Black wire blues.....

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Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: Black wire blues.....

Post by Pchristy »

Bluejets: That is almost certainly down to the environment. All TVs - and most VHF radios - in the UK use 75 ohm co-ax rather than 300 ohm. A 300 ohm feeder is supposed to be a balanced system, but a lot of manufacturers took the easy way out and connected one end to ground. If you then add in a coastal environment (lots of salt water about - very corrosive!), or other similar pollutants, static in the air will favour the easiest path to ground and drag the corrosive particles in the air with it. (Think lightning conductors - the most extreme static discharge!)

Its a similar scenario, but just involves different chemicals.

As Mike points out, the finer the wire, the greater the surface area of the bundle, and the more susceptible it will be.

In case you are wondering why some use 75 ohms, and others 300 ohms: 50-75 ohms is the typical impedance of a half wave dipole - or more accurately, a quarter wave monopole over a perfect reflector (ground plane). 300 ohms is the typical impedance of a folded dipole - a popular configuration in the early years of VHF broadcasting. All these figures are affected by adding director and reflector elements, but make a good starting point.

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Pete
bluejets
Posts: 316
Joined: 19 Jun 2019, 04:09

Re: Black wire blues.....

Post by bluejets »

Hi Pete,
Yes it was in the days of black and white TV on VHF and most of our area is within 20Klm of the coast but it used to happen inland as well.(outback)
Perhaps prevailing winds.
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