Does anyone know if Twinax cable (as used in older ethernet computer networks) is compatible with domestic coax cable as used for TV's etc. I have quite a bit of this wired into the structure of the house and it would be very convenient if I could use this for wiring up the TV's
Many thanks
Mike
Do you mean 10base2 (Thin black stuff as opposed to fat yellow stuff)? Some of that was referred to as twinax, but there's another twinax used for gig networking (1000baseCX).
TV aerials have a characteristic impedance of 75 ohms - and 10base2 was IIRC 50 ohms. So it may not work that well unless you correct with a balun at each end, as to whether that would work, I've not idea.
Are you in a position to test a bit out of curiousity?
Tim
If you really mean Twinax cable as was used for hooking up terminals on IBM System 3x and AS400s then it is its own thing and not Ethernet.
There are two versions of Twinax - 78ohm and 95 ohm. 78ohm would be reasonably close to the required 75 ohm for TV distribution, but 90 ohn would not. Added to this, the cable has a poor high frequency response, so for UHF TV is likely to be poor.
There was also a twin coax used for 10base2 ethernet together with special connectors from Amphenol. The concept was that you could unplug the connector from the wall and the network bus would be maintained. These were horrendously unreliable and if one failed, the whole segment would go down. The cable, like all 10base2 was 50ohm.
The short answer is that you counld try this because the runs are likely to be short, but don't be surprised if the results are poor, especially on high numbered TV channels.
.andy
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Nice answer ;-)
T i m
Yes the thin black stuff known as 10base2
Could you please expand on what a balun is and where these are to be found It might be worth experimenting if there is a reasonable chance of success
Yes but I wanted to know first if it was worth the effort
Mike
They're a balance to unbalance trasformer. Check the usual component suppliers. Might even be possible to make them, given the frequencies involved - but that's a guess.
But to be honest, I'd just use video coax for baseband - it's cheap enough.
Using transformers to drive composite down a balanced line is bound to introduce losses, so you'd be into some form of amp to sort it out as well.
OK - then I'm familiar with this.
A balun is an impedance balancing transformer. I'm not an RF expert but I did do physics. A TV arial is designed assuming a 75 ohm circuit. If you wack a bit of cable on that is rated to a difference impedance you will get reflections (exactly as light reflects off water - that's an impedance mismatch) and the system won't resonate at the desired frequencies.
In short it's sub optimal and you get a crap signal down the other end.
You can get baluns from Maplin etc. but I'm not sure if 75-50 ohm is a normal type or not.
Well - if you had a strong signal to start with it might work. Or might not. It's an inherently suboptimal system - but it might be good enough.
I've never tried it so I can't tell you whether it will just work or not - sorry. Need an RF bod or someone who's actually tried it for that.
Sorry to sound unhelpful but I honestly don't know. If you can experiment, then that's the answer. If you have to commit a lot of time, then first see if you can find a pair of 50-75 ohm baluns becaus eyou'll probably be able to save the day with them.
Cheers
Timbo
PS Sorry for any typos - I'm a bit p*ssed.
At the end of the day I'd just buy a reel of RG6 or CT100 which is the right stuff for domestic aerial installation. I'm an RF engineer and I wouldn't use the system what you're discussing!.....
Its wrong imedance - 50 ohms IIRC.
Better than a piece of wet string, but not a patch on stuff desined for teh job.
"balun" is also (mis)used to refer to an impedance transformer. In this context it's possible to make something using cut lengths of the
50 and 75 ohm coax. I expect digging about on the web would find many references.I think you have your threads crossed Dave. This one is about using old coax based network cable to carry RF, not the one about wiring a place with new Cat5 and using that for "TV" (not sure if that is baseband or RF...)
Wander over to the Canford site, 675m is the distance quoted for those baluns before "significant degradation of the signal" occurs. As Canford are broadcast suppliers I expect that quoted phrase to mean that there is very little ringing or HF loss until you get a fair bit further away than 675m. Not played with a pair though...
The impedance is in the terminators which are plugged into either end of the network, and not in the cable itself.
Apart from that, I couldn't say whether it would be suitable for TV.
Hth Bill
In article , invalid writes
For info, the cable has a 'characteristic' impedance too, which is based on the combination of its inductance and capacitance. At the sort of frequencies involved in TV, satelite (& computer networks) a mismatch between the source and/or cable and/or receiver termination is likely to cause significant problems. As many have said, this cable is not the right stuff for TV.
No that isn't correct. The cable has a characteristic impedance and if terminators are used they should match thst impedance.
So if 50 ohm cable is used for TV, there will be a mismatch with consequent deleterious effects.
.andy
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Hmmm, just went out to the garage and pulled out some cable and terminators.
The cable is "Thin net 50 Ohm 10-base2" it measures 0 Ohms as near as dammit. The terminator measures 50 Ohms as near as dammit. Admittedly there is no signal being passed down it, and to determine what the impedance would be with a sine wave (video, data etc) passed down that same piece of cable is a different matter. The impedance will vary according to the frequency of the signal passed down it.
The quoted impedance is based on it being used for a specific technology or application, and will be different under different applications.
I have no idea whether this would be an issue for television signals, and over what sort of distances.
I would suggest the OP plugs in an aerial/tv and tries it.
Regards Bill
You cant measure impendance with a normal dmm. Its not resistance in the normal sense, its made up of capacitance, some resistance and inductance. Take a look at a good amateur radio book, it should show you a theoretical circuit of coax using lumped C AND L.
Dave
Yes, but presumably this is with a DC meter. It will measure 50ohms for the terminators and zero along the conductors.
There is the impedance mismatch and also that this type of cable will show high losses at higher frequencies.
The combination of these may produce quite uneven results across the range.
.andy
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Or impedance even... B-)
And as the capacitance and inductance are influenced by the physical cable dimensions you can tell if you have 50 or 75 ohm cable by looking at the end. Fairly sure that for same screen diameter, 50 ohm has a larger dia core and also fairly sure that frequency doesn't have anything to do with the characteristic impedance but it's a very long time since I did AC Theory...
No, it depends on many things - including the core conductor, the quality of the cable, whether it is solid core or multi core etc.
The frequency influences the impedances, info about it here...
the reactance of an inductor - merely part of the story.
The characteristic impedance of a cable is what you'd measure between the wires with your multimeter if you could connect it to an infinite length of the cable. Its value is the square root of the ratio of the inductance and capacitance per unit length of the wires: Zo = sqrt(L/C)). To a very good approximation, in most cases, it *doesn't* vary with the frequency, except at very low frequencies where the resistance of the wires becomes significant compared to the inductive reactance.
This is a more relevant URL:
In message , invalid writes
No it's not, the cable has a characteristic impedance which depends on the geometry and dielectric of the cable.
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