TV Coax--300 vs 75 ohm

[Oh good, something that I know just enough about to be dangerous ;-) ]

The 300 ohm stuff that you remember was not coax, it was the old twin-lead cable. That configuration has the advantage of somewhat lower losses than coax, and also a better impedance match to folded dipole antennas (a common TV antenna configuration). A disadvantage is its greater susceptibility to interference and also to losses caused by neighboring conductive surfaces (e.g., aluminum siding).

When TV sets went coax, along with the rise of cable systems, twin-lead could still be run from an antenna but it needed a balun (balanced (twin-lead) to unbalanced (coax)) converter between the two.

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Rich Webb     Norfolk, VA

Perhaps I should have mentioned that what I have from The Shack uses BNC at both ends, and that I'm using a yagi.

The really old stuff I suspect is flat like a ribbon.

There was shielded 300 Ohm Twinlead, but it was rare. The last I saw was 45+ years ago.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

300 ohm coax sounds to me not so easy to get. Coax is usually 50-52 ohms or 75, much less commonly other values generally less than 300 ohms. TV coax is 75. Coax connectors for TV and video are for 75 ohm coax cable. 300 ohm TV cable is twinlead, not coax. What twinlead connects to is generally a pair of screws.

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 - Don Klipstein (don@misty.com)

I wired the outlaws' antenna system with that stuff, um, 40 years ago. ;-)

No. TVs use 75ohm coax. Antiques may have 300ohm twinlead inputs. You can convert from twinlead to coax, or verse visa, using a Balun.

I'm sure they sell RG6. I'd be surprised if RadioShaft sold anything else these days.

Good move! You still got ripped a new one, but not quite as large. ;-)

What I saw was early foam insulation over standard twin lead, with a layer of foil. It was a pain in the dimbulb to work with, and was quickly replaced by RG59.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

Same stuff. RG59 was around but I would have needed two baluns and eaten the insertion loss.

A BNC coax cable from radio shack was probably an old thin ethernat cable

50 ohms.

Yeah this stuff:

--
?? 100% natural

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

A good bauln has less than a half dB loss, but some of the early stuff was crap. One thing a lot of people don't know is that there was a balun in the TV tuner so you could use cheap twin lead.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

Half a dB here and a half there and pretty soon you have a snow storm. The set only had screws on the back. I wasn't about to hack into the set.

]

lower

twin-lead

;-)

the

The problem was that early RG59 had a copper braid for a shield, and sold poly insulation with had a lot higher loss than a pair of baluns. It wasn't undil foil shielded RG59 & RG6 was availible that 75 ohm feeds made sense. Early VHF CATV systems used RG11 between amplifiers, and horrible tilt.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.