[time-nuts] 50 vs 75 ohm cables
John Day
johnday at wordsnimages.com
Thu May 10 23:26:57 EDT 2007
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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