Co-ax V 300 ohm flat twin.

[Hartley118]

Did we adopt coax in the UK because we were preoccupied with ignition interference problems?  Presumably the coax shield helps to reduce feeder-induced intereference

I also wondered about that, Martin, but dismissed it because of the poor screening provided by the coax used - more like leaky feeder than screened - and wondered if the inherent interference cancelling ability of a balanced feeder would not have performed better under such circumstances?

Obviously, the feeder would have to have been balanced at both ends but the cost savings from using twin feeder as against coax would surely have cancelled out the cost of the extra balun at the aerial end?

My distant memory of aerial coax in the 1950s is of a much heavier braided shield than on today's UHF stuff, so perhaps it did offer some useful shielding.

Martin

[ppppenguin]

Today's decent cable (CAI approved, often generically if incorrectly called CT100) has fair bit of braid plus a foil shield. Much better than anything we routinely used before.

I've seen 1950s co-ax with reasonable briad coverage. Most of it was horribly high loss, just about acceptable for Band 1, hopeless for anything higher frequency.

[Terry]

I've seen 1950s co-ax with reasonable briad coverage. 

Not on most that I saw!

In fact, the braid was so loose that a common way, although incorrect*, of fitting a coax plug was to ease the braid back slightly - this really opened it up! - and stick a screwdriver blade though the braid at the point where it exited the sheath. Now hook the blade under the dielectric and ease the centre conductor out through the hole in the blade. Pull the braid to close it up again and fold back along the sheath so that it gets trapped by the cable clamp. An 'expert' can do the inner-through-the-braid trick in a couple of seconds - the proper way* takes a bit longer!

* The proper way to terminate coax in a coaxial plug is to comb out the braid  and bend it at right angles to the inner in a radial fashion. Then trim it back and ensure it is jammed between the cable clamp and the plug body when the plug is assembled to make a true coaxial connection. See http://www.megalithia.com/elect/bellinglee/ for a pictorial guide.

[Hartley118]

I wonder what proportion of the millions of Belling Lee aerial plugs feeding UK TV sets are actually correctly wired according to the above instructions with a tip-soldered centre contact.

5%?

Martin

[pwdrive]

I read the instructions, looks like one soldered joint and one dry joint...

Lawrence.

[Synchrodyne]

Regarding the US use of 300 R flat twin in the early days, Fink (in “Television Engineering”, 2nd, 1952; see pp.382,383) stated that most TV receiver makers adopted 300 R balanced inputs, for use with 300 R flat twin, because it was cheaper and of lower loss, particularly at higher frequencies, than the available coaxial feeders.  Those of the latter might be used in domestic installations were more costly and had higher losses than was the case for flat twin, the more so at UHF, and larger diameter, lower loss coaxial cables were prohibitively costly.

In the European case, Kerkhof & Werner (in “Television”, 1952; see pp.348-350) did not comment upon receiver maker choices, but the detailed discussion was about balanced feeders and also the advantages of using balanced RF and mixer/oscillator stages, with conversion to unbalanced thereafter, rather than at the input.  The closing comment in that section was:  “An unbalanced (coaxial) aerial feeder may also be used, but in that case the aerial must also be unbalanced or a balancing transformer must be incorporated between the dipole and the coaxial cable”.

In respect of the US change to 75 R coaxial, these pages from a 1969 Heathkit catalogue give some sense of its timing.  Evidently it was a relatively new idea (for the US market) in 1968-69.

       

I imagine that the rapid growth of cable TV after about 1970 also helped with the change, given that the cable systems mostly use coax as far as I know.  When I was living in the USA, RG6 seemed to be the norm for cable installations and also for domestic aerial installations.  The thinner and lossier RG59 seemed to be used more for short interconnects.

And one might also observe, in respect of the “righteousness” of using baluns to connect balanced aerials to coaxial cables, given that most aerials had a nominal impedance of 300 R, some kind of impedance matching device was required anyway to get to 75 R, and that being the case, it was no more difficult to make it a balun than say a “bal-bal”.


Cheers,

Steve

[colly0410]

I'm wondering if the UK used co-ax almost from the get-go was because we used vertical polarisation for the early main transmitters? When the horizontal TX's like Divis & Pontop Pike fired up co-ax had sort of become the norm...

[ppppenguin]

Can't see it's got anything to do with polarisation.

[Mike Watterson]

Nothing to do with polarisation.
Decent coax needs more expensive plastic core for low loss at higher frequencies, the solution to that initially was the semi-air-space core. Then foam.
Braid coverage is about screening. You don't need as much at VHF as UHF. Sat boxes use up to 2.2GHz, so foil is needed.
The ratio of internal diameter of braid and outer diameter of core sets the impedance. The loss rises with frequency due to skin effect, thus Cable TV cable is often copper plated steel inner. Lower loss cables for satellite box feeds or distribution or commercial masts thus use progressively fatter coax. I have some where the inner is a copper tube. Very low loss. I'm going to use inner as an AM loop and outer as screen. It's pretty rigid.

The 300 Ohm twin was maybe cheaper in USA & Japan than coax in late 1930s to 1960s and loss can be lowered by slots in the plastic. Lower loss also needs fatter wire and then change spacing to maintain characteristic impedance as it's ratio of spacing and wire diameter.

The screening is poorer on twin feed as it needs to be in free space. Loss & balance (screening) affected by surfaces and pipes.

Decent coax is far better than twin at Band III and certainly UHF. Certainly 300 Ohm twin was cheap for band 1 (USA Radio started there and moved to band II in 1945). Japan was/is 76 to 90 for FM radio, so twin feed a cheap solution.

Cable TV was 42MHz to 300MHz, then 500MHz, so used coax. Also easier to put in pipes. Band II added in many countries for FM, then 5MHz to 65MHz added for Broadband return path instead of Band 1 TV. Upper limit gradually moved from 500Mhz in a couple of steps to 860MHz or 1200MHz.

[pwdrive]

Just out of interest, dipole and coax etc:

http://downloads.bbc.co.uk/rd/pubs/reports/1957-26.pdf

Lawrence.