Why are Coaxial cables 75 or 50 Ohm?
This is a question that has been annoying me for some time...does
anyone know whay the characteristic impedance of caxial cables was
chosen to be 50 or 75 Ohm? Is it somekind of trade-off?
After working in the RF realm for many years, I had begun to wonder about
this same question... after looking around on the WWW for a while seeking
an answer, I came across a document which explained a little about this
issue... I can't say that this is the final answer, I'm not even sure
it's the actual truth of the matter, it's just what my research turned up,
and I'd certainly like to hear if any radio 'old-timers' know anything
about the true history of coax...
to the best of my recollection -
in the 'early' days of radio engineering, transmission lines were
constructed from those materials already "at-hand"... coaxial lines were
built using existing pipe and tubing sizes, with air as the dielectric, and
the inner conductor supported by bakelite discs at regular intervals.
(wonder how they made 'bends' in the line, used a junction box of some
the "characteristic impedance" of any coax line is a function of the outer
diameter of the inner conductor, the inner diameter of the outer conductor,
and the dielectric material between them; be it air, teflon, or goat milk
cheese (probably not a good choice for low-loss cables).
as it happened, due to the sizes of the materials originally used, the
impedance fell right at about 50 ohms, and it sort of stuck as a standard.
later research is said to have revealed that lower impedance designs
produced transmission lines with greater bandwidth, while higher impedance
designs made for lower losses (less unit capacitance per unit length?)...
the choice of 75 ohms over 50 for systems such as cable television and
video (yes, Virginia, there *are* 75 ohm BNC connectors) was supposedly due
to these issues, a choice of less loss over greater bandwidth. seems kind
of odd, tho, I'd have guessed greater bandwidth to be more important than
loss for a multi-channel distribution arrangement.
the same article also claimed that the 'european' standard was 60 ohms.
I've never heard of that before, and from what I recall of working with
european equipment, it was always on a 50 ohm standard. the article was
also apparently written by an English fellow who may be having a bit of a
laugh over it all. anyone ever seen a Rohde & Schwartz RF signal generator
with a 60 ohm output? I haven't.
my best guess is that the idea regarding available materials is close to
the real story. the rest of the information... well, I'd like to do more
research before accepting it as gospel.
I'll welcome any comments, dissention, clarification, love/hate mail or
lunatic ramblings on the subject.
> After working in the RF realm for many years, I had begun to wonder aboutSince you didn't quote or paraphrase I have no idea what question you've been
> this same question...
- --- In Electronics_101@y..., Jim Purcell <jpurcell@w...> wrote:
> Steve,uhhh.... yeah... well, the uhh, "subject" header does sorta state the question pretty plainly.
> > After working in the RF realm for many years, I had begun to wonder about
> > this same question...
> Since you didn't quote or paraphrase I have no idea what question you've been
> wondering about.
but I do get your point, I did sorta forget to address my response to the person asking the question and to the question itself.
I'll make it a point in future posts to be more concise. please don't tell Miss Manners about my egregious omission?
>uhhh.... yeah... well, the uhh, "subject" header does sorta state the question pretty plainly.I have a bad habit of not reading subject headers since threads often go far
afield from the original topic. I recall that on my favorite BBS they could go on for