Long Freeview TV aerial extension cable - use F plugs & satellite cable?

I want to re-route our existing TV aerial cable which presently drops from the aerial on the central chimney down the roof and front of the house. Rather than try to sling the cable over the roof to hang down the back I plan to draw it into the attic and make an extension. We're in a weak signal area (Reading) which is not even suposed to be able to get Freeview (though we can - I've tried it). To keep losses down and make termination easier (than bloody BL plugs!) I'm thinking of using 'satellite' type cable (as used by NTL^H^H^HVirgin) and F plugs.
Does this seem like a good idea? And what type of cable? Some F plugs are specced to fit "PH100, PF100, RG6 and CT100" cable: what's the difference between them? Has CT100 been superseeded by H109F and WC100?
http://www.satcure.co.uk/accs/page8.htm seems to have much lower prices on cables than anywhere else I've come across - anyone had any dealings with them? They suggest RG6 "for connecting additional TV sets via the RF out socket" which seems to indicate it's OK for UHF (though maybe not low-loss?)
And what about twist-on connectors versus crimp connectors, and if crimps are the bee's knees what sort of crimp tool?
tia
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John Stumbles

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I expect someone will be along shortly who can answer on all points. In the meantime, if you have not seen it already, you might care to read Bill Wright's excellent (dare I say seminal?) article on cables http://www.wrightsaerials.tv/coaxcablequalityhmdim.htm to see where RG6 stands in the rankings.
HTH
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Robin



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On Wed, 09 May 2007 23:00:27 +0000, Robin wrote:

It does indeed - excellent, thanks. I've started a wiki page on it here
http://wiki.diyfaq.org.uk/index.php?title=TV_and_Satellite_Aerial_Installation
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John Stumbles

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John Stumbles wrote:

Very good.
Just to add that 50 Ohm cable is used, almost exclusively, in radio communications and this is its main use.
Steve
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Steve wrote:

Odd. I have only ever uses 50 ohm cable around the lab. Or for Ethernet. All anteannae are fed via 75ohm cable, as this matches the natural impedance of a dipole.
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The Natural Philosopher wrote:

Maybe I should have made that clearer and said commercial radio communications. 50 ohm is the industry standard.
All my aerials (yagis) were fed with 50 ohm coax 7/8" & 1/2" Heliax before I moved, but often, as you say, people do use 75 ohm for things like dipoles but rarely in the commercial world.
Steve
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Steve wrote:

Ah, all now makes sense..yes for some reason 50ohm is the de-facto standard in high quality stuff. Where the cost of matching it to the antenna is trivial compared with the installation costs. It is a complete nono for domestic installations of aerials though. Without a balun etc anyway. Gets you nasty reflections and standing waves..
Once past the first amplifier..if the kit is designed for it, yep, BNC or better and 50ohm all the way..
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The Natural Philosopher wrote:

Only if you are considering dipoles where the impedance will be about 73 ohm and only if that dipole is at a half wavelength or multiple thereof above average ground. Most dipoles are anything but 73 ohm in reality, depending on their surroundings. So most aerials need matching to their feeder anyway.

A lot of it is historical, dating back from WW2, as a compromise between loss, power handling and saving copper.
75 ohm cable uses less copper for a given cable diameter to achieve the same loss, because the inner conductor is thinner and the dielectric is correspondingly thicker. This makes 75 ohm cheaper to make than the equivalent 50 ohm version... all other things being equal of course.
Because of the fatter inner conductor and especially because of the "skin effect", 50 ohm cable can handle more RF power than 75 ohm of the same overall diameter for a given temperature rise. This is despite the additional current that must flow in a 50 ohm system for a given power and is all due to the increased surface area of the coax inner and with slightly less thermal insulation from the dielectric.

Once past the first amplifier, for household receiving purposes it makes little difference! In fact a properly made-off good quality F connector would probably outperform a BNC (given that both used comparable coax) which ultimately relies on friction for connection surface on both inner and outer whilst a F connector has only one friction connection.
Steve
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Most pro aerials use 50ohm. After all, there are very few simple dipoles in use and many of them are folded with a natural 300 ohm impedance.
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From KT24 - in "Leafy Surrey"

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On Thu, 10 May 2007 10:29:36 +0000, Steve wrote:

It's used for Ethernet too, but I was writing from the POV of TV+Sat
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John Stumbles

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Would old ethernet cable be any good as a TV cable? I've got loads of that left over from when I switched to Cat5.
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On Fri, 11 May 2007 00:53:28 +0100, Edster wrote:

No. It's the wrong impedance.
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Yes, the ethernet cable would be 50 ohms. I wouldn't use it out of choice, but there's a good chance it would still work. Try a run of it, and find out! Ian.
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Ian Jackson wrote:

Not good. You will get standing waves - peaks and nulls
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Please quantify. Ian.
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Ian Jackson wrote:

Not possible without knowing cable length, specific losses and reception frequency.
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OK. Let's say 10m, 20dB/100m @ 650MHz. F = 650MHz. Ian.
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Ian Jackson wrote:

Bugger..its been 30 years since I had that sort of question in an exam..
Mm,. need to konow the relative propagation speed in the cable as well..assume its about .9c..so a quarter wave at 650MHz will be around 128cm..
So the 10m will be almost exactly 78 quarter waves..Hmm. You should get a modest peak around that length.
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But how big will the peak-to-trough amplitude be?
What will be the worst-case slope across an 8MHz channel?
How much signal level will you lose compared with using 75 ohm cable with the same loss>
[To answer these question, you might choose to assign a notional worst case value to the STB input RLR .]
Will any of this have a noticeable impact on the quality of a digital signal?
Ian.
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I swapped a 5m 75ohm TV cable for a 5m 50ohm ethernet cable with the ends cut off and twist on connectors on it, and didn't notice any difference.
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