QUESTION

Question

Most of the resources, say that the return loss is a positive parameter (+ve dB) , considering that the system is matched well. For example, refer this site : https://resources.pcb.cadence.com/blog/what-is-a-good-return-loss-value-and-how-can-it-be-calculated ;  A higher Return loss indicates that the reflected power is very low, hence a decently matched network. Whereas a negative return loss says that the reflected power is more than the incident power, which is an undesirable condition. But, almost in all the antenna datasheet, the return loss graph is a negative dB graph.For example refer the datasheet of a antenova antenna : https://www.antenova.com/wp-content/uploads/2019/05/Mutica-SR42W001-PS-1.5.pdf . Could you please explain this controversy ?
Thanks & regards,
Gayathri G

Answer 1

Hi Gayathri,

You are correct by textbook/definition standard that return loss is a positive parameter. However,  in the antenna industry it is notated as a negative this is to help our customer understand that return loss is a loss and not a gain to the system, it also aligns with other values such as insertion losses.

Mathetically they are the same as the formula for Reflection Coefficient(S11) requires us to use the absoluate value of the return loss.

Best regards,

Yu Kai Yeung

Comments

Hi Yu Kai Yeung,
Thanks for the explanation. Now that the return losses are mentioned as a negative value for the ease of industry usage, what does a negative RL value signifies here ? Because, as per theoretical meaning of return loss, if a system has a RL of 30dB, it means that the reflected power is 30dB lesser than the incident power, which signifies that the reflected power is very less. Like wise what does this negative indication of return loss signify in the industry use case ? And what is the considerable value of RL in antenna, while selecting for RF mesh applications in Sub-GHz band ?
Thanks & regards,
Gayathri G

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Hi Gayathri,

30dB/-30dB return loss is very good this means very little of the signal is being reflected. It is negative because it aligns with other metrics for passive antenna performance. A good return loss would be 10dB but 5db+ would be considered adequate in some cases

Best regards,

Yu Kai Yeung

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Hi Yu Kai Yeung,
Thank you for the clarification. That solves my query.
regards,
Gayathri G