[time-nuts] 50 vs 75 ohm cables

[John Day]

At 02:31 PM 5/10/2007, SAIDJACK at aol.com wrote:
>Hi guys,
>
>some practical comments:
>
>    * 50 Ohms transmission lines are much easier to fabricate  on standard 4,
>6, or 8 layer PCB's. 75 Ohms traces are very thin and thus have  issues in
>manufacturing accuracy.
>
>    * Feeding a 50 Ohm source into a 75 Ohm load gives a VSWR of  1.5, a
>mismatch loss of only 0.177dBm, and a return loss of -13.98dBm, so 
>not  much power
>is lost due to the mismatch loss. Of course at high power the 
>-14dB  return is
>a problem.
>
>    * 50 Ohm connectors are mechanically more stable and easier to
>manufacture. 75 Ohm BNC for example removes the internal dielectric 
>and  leaves the
>center pins "floating" in free air so they can break more  easily.
>
>    * 75 Ohms requires thicker dielectrics, or higher dk  dielectrics -
>tougher to manufacture.
>
>    * 75 Ohms cables usually use dirt-cheap and flaky  F-connectors. 
> These get
>jammed easily, and every F-Connector I have seen so far  has a different
>center pin length. Whoever designed F-Connectors (and S-Video  connectors!!)
>should be held accountable in my opinion. There are so  many better 
>ways to design
>a (cost-effective) connection.

Excellent comments Said. Having spent the last 37 years designing RF 
and microwave equipment I am rather glad that I don't have to use too 
many things like 75 ohm BNC. Of much better robustness and 
performance are things like the 1.6/5.6 and other European type which 
I have seen precious little of in North America.

For years I would not use 'F' connectors at all. But then I didn't 
live in North America for most of that time and although recent years 
have seen 'F' types become ubiquitous throughout most of the world, 
they were thankfully almost completely unknown in the UK, Europe or 
Australia until then.

Because of the smaller centre conductor in 75 ohm cables the support 
offered by the dielectric is not as great as for 50 ohm and often 
inadequate to maintain constant impedance. Some types use a foam 
dielectric which is even more of a pain! Even worse is the poor 
quality of the cable itself. Often the outer braid has very poor 
coverage and the cable often behaves more like "leaky" coax or an 
antenna array!

Fortunately we are seeing more and more DVI and HDMI around here and 
they seem to have much better characteristics, the S-Video 
interconnec system is, to put it mildly, worthy of the junk bin!

John


>
>bye,
>Said
>
>
>
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