Return Loss to Mismatch Calculator

The calculator converts between return-loss, VSWR, reflection coefficient, and mismatch loss. It also shows the percentage of forward to reflected power. Particularly, reflected power and mismatch loss help describe the need for the antenna (with its matching network) to provide a suitable return loss to the active electronics. Note that VSWR and reflection coefficient are just different ways to represent return loss. We conclude with some performance recommendations & an explanation of parameters used in the calculator.

Return Loss Calculations

Enter the return loss, VSWR or reflection coefficient using drop-down menu to calculate how much power is reflected from or delivered to the load and mismatch loss.

Conducted Antenna Measurements - Return Loss

The three most important performance criteria when selecting an antenna are radiated efficiency, pattern shape, and conducted return loss. Regarding return loss, in most cases we recommend a minimum of 6 dB and an upper goal of 10 dB where:

6 dB Return Loss

This results in ~ 25 % reflected power with ~ 1.25 dB mismatch loss.

The wireless community commonly accept that 6 dB should be the worst-case return loss at any frequency within your operating band. In other words, greater than 25% reflected power is undesirable and life can become very difficult, project timescale extend and compliance increasingly expensive. For example, it may cause complicate power amplifier stability or that the receiver noise figure is adversely affected. Therefore, any antenna presenting less than 6 dB return loss should be avoided.

10 dB Return Loss

This results in ~ 10 % reflected power and ~ 0.46 dB mismatch loss.

The bottom line for most communication systems is that a return loss much above 10 dB represents a diminishing improvement in mismatch loss. Therefore, striving for a higher value is uneconomical. The caveat to this is when a low standing wave (gain variation), or group delay (variation of phase with frequency) is required across the operating frequency band often required in measurement systems.

Return Loss Table

Return Loss (dB)	Reflected Pwr (%)	Mismatch Loss (dB)	VSWR	Reflection Coeficient
0	0.00	∞	∞	1.00
1	20.57	6.87	17.39	0.89
2	36.9	4.33	8.72	0.79
3	49.88	3.02	5.85	0.71
4	60.19	2.2	4.42	0.63
5	68.38	1.65	3.57	0.56
6	74.88	1.26	3.01	0.50
7	80.05	0.97	2.61	0.45
8	84.15	0.75	2.32	0.40
9	87.41	0.58	2.10	0.35
10	90.00	0.46	1.92	0.32
12	93.69	0.28	1.67	0.25
15	96.94	0.14	1.43	0.18
20	99.00	0.04	1.22	0.10
30	99.90	0.00	1.07	0.03
∞	100.00	0.00	1.00	0.00

Return Loss Explained

The ratio of a sinusoidal signal reflected back from the load to the formward signal absorbed by the load in dB is called return loss. The term ‘forward power is synonymous with powered deliver to the load, or thru power. Thus with ~ 25% reflectd power:

Return Loss = -10 x Log [ 100/25.12] = 6 dB

Note: The historical definition of return loss results in a negative sign. More recently this has been dropped/corrected as seen in the table provided here.

Mismatch Loss

The mismatch loss is another way of representing reflected power. It is caused by any difference between the source and load impedance. For example, in the case of a modular cellular power amplifier, this generally requires a 50 Ohm load for maximum power transfer. Mismatch loss should be minimized in cascaded RF system calculations and can represent a significant loss of power delivered to the antenna. Consider the 6dB antenna return loss case discussed here with ~ 1.25 dB mis-match loss.

Mismatch loss (%) = 100 x [1-10^ ^(-0.5/10)] ~ 25%

Maximum Power Transfer

The maximum power transfer from source to load occurs when the source impedance is equal the load impedance. For example, a 1 Watt power amplifier (source) optimized for use in a 50 Ohm system will deliver 0.75W into an antenna with 6 dB return loss.

Power = Source Power (W) x Forward Power (%) = 1W x 75% = 0.75W

Voltage Standing Wave Ration (VSWR)

The voltage standing wave ratio is the magnitude of a standing wave caused by a mismatch impedance. It is defined as the ratio of the standing wave amplitude measured at maxima and minima along a transmission line. In the case of a 6 dB return loss, this is equivalent to 0.5 reflection coefficient:

VSWR = (1 + | Γ |^ ²) / (1 – | Γ |^ ² = (1 + | 0.5 |^ ²) / (1 – | 0.5 |^ ²) = 3

Antenna Return Loss Take Away

This text is intended to illustrate the impact of return loss on antenna selection for non RF engineers. The key take aways are:

When selecting or matching an antenna make sure the return loss does not drop below 6 dB at any frequency in your operating band.
An antenna return loss above 10 dB typically represents diminishing returns in performance.

If you need help to match an antenna to improve performance or pass PTCRB, then take a look at our comprehensive antenna matching service here.

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