[Elecraft] TxMon and K3 (tuner bypassed) SWR missmatch
Wes Stewart
wes_n7ws at triconet.org
Sun Nov 6 11:40:34 EST 2016
Line loss decreases the SWR measured at the source, not the load.
In this case, thinking in terms of return loss makes this thought experiment
easier. The line loss adds to the return loss of the load, increasing the input
return loss by a factor of 2 X line loss (dB). Higher return loss = better match.
While on this subject, let me introduce you to Dan, AC6LA's fabulous software
and tutorials. Before the death of my wife changed some priorities, I did a lot
of beta testing for Dan after he was gentleman enough to ask permission to
include ladderline data that I developed in his software.
For a discussion of increased loss due to SWR this is a must read:
http://ac6la.com/swrloss.html
If you want to use a wonderful tool to help understand some of this, this
(free!) one can't be beat: http://ac6la.com/tldetails1.html *
His Zplots tool is also useful, and free, but does require that you have some
version of MS Excel. If you use EZNEC, then I've found AutoEZ to be indispensable.
Wes N7WS
* If you download and play with this tool, here are some starting values that
are illustrative.
Set
Type = Belden 9913.
Freq = 125 MHz
R = 50
X = 0
R and X = At Load
Length = 100 feet
Results:
At the bottom of the frame click the "Return Loss"
Loss (total) = 1.5 dB (this is the matched line loss)
If you look at the "At Input" and "At Load" columns you can see that:
RL (true) is 61.238 dB and 58.238 dB respectively. The input RL is exactly 3 dB
greater than the load RL. (2 X the line loss)
The true values are different from the 50 Ohm values. This is because the line
Zo isn't 50 Ohm as seen in the "True Zo" box. The "Plot |Zo|" and "Plot VF"
buttons will open another screen where these values can be seen to vary with
respect to frequency.
Return to the first screen and click Show SWR. Change R to 1000. Note that the
At Load SWR (50) = 20, exactly as expected. The input SWR is ~4.5. showing that
a moderately lossy line can have a profound effect with highly mismatched loads.
In the loss column the total is 6.599, ~5.1 dB greater than the matched loss.
But selecting Show Return Loss again shows that the input is still 3 dB higher
than at the load, they're just much smaller values.
If you can measure input SWR and want to know load SWR and you've characterized,
or believe the manufacture's specifications for your matched line loss, you can
convert the input SWR to RL, subtract 2 X matched line loss and derive the load
RL. Convert this back to SWR. If you don't want to do the math, use Dan's tool
and trial-and-error.
On 11/5/2016 12:57 PM, Ron D'Eau Claire wrote:
> And the losses are low. Losses go up with increased SWR on a given
> transmission line and losses reduce the SWR measured at the load.
>
> (I neglected to make that point in my last post and couple of sharp-eyed
> readers caught it.)
>
> 73, Ron AC7AC
>
>
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