antenna-theory.com

[antenna9071]

I read on the homepga antenna-theory about s11

http://www.antenna-theory.com/definitions/sparameters.php

"If S11=-10 dB, this implies that if 3 dB of power is delivered to the antenna, -7 dB is the reflected power. The rest was "accepted" by the antenna. This accepted power is either radiated or absorbed as losses within an antenna. Since antennas are typically designed to be low loss, the majority of the power delivered to the antenna is radiated."


I try to calculate the refelctoed power (-7dB) with this formula

s11(db)=10log(Preflected/Pincident)

How can they know that reflected power is -7db. I get someting else.

[bigSteve]

Make sure you don't confuse linear units and dB units.

s11 [dB] = 10* log( Preflected / Pincident )

If s11 = -10 dB, and the incident power is 3dB, we must first convert Pincident from 3 dB to linear units, which is 2 Watts. You can never divide dB. So:

-10 = 10*log( Preflected / 2 )

==> Preflected = 0.2 W = -7 dB

Another way of working with the above equation is to use only dB:

s11 [dB] = Preflected [dB] - Pincident [dB]

Then it is even simpler:

-10 dB = Preflected [dB] - 3 dB

Preflected [dB] = -7 dB

[antenna9071]

Tank you. Forget to convert to watt.

[R. Fry]

[quote="bigSteve"]

>If s11 = -10 dB, and the incident power is 3dB, etc
_____________

Just to note that decibels are used to show the logarithmic relationship of one parameter to another. It cannot be assumed that "3 dB" means 2 watts unless the zero decibel reference value is given to which 3 dB applies. The correct way to express this for a power of 2 watts would be 3.01... dBW.

Likewise in the statement "-7 dB is the reflected power" in link quoted by the OP, -7 dB is not a specific power because the reference for it is not given. If it is -7 dB with respect to 1 watt (-7 dBW), and the incident power is 2 watts, then the S11 of the system would be -10 dB, not -7 dB.*

But, assuming that we want to know what the reflected power will be for an incident power of 2 watts if the S11 is -7 dB, the equations needed for this are shown at the bottom of the chart at the link below.

In this case the reflected power would be about 0.1995 * 2 watts = ~0.4 watts, rather than 0.2 watts.

* Proof: -7 dBW = 0.2 watts, 3 dBW = 2 watts, and 10*log(0.2 / 2) = -10 dB = S11.

http://www.markimicrowave.com/menus/data/return%20loss%20to%20vswr.pdf