Application Note – 39
Impedance Matching Explained
Introduction
In a number of applications it is necessary to distribute high quality RF signals to various items of equipment that can be some distance from the source.
One example of this is the ptf 1226 Auto Switch and Distribution Amplifier which takes in redundant pairs of signals (primary and backup) and can provide up to 36 outputs.

ptf 1226 Auto Switch and Distribution Unit
Interconnections and Installation
Typically the cables used for interconnection will be low cost coaxial cables such as RG 58 and on the face of it the task is quite simple, however there are some pitfalls that should be noted as detailed below.
In practice, a coax cable is a transmission line where impedance matching can be critical. For instruments such as the ptf 1226 this is important not only for the integrity of the signals being distributed, but also because there are special sensing circuits designed to indicate fault conditions in case there is a problem with the signal.
The output of the ptf 1226 distribution amplifiers is designed to drive a 50 ohm termination impedance. If the cable is incorrectly terminated, some strange results can occur. These results are actually quite predictable and a result of the characteristics of RF signals.
An explanation of this is shown below using RF signals at the standard 10MHz reference frequency:
At 10MHz the wavelength: lambda = C/f
Where: C is the speed of light approximately 3e8 meters / second
f is the frequency of operation = 10MHz = 1E7Hz
therefore:
wavelength at 10MHz = (3e8/1e7)
= 30 meters (98.4ft).
This means that an un-terminated cable 30 meters long will have one full standing wave cycle within it. The source at the beginning of the coax will see a high impedance and the end of the coax will have 6dB higher signal than nominal at 10MHz.
If the coax is 30/4 or 7.5 meters long (24.6ft), only a quarter standing wave will be able to appear. The source at the beginning of the coax will see a very low impedance and the un-terminated end will be at a high impedance. The 10MHz level seen at the source will be zero causing a fault and the un-terminated end will have 6dB higher level than the terminated output.
If the length of coax is << 24ft then one does not have to necessarily terminate it otherwise it is always a good idea to properly terminate a coax. This is especially true when the frequency is higher than 10MHz.
In practice, it is always a good idea to terminate unused outputs, and definitely to terminate connected cables that otherwise would be un-terminated.