s-parameter s11 definition

check out this link which defines s-parameters http://antenna-theory.com/definitions/sparameters.php

It states towards the bottom:
"If S11=-10 dB, this implies that if 3 dB of power is delivered to the antenna, -7 dB is the reflected power."

This statement is incorrect, as S11 = -10 dB indicates that 10% of power is reflected, the rest is radiated. So, 10% of 3 dB is 0.3 dB is reflected, not -7 dB. Could someone confirm my understanding.

Thanks.

No. 10% of power correponds to 10 dB attenuation. 3 - 10 = -7 dB.

lol ya beat me to it ;)

D

This is the cause of possible misunderstanding ... "3dB of power" makes no sense, because dB is always relative to some reference value. Something like 3 dBm would make sense as a power level.

"If S11=-10 dB, this implies that if 3 dBm of power is delivered to the antenna, -7 dBm is the reflected power."

Yes, of course. The calculation would be the same, if it refers to a different reference level, e.g. dBV.

how could that be? it doesn't make any sense. 7 dB is reflected when the input power was only 3 dB?! I must be missing something. Please elaborate.

S11 = -10 dB is derived from S11 = 10 * log (Preflected/Pincident) = 10 * log (10/100) = -10 dB!

This means 10% of power is reflected. 10% of 3 dB incident power is 0.3 dB, not -7 dB.

pls study the dB

Got it....Thanks.

3 dB = 200
-7 dB = 20 (which is 10% of 200)

The advantage of the dB is that you can add and subtract them and not have to multiply it.