Return Loss vs Insertion Loss: What Each Number Tells You
Return loss, from S11, tells you how much power bounces back. Insertion loss, from S21, tells you how much fails to arrive. A 10 dB return loss reflects 10 % of the power, yet a matched 10 dB attenuator still has 10 dB of insertion loss.
For practitioners
Two numbers, two questions
Return loss and insertion loss are often mixed up because both are quoted in dB and both look like "loss". They answer different questions.
- Return loss asks: how much of the signal I sent in comes back out of the same port?
- Insertion loss asks: how much of the signal I sent in does not arrive at the other port?
The first describes the match at a port. The second describes the cost of passing through the device. A device can score well on one and badly on the other.
Where they come from in the S-parameters
Take a two-port device with ports 1 and 2. Its four S-parameters describe how waves move in and out of it.
For a signal applied at port 1 with port 2 terminated in the reference impedance:
- S11 is the wave leaving port 1 divided by the wave entering port 1. It is the reflection.
- S21 is the wave leaving port 2 divided by the wave entering port 1. It is the transmission.
S22 and S12 are the same two measurements made from the other side. This page uses S11 and S21 only.
Written as positive numbers, the two losses are:
A network analyzer or an S2P file shows S11 and S21 in dB as negative numbers, for example −20 dB. Return loss and insertion loss are the same values with the sign flipped. A data sheet that says "return loss −15 dB" is mixing the two conventions, so check which one it uses. A larger positive return loss always means less signal comes back.
Reading return loss
Return loss converts directly to reflected power, because power goes with the square of amplitude.
- 10 dB: |S11| = 0.316, so 10 % of the incident power is reflected.
- 20 dB: |S11| = 0.1, so 1 % is reflected.
- 30 dB: |S11| = 0.0316, so 0.1 % is reflected.
Every 10 dB of extra return loss cuts the reflected power by a factor of ten.
The same information can be written as VSWR. A return loss of 10 dB corresponds to a VSWR of 1.92, and 20 dB to a VSWR of 1.22. The VSWR, Return Loss and Mismatch Loss Calculator converts between them.
A reflected signal costs something. Power that is reflected never enters the device. This is called mismatch loss, and it follows from the reflected fraction:
At 10 dB return loss this is about 0.46 dB. At 20 dB it is about 0.044 dB. So a poor match does cost signal, but only a small amount once the return loss is above about 10 dB.
Reading insertion loss
Insertion loss is the other side of the ledger. An insertion loss of 3 dB means |S21|² is about one half: only half the power reaches port 2. Insertion loss tells you nothing about why the power is missing. It may have been reflected, absorbed, or radiated.
Where the rest of the power goes
For a passive two-port, power cannot be created. The reflected power plus the transmitted power cannot exceed the incident power:
Whatever is left over is dissipated as heat in the device. That leftover is the key to the difference between the two numbers. Return loss accounts for the first term. Insertion loss accounts for the second. Neither one alone tells you how much heat is produced.
Worked example: a matched 10 dB attenuator
Take a 10 dB attenuator in a 50 ohm system.
- Insertion loss is 10 dB, so |S21| = 10^(−10/20) = 0.316.
- Transmitted power is |S21|² = 0.1. Only 10 % of the power arrives at port 2.
- The attenuator is well matched, so |S11| is small and the reflected power is small. It has a high return loss.
- The power balance leaves most of the input unaccounted for. Because reflection is small, nearly 90 % of the incident power is dissipated as heat in the resistors.
This device has a high return loss and an insertion loss of 10 dB at the same time. The match is good. The loss is high. That is exactly what an attenuator is built to do.
A high return loss does not mean a low-loss device. Always read S11 and S21 together. If you only look at S11, a device that swallows 90 % of your signal can look perfect.
Why a good match does not mean low loss
Matching decides whether power enters the device. Loss decides what happens to the power once it is inside. These are independent properties.
- Good match, high loss: the attenuator above. Power enters, then turns into heat.
- Poor match, low loss: a lossless reflective part, such as an open or a reactive element at the wrong frequency. Very little power is absorbed, so heat is small, but |S11| is large and |S21| is small because the power is sent back.
- Good match, low loss: what you want from a cable or a connector. Both |S11| and |S21| are favorable.
When a signal path loses more than expected, S21 shows how much, and S11 helps tell you whether reflection is part of the cause. If |S11|² is small and |S21|² is also small, the missing power is being dissipated. To see both traces, open the device's file in a viewer. The guide How to Open an S2P (Touchstone) File lists ways to do it.
In plain words · for students and beginners
Return loss vs insertion loss in plain words
An everyday picture
Think of a window. When you look at it, some light bounces off as glare. Some light goes through. Some is soaked up by the tint.
- Glare is like return loss: the part that bounces back.
- Light that fails to get through is like insertion loss.
- The tint that soaks up light is like heat inside the device.
A window can have a coating that removes all glare and still be so dark that you see almost nothing through it. That is a good match with high loss. The picture breaks down because real devices work with radio waves, and the numbers are given in dB, a logarithmic scale, not in simple fractions.
How loud is the echo?
Shout into a canyon. A big return loss means a quiet echo, because little of your shout comes back.
The same example in simple steps
Take a 10 dB attenuator, a part that deliberately weakens a signal.
- Its insertion loss is 10 dB. That means only 10 % of the power arrives at the output.
- It is well matched, so very little power bounces back from the input.
- So where did the other power go? Into heat inside the part.
- Result: great return loss, yet high insertion loss.
Mistakes beginners make
- Using S11 to judge loss. S11 only tells you what bounces back. Loss through the device is in S21.
- Mixing up the sign. On a screen S11 and S21 are negative numbers in dB. Return loss and insertion loss are the same numbers written without the minus sign.
- Thinking a bigger number is always worse. For return loss, bigger is better. For insertion loss, smaller is better.
Remember
- Return loss comes from S11 and says how much comes back. Bigger is better.
- Insertion loss comes from S21 and says how much fails to arrive. Smaller is better.
- A good match does not mean low loss. A part can be well matched and still dissipate most of the power.
Related pages on RF View
Frequently asked questions
What is the difference between return loss and insertion loss?
Return loss measures the signal that is reflected back toward the source and is read from S11. Insertion loss measures the signal that does not reach port 2 and is read from S21.
Does a good return loss mean low insertion loss?
No. A well matched 10 dB attenuator in a 50 ohm system has a high return loss and an insertion loss of 10 dB. Return loss says how well the port is matched, not how much power the device dissipates.
Is a bigger return loss better?
Yes, when return loss is written as a positive number. A return loss of 20 dB means 1 % of the incident power is reflected, while 10 dB means 10 % is reflected.
What return loss equals a VSWR of 1.92?
A return loss of 10 dB corresponds to a VSWR of 1.92. A return loss of 20 dB corresponds to a VSWR of 1.22.