[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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