18-08-2019, 02:07 PM
(This post was last modified: 18-08-2019, 02:11 PM by 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.
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.
