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- Path: sparky!uunet!europa.asd.contel.com!emory!athena.cs.uga.edu!mcovingt
- From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
- Newsgroups: sci.electronics
- Subject: Re: Coax Question
- Summary: 50 and 75 ohms explained
- Message-ID: <1992Jul22.052519.901@athena.cs.uga.edu>
- Date: 22 Jul 92 05:25:19 GMT
- References: <1992Jul20.140143.19341@yang.earlham.edu>
- Organization: University of Georgia, Athens
- Lines: 66
-
- In article <1992Jul20.140143.19341@yang.earlham.edu> jeremiah@yang.earlham.edu (Jeremiah Wilton) writes:
- >I am wondering what kind of coax is used in Ethernet thin and thick wire
- >connections. I am unconvinced that some really expensive special stuff
- >is necessary. Might not RG-58 or 59 work for thinwire? How about
- >composite video cable (the thin cheap stuff)?
-
- Thinwire Ethernet is RG-58 (52-ohm).
-
- >Why is RG-58 used in low-power FM applications (like receiving) while
- >RG-59 is used for TV? I know about 50 ohms vs. 75 ohms, but what does that
- >mean, and why are they different sizes?
-
- Well, let's see.
-
- Cable impedance depends on the diameter of the central wire, diameter of
- the shield, and properties of the dielectric. So you can have the same
- impedance in various diameters (larger ones have lower loss, generally).
-
- 75 ohms is the characteristic impedance of a dipole antenna. TV and FM
- receivers generally use horizontally polarized dipole antennas, and hence
- 75-ohm cable (or 300-ohm cable for folded dipoles; actually the most
- common setup nowadays, due to haphazard evolution, is a 300-ohm antenna
- and a transformer matching it to 75-ohm cable).
-
- In commercial 2-way radio setups, the antenna is not a dipole, but rather
- a "ground plane" antenna (1 vertical segment on one side, and 3 or 4
- drooping almost-horizontal segments on the other side). The characteristic
- impedance is somewhere around 50 ohms, hence the popularity of 52-ohm
- cable.
-
- What _are_ these ohms? Well, imagine a cable that is millions of miles
- long. Connect a battery at one end. Obviously, a current flows into
- the cable so that the battery voltage can get to the other end. This
- current is proportional to the voltage (of course) and is determined by
- the inductance and capacitance along the cable.
-
- That is, from the battery's viewpoint, the cable looks like a resistor
- with a particular resistance.
-
- This is only true _until_ the voltage gets all the way to the other end.
- (That's why I asked you to imagine a tremendously long cable.)
- When the voltage finally gets to the other end, either of 3 things will
- happen:
-
- - if the end is open-circuited, current flow will stop, and the effect
- will be as if a voltage source had suddenly appeared there
- (a "reflection" of the original voltage source);
-
- - if the end is shorted, current flow will increase, as if a voltage source
- of _opposite_ polarity had suddenly appeared there (again, a "reflection");
-
- - if the end is terminated with a resistor of exactly the same resistance
- as the characteristic impedance of the cable, then the current will just
- keep flowing same as before.
-
- That's why we terminate Ethernet cables with resistors. It prevents
- reflections.
-
-
-
-
- --
- ==========================================================================
- Michael A. Covington, Ph.D. | mcovingt@uga.cc.uga.edu | ham radio N4TMI
- Artificial Intelligence Programs | U of Georgia | Athens, GA 30602 U.S.A.
- ==========================================================================
-