Pardon the lecture that follows, but I think you have been misled.
This really is worth getting into!
> Whoa. Thanks for all of the fast responses. The most popular suggestion
> appeared to be looking in the ARRL Handbook. I checked out the handbook
> and with some consideration I figured out the following formula to get
> equivalent coax db loses from an SWR value:
>
> ((SWR - 1) / (SWR + 1)) ^ 2 * 100% = % Lost
> 10log(100/(100 - % Lost)) = Loss In dB
>
> So if your SWR's 3.0/1:
>
> ((3 - 1) / (3 + 1)) ^ 2 * 100% = 25% Lost
> 10log(100/(100 - 25)) = 1.25db Loss
A 3:1 SWR does NOT cause a 25% power loss. It might cause an
increase in loss, but that increase can be anything. SWR can even
cause a DECREASE in line loss!
The ONLY case where that formula is correct is the special case
where the source is a perfect resistive source without any
shutdown circuits. The source Z must also match the line
impedance exactly, and the line itself must be lossless.
One example of where this formula "works" is with a signal
generator feeding a high value 50 ohm attenuator pad, and that
attenuator pad feeds a load that we "mismatch" to 3:1 VSWR from
50 ohms j 0.
In that case, or a case where the line itself is so lossy it "looks
like" the high value attenuator pad in the example above, the load
power reduction caused by a 3:1 VSWR will indeed be an
additional ~1.25dB.
In cases where the line is electrically short, and if we have a
generator (transmitter) that has a matching circuit (antenna tuner)
or a transmitter that "likes" a load above 50 ohms (many or most
do) a 3:1 SWR can cause a system loss REDUCTION when the
mismatch is in a direction that reduces line current!
That's right, the system can be more efficient with a mismatch in
the line than without it!
So... unless you are talking about a source that is a perfect
dissipative 50 ohm source without SWR shutdown or a very high
matched loss line the answer you came up with is incorrect.
What most people want to know is the transmission line loss
increase with SWR, and that is complex for short lines. For long
lines, the Handbook gives formulas that work....but not the one you
are using. Look in the Handbook under transmission lines, and at
"additional losses caused by SWR".
I fought hard to have this formula (displayed on the screen) NOT
included in the MFJ SWR analyzer because even engineers
commonly misuse or misunderstand it. I didn't want to promote the
misconception!
73, Tom W8JI
W8JI@contesting.com
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