shield which in turn could/would induce a signal in the centre conductor of the second cable. I hope that makes sense - it was a long time ago. I remember making physical models of two different types of semi-rigid coax to try and verify the theoretical results but I do not think I had much success as the cables themselves were by manufacturing necessity poor approximations to the ‘perfect’ mathematical cables.

We made two different types of semi-rigid cables .174 and . 375 (surface area in sq inches) that were air spaced, I recall the air spacers were about the size of shirt buttons on the larger cables. The biggest cable we made were .375 times 18 ‘tubes’ which all told came in at about the size of a chaps biceps. We laid it between the GPO research place in Martlesham and the Birmingham PO tower. Each tube was designed to work up to 500Mhz. Great fun whizzing around the streets of Birmingham in a Land Rover lifting up manholes and testing these tubes out in situ. TDR etc. I wonder if it’s still there and in use?

In the labs we had a/the Italian machine that made flexible coax, the sort of cables used for domestic TV’s etc. To see that in action was something to behold, we could never work out how it was putting the copper braid over the insulation, it seemed very fiendish and no matter how closely we peered at it we just couldn’t work it out. Happy days indeed.
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