50 Ohms: The Forgotten Impedance

If you play with coax, short for coaxial cable, you probably know this it is available in a number of different impedances. The most common is 75 ohm, like video cable or antenna cable, but in fact our products range from 32 ohms up to 124 ohms.pb299-cable-300px

Why all these different numbers? It’s not an accident of course, and there is a reason for each one. Today, we’re going to take a quick look at 50 ohm coax cable.

Belden makes hundreds of 50 ohm cables, including a whole line of ultra-low loss versions (Belden 7805 to Belden 7977). The two largest versions (Belden 7976 and 7977) are shown in the photograph below. They are HUGE. The 7977 has a diameter of .600″ six-tenths of an inch! This is the largest coax cable that we make.

But first of all, why 50, or any other number? The answer can be shown in the graph below. This was produced by two researchers, Lloyd Espenscheid and Herman Affel, working for Bell Labs in 1929. Read more »

How Far Can I Go? – Part 2

Last time I promised to tell you how to calculate a voltage drop. This could be very valuable if you have a cable that is carrying both signal and DC power such as control cable, broadband cable for powering satellite dishes or fancy cables like SMPTE 311M that contain power conductors to remote power video cameras. 


Phantom powering microphones is a bit more complicated and I will talk about that in a following blog. The hard part of all this will be finding out all the things you need to know before you even do any calculations. Don’t be scared of the math. It’s simple algebra. If you can add, subtract, multiply and divide, you’re good to go. Read more »

How Far Can I Go? – Part 1

One day, Georg Simon Ohm did an experiment. He built a voltaic pile – what we would now call a battery. To judge the voltage, he attached a wire to each end, held one in his hand and touched the other to his tongue. Ouch! He did notice one interesting thing. If he stuck with copper wires and he went to a larger wire, his tongue hurt more! I am not making this up! You can Google it. 


That told him there must be some relationship between the size of the wire and the voltage running down that wire. He soon realized that this could be described in a formula, the formula we now call Ohm’s Law. He told this to his scientific friends and they were aghast! The idea that there was a relationship between voltage and current and the size of the wire (resistance) was so controversial, that they begged him not to reveal it. And so these results were not published until his death. It is pretty much the same story as Copernicus and the sun-centered planets. And we’re still using Ohm’s Law today. Read more »

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