[aprssig] Why is coax 50 ohm ?

[Dave Baxter]

Good question, well presented, ?????????

50 Ohm is generally used in communications and many instrumentation
systems.

75 for civilian broadcast, and many landline telecoms.

92 for some high speed networks, and some instrumentation.

Then you have the realy wierd stuff, used for auto AM antenna's, that
has a very thin center conductor, in a semi air spaced tube, but not too
well located.  Goodness knows what the Z0 of that is, but it loads the
front end of the AM radio's very little.

The "Characteristic impedance (Z0)" is a function of the ratio between
the size of the inner conductor, and internal size of the outer, and the
dielectric constant of the insulation between them.  You can find the
formula on the web quite easily if you look.  As well as the forms of
all sorts of "Co-Ax" transmission line.  Round conductor in square
section tube etc.

"Co Ax" is of course Co Axial.  The centers of each conductor are
coincident, or the same in other words.

For a given length, of the same sort of size (outer diameter) 50r tends
to be the lossier, but can handle higher power (heavier conductors
perhaps?)  92r the least loss, but only seems good for relatively low
power.  Real flimsy conductors on the stuff I've seen, and we have here.

How they came to be used in different industries, historically, who
knows?

I can appreciate that to drive a load near 100r with high speed digital
stuff, takes less power than to drive say 50r cable, but?.....

75r was supposed to have come about so it is said, as a good match to a
folded dipole, for domestic TV/FM radio, but again???   It depends on
who you talk to, and what books you read.

There are also semi-rigid cables about of 25 and even 12 Ohm impedance.
We use that in the matching sections of the wideband QRO amplifiers we
handle, and the combiners/splitters too.

You can also do some magic things with mixing different lengths of two
types of co-ax.  Matching (narrow band) from say 50 to 75r is quite
easy.  There are many books and articles on the subject.

Just one perspective from the UK/EU side of the pond.  No doubt there
are other perspectives from other places/industries/walks of life.

73.

Dave G0WBX.



________________________________

	From: Andrew Rich [mailto:vk4tec at tech-software.net] 
	Sent: Tuesday, March 28, 2006 12:10 PM
	To: TAPR List
	Subject: [aprssig] Why is coax 50 ohm ?
	
	
	Is there a reason why coax is 50 ohm ?
	 
	And 75 ohm ? 
	 
	And 92 ohms ?
	 
	Is it historical or is there a scienific reason why ?

	-----------------------------------------
	Andrew Rich
	Amateur radio callsign VK4TEC
	email: vk4tec at tech-software.net
<mailto:vk4tec at tech-software.net> 
	web: http://www.tech-software.net
	Brisbane AUSTRALIA