Inspired by Rodrigo , one of this blog reader, who ask about why used 50 Ohm impedance ?
The answer can be shown in the graph above. This was produced by two
researchers, Lloyd Espenscheid and Herman Affel, working for Bell Labs
in 1929.
They were going to send RF signals (4 MHz) for hundred of miles
carrying a thousand telephone calls. They needed a cable that would
carry high voltage and high power. In the graph below, you can see the
ideal rating for each. For high voltage, the perfect impedance is 60
ohms. For high power, the perfect impedance is 30 ohms.
This means, clearly, that there is NO perfect impedance to do both.
What they ended up with was a compromise number, and that number was 50
ohms.
You will note that 50 ohms is closer to 60 than it is to 30, and that is
because voltage is the factor that will kill your cable. Just ask any
transmitter engineer. They talk about VSWR, voltage standing wave ratio,
all the time. If their coax blows up, it is voltage that is the
culprit.
So why not 60 ohms? Just look at the power handling at 60 ohms -
below 50%. It is horrible! At the compromise value of 50 ohms, the power
has improved a little. So 50 ohm cables are intended to be used to
carry power and voltage, like the output of a transmitter. If you have a
small signal, like video, or receive antenna signals, the graph above
shows that the lowest loss or attenuation is 75 ohms.
Still, I get a lot of feedback from people who use 50 ohms for small
signals; you can see above that they are taking a 2-3 dB hit in
attenuation. Excuses I hear are
“It's too late to change now!” or “That's the impedance of the box itself.”
This is especially true of most test gear, which is universally 50
ohms. You have to buy a matching network to use it at 75 ohms or any
other impedance. But there are lots of applications where 50 ohms is the
best choice.
( Source : Belden.com)
