[time-nuts] 50 vs 75 ohm cables

Dave Brown tractorb at ihug.co.nz
Thu May 10 10:01:50 EDT 2007


Peter
I believe most of the telco world adopted 75 ohms back in the early 
days of wideband analog multi channel transmission - this was before 
WW2 when 50 ohm transmission lines were virtually unknown. 75 ohms 
reigned (almost) supreme.  But remember that coaxial transmission 
lines were not widely used back then. It was developments in WW2 
(primarily radar related) that led to 50 ohms being adopted as a 
standard vhf/uhf/microwave cable impedance.  There are good reasons 
that govern the choice of cable impedance that have to do with 
minmiising loss and/or maximising breakdown voltage, etc. And it so 
happens that a 75 ohm air dielectric line, when filled with what was 
back then a very common dielectric, becomes an ~ 50 ohm line.
Suffice to say, a 50 ohm cable provides minimum loss for a given size 
outer conductor.  Google for 'why 50 ohms' and you will find out some 
of the background.

So, post WW2, the RF comms industry developed rapidly and so did the 
use of 50 ohms with it. But telcos (and video/broadcasting 
organisations) were already using 75 ohms and had no reason to change. 
So they didn't.
The transition to digital transmission in the 1970/80s was no reason 
to change from 75 ohms - so it stayed as the unbalanced  transmssion 
line standard in the telco world.  Balanced 120 ohm transmission lines 
were introduced for telcos soon after the first digtal transmission 
systems came into use but are only used at the lower line speeds such 
as E1/T1 etc.  Higher data rate systems still use 75 ohm unbalanced 
lines almost exclusively where they are transported on copper. But 
most are on optical fibre these days.  There is still a lot of 75 ohm 
copper at system interconnect points though.

HTH
DaveB, NZ

----- Original Message ----- 
From: "Magnus Danielson" <cfmd at bredband.net>
To: <time-nuts at febo.com>; <pvince at theiet.org>
Sent: Friday, May 11, 2007 12:53 AM
Subject: Re: [time-nuts] 50 vs 75 ohm cables


> From: Peter Vince <pvince at theiet.org>
> Subject: [time-nuts] 50 vs 75 ohm cables
> Date: Thu, 10 May 2007 13:39:27 +0100
> Message-ID: <v04210100c268c17d2911@[10.0.0.3]>
>
>> I came across some telecom equipment the other day which had
>> reference outputs marked as 75 ohms.  I work in television, not
>> telecoms, and we use 75 ohm connections for video, but with most RF
>> stuff being (I believe) 50 ohms, and certainly all the HP and other
>> counters seem to have 50 ohm inputs, I rather assumed telecoms used
>> 50 ohms - obviously not!
>
> No, it is 75 Ohm for unbalanced PDH signals such as E1, E2, E3 and 
> E4 as well
> as the american DS1, DS2, DS3 and DS4.
>
>>  Can anyone tell me how and why the 50/75
>> ohm distinction came about?  Was it perhaps a VHS/Betamax type 
>> issue
>> with different manufacturers going their own ways, and then with so
>> much equipment out in the field,  neither side was willing to 
>> change?
>
> There where something about damping etc.
>
> Cheers,
> Magnus
>
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