[solved] 50 Ohms (coax) cables [10Base2/5 Ethernet] - Page 1
Forum section: electronics / beginners

author: faekjarz
author_rank: Regular Contributor
timestamp: April 29, 2017, 03:57:13 pm
content: Hey there, beloved enginerds!A while ago, i offered an ole Thinnet set (10Base2, 10 mbps) with a coaxial cable on ebay. I received an enquiry whether that cable is a "50 Ohms cable". That other dude might have been as clueless as i am. I measured the centre conductor, as well as resistance between centre and shielding, of that coax cable, and it was not 50 Ohms, way below actually (centre to shield was O.L.).1) So what makes a 50 Ohms cable a 50 Ohms cable?2) Is there even something as a "50 Ohms Cable"™?I suspect a misnomer of some sort …i mean, a conductor is a conductor, and its purpose is to conduct those little pixies, as efficiently as possible. I guess those 50 Ohms aren't a property of the conductor itself, rather one of the edges of the (10Base2/5 Ethernet) physical layer, the whole system, which must be terminated with a 50 Ohms resistor (Terminator). ...to prevent signal reflections.

author: Unknown
author_rank: Unknown
timestamp: April 29, 2017, 04:06:43 pm
content: Quote from: faekjarz on April 29, 2017, 03:57:13 pmSort of; it isn't a property of any single conductor, but rather the system of the center conductor of the coax and it's shield, as a transmission line. The "50 ohms" refers to the AC impedance of the cable:https://en.wikipedia.org/wiki/Transmission_line#Input_impedance_of_transmission_line. This comes into play when the wavelength of the signal you are sending is small in comparison to the length of the cable. However the cable itself will not present 50 ohms of DC resistance from end to end or center conductor to shield.The 50 ohm terminators on each end are loads that match the impedance to prevent reflections of the signal:https://en.wikipedia.org/wiki/Reflections_of_signals_on_conducting_linesTo my memory, 10base-2 was always run over 50 ohm coax.

author: NottheDan
author_rank: Frequent Contributor
timestamp: April 29, 2017, 04:11:58 pm
content: This should help:

author: faekjarz
author_rank: Regular Contributor
timestamp: April 29, 2017, 04:47:25 pm
content: Oh, what a captivating accent you've discovered, @NottheDan! *chuckles* (I'm from Germany, so i recognise that accent 10 miles against the wind)The title looks promising, thank you.That will be part of the next 30 minutes of my life.…edit: (I'm 15 minutes in now.) That video comes with a formula and a demonstration. Excellent! Thanks again!…edit2: to conclude in text form; the Ohms rating of a conductor Z = sqrt( L("inductance") / C("capacitance") ). …so i would've needed an inductance measurement device (my DMM only does C) to answer the Dude's question.

author: NottheDan
author_rank: Frequent Contributor
timestamp: April 30, 2017, 09:31:34 am
content: Quote from: faekjarz on April 29, 2017, 04:47:25 pmAyup. One of the cheapish ones from eBay should be good enough for the job. Though the very cheapest may fail for all the usual reasons.But there's another angle to this. If it is the proper thing and not something hacked together on the it will be 50Ohm coax. That's what the specifications for 10Base2/5 call for after all.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 30, 2017, 11:13:17 am
content: 10BASE2 uses 50\$\Omega\$ coaxial cable BY DEFINITION. If it is a proper 10BASE2 then there is no need to measure anything.There is also 10BASE5 which uses larger and thicker coaxial cable like RG8, but also 50\$\Omega\$ impedance.Cable impedance is very important for proper transmission of high-speed data (or Radio Frequency signals) But it is not something you can measure simply. You should be able to tell what impedance a cable is designed for by looking up the number. For example RG58 cable (typically used for "Thinnet") is 50\$\Omega\$ by design. While RG59 (and RG6) is 75\$\Omega\$.10BASE2 ("Thinnet"):https://en.wikipedia.org/wiki/10BASE2RG-58:https://en.wikipedia.org/wiki/RG-58Coaxial cable:https://en.wikipedia.org/wiki/Coaxial_cableI like "enginerd". Since I live in Oregon, perhaps I can use that title without expecting to be fined $500 by the government.