[time-nuts] 50 vs 75 ohm cables

[SAIDJACK at aol.com]

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.
 
bye,
Said



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