Dealing with Impedance Mismatching


When selecting coax cable or certain types of coaxial connectors and adapters there are a variety of factors to consider such as jacket type, shielding, and loss.  Today we want to focus on impedance, specifically what to do if you encounter differing impedances.

In coax, there are two categories of impedance 50 Ohm and 75 Ohm.   Ohm is the measurement of electrical resistance.  The physical properties of the cable determine if it's impedance.  The hallmarks of 50 Ohm coax are high power handling and low attenuation and are used in RF applications.  75 Ohm coax is designed for low power, signal transmission efficiency, which is typically used in audio/video applications.

An easy-to-overlook aspect of cable choice is impedance.  In alternating current (AC) circuits, impedance (measured in ohms) represents how effectively voltage may be transmitted through the cable, before meeting the counterforces of resistance, inductance, and capacitance.  In very general terms, a low impedance rating indicates that the cable is designed to transmit a higher degree of power.  Cables rated for higher impedance, on the other hand, are often more appropriate choices for data applications that prioritize signal coherence above high voltage transmission.  

The problems begin when a cable with one impedance rating is coupled to a device with a different one.  Called impedance mismatching, this creates a situation in the circuit where only part of the intended voltage is reaching the load.  The rest rebounds back through the cable as standing waves - a phenomenon called reflection - which in turn creates echo problems for the signal still passing through the cable. 

Reflection is something you don't want, as it creates signal loss and distortions.  In a coaxial analog audio-visual application, for example, reflection causes echoes, ghosting, and decreased transmission efficiency.  In data, it gets even worse: packet loss, jitter, and general network degradation, all conditions that represent significant operational liabilities to any digital application.  And once the mismatch has been installed, it may be very tricky to later diagnose and fix.

So how do you avoid this risk?  First, always check your cable's impedance rating, and make sure it matches the devices you plan to use it with.  Your Show Me Cables representative can help you determine the specific cable that you need for your application, ensuring that you avoid an impedance mismatch scenario that could later spiral into an expensive and frustrating ordeal.

impedance matching transformerSecond, if you do encounter an impedance mismatch an impedance matching transformers are designed to bridge the difference with minimal reflection.  They are available in a variety of connector types to work with your system.

For a less sophisticated solution in A/V systems, which depends upon distance and signal strength, the system may work even with an impedance mismatch.  While this isn't preferred, it may work and save you from rerunning cable.

Would you like to learn more?  Ask a ShowMeCables representative today about how our commitment to cable design and manufacturing quality can help you achieve better, more reliable project outcomes.