Readers of my articles occasionally ask for explanation of the distinction between meanings of:
- Insertion Loss;
- Mismatch Loss;
- Loss (or Transmission Loss).
These terms apply to linear circuits, ie circuits that comply with linear circuit theory, things like that impedances are independent of voltage and current, sources are well represented by Thevenin and Norton equivalent circuits.
Insertion Loss
Insertion Loss is the ratio of power into a matched load (to mean that the load impedance is the complex conjugate of the Thevenin equivalent source impedance) to the power in the load with the subject network / device inserted.
Insertion Loss can also be expressed in dB.
In practical passive networks, Insertion Loss must greater than 1, and positive dB.
Mismatch Loss
Mismatch Loss is the ratio of output power of a source into a matched load to the output power under a given mismatch.
Mismatch Loss can also be expressed in dB.
In practical passive networks, Mismatch Loss must greater than 1, and positive dB.
Loss
Loss is simply \(\frac{Power_{in}}{Power_{out}}\).
Loss can also be expressed in dB.
In practical passive networks, Loss must greater than 1, and positive dB.
Loss is sometimes called Transmission Loss to distinguish it from other qualifications, but it is unnecessary.
Caveats
Most ham transmitters are not well characterised by an equivalent Thevenin circuit, and so the foregoing does not apply literally. Results might be quite similar, but the extent of the difference is dependent on the transmitter and is uncertain.
Does it matter?
Difference scenarios may see some of these factors less important than others, or even unimportant.
A common mistake is to take a VNA, measure |s21| and calculate Insertion Loss, and to infer from that alone, the conversion of RF energy to heat. This can have gross error.
Another common mistake is to come up with -ve dB loss values for passive networks, the user is making up his own definitions and their work is questionable.
Understand the meaning of each, be clear about what is needed and what is being measured and how to get from the latter to the former.