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- 20th June 2012, 22:41 #1

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## formula for standing wave ratio

Hello, in order to solve the SWR for transmission line with:

1) forward measured power of -0.5dBm

2) reflected measured power of 1dBm

I want to use the formula:

SWR = 1+sqrt(Pr/Pf) / 1-sqrt(Pr/Pf)

My question is do I have to covert the measured powers out of decibels first i.ie.

-0.5dBm = 10logx therefore Pf= 10^-0.05

1dBm = 10logx, therefore Pr = 10^0.1

or can I just plug them into the SWR formula as their measured dBm format ?

Thanks for any help.

- 20th June 2012, 22:41

- 20th June 2012, 23:10 #2

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## Re: formula for standing wave ratio

Powers must be in linear units (i.e. watts), doesen't matter if you use multiple or submultiples but Pr and Pf must be espressed in the same units. f.i

Pr in W and Pf in W

Pr in uW and Pf in uW

and so on.

However your case is not a real one, since the reflected power is higher than the forward power. If, instead we have Pf(dB)=1 dBm and Pr(dB)=-0.5 dBm we will have:

Pf = 10^0.1 = 1.26 mW

Pr = 10^-0.05 = 0.9 mW

then SWR = [1+sqrt(0.9/1.26)]/[1-sqrt(0.9/1.26)] =11.9

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