You may have heard the term impedance matching if you have ever worked with radio frequency (RF) signals, such as when planning a home antenna, in broadcast, or when designing an AV system. It may be technical (it is), but it is not the obsession of an engineer alone. That is one of the main reasons that your signals get through or don’t.
And right in that equation? 75-ohm coaxial cable.
We’ll explain this in simple terms: Picture yourself screaming into a tunnel. When the tunnel remains with the same shape all through, your voice flows smoothly. However, when it just suddenly shrinks or expands, you get the form of the echo or distortion.
That is essentially what RF signals turn into when not properly impedance matched. They reflect back, giving what is referred to as signal reflection.
So, what is impedance anyway?
This is the resistance that a cable presents to an alternating current, which is the RF signal in this context. It is expressed in ohms. The operative word in dealing with clean signal transmission is maintaining consistency in impedance throughout the system:
- At the source, such as an antenna or camera
- Through the cable
- Ultimately, the device being used or connected, such as a TV or tuner.
What happens now when you introduce a cable of a different impedance into the situation? The signal does not know what to do with it. Some of it goes on, some is reflected. You end up with ghosting signals, poor signals, pixelation, or, in extreme cases, full signal loss.
Why 75 Ohms? Why not something different?
75-ohm coax is a sweet middle ground between video signal/digital signal and minimum signal loss. Indeed, 75 ohms is the impedance around which almost all consumer-level and broadcast video is built, as well as many RF systems.
The special part of 75-ohm coax entails its composition: the diameter of the central conductor, the dielectric material used to separate it (the insulation), and how it is all spaced. This exact geometry maintains the impedance level and adapts very well to high frequency, with particular application on video and digital audio.
Signal reflection in the real world
Perhaps you have done this without knowing it. You install your antenna, and everything is fine until some other channels go off air or even distort. Or you swap out one of your cables in your AV rack, and then all of a sudden, you are dealing with strange interference that you can barely explain.
The problem is most likely to be impedance mismatch, and the signal is fighting with itself.
When you run a 75-ohm coaxial cable (impedance-matched to your gear) anywhere, you essentially make a smooth tunnel through which your signal travels, without echo or other bouncebacks.
Wrapping up
Your choice of cable makes a difference. 75-ohm coax can appear to be simply a wire, but behind the scenes, there are many things that it gets right when more is at stake.
The next time you want to wire up a system, whether it’s your home theater system or a broadcast studio, don’t just pick anything that you have lying around. A 75-ohm coax is designed to keep the signal moving in one direction.
