[Elecraft] [w2/K3] High SWR

[Jim Wiley]

Don -

A 50-ohm directional wattmeter can only read correctly when it is 
connected to a 50-ohm source and a 50-ohm load.  Anything else will 
change the readings.  For example:  Suppose you have a 10-watt 
transmitter that feeds an antenna that has a SWR of 3:1.  That means 
about 2 watts will be reflected.  Assuming the source provides what is 
termed a conjugate match, a directional wattmeter in that system will 
now show 12 watts forward and 2 watts reflected.  You still have a net 
power to the antenna of 10 watts  ( 12 - 2  = 10).

A conjugate match usually means that the source (the transmitter) has a 
tuned output network.  This can be either a tunable final amplifier, as 
is found with most vacuum tube amplifiers, or it could be a broadband 
untuned amplifier (most solid state rigs) followed by an antenna 
matching unit, such as the KAT2 or KAT3 in an Elecraft rig.

In simplified terms, without having to get wound up in all sots of 
mathematical talk, the conjugate match may be thought of as a perfect 
one-way mirror for  RF energy.  It passes all power in the forward 
direction without loss, but is 100% reflective for energy coming back 
down the feed line.  Any RF energy reflected from the load (the antenna) 
comes back down the feed line, encounters the conjugate match, and 
starts back up the lint toward the antenna.  It eventually all gets 
radiated, even if it has to make a few trips back and forth.  This 
disregards feed line losses, which burn up a bit of the power  (by 
converting it to heat) each time it goes up or down the line.  This is 
why SWR can be a problem at VHF or UHF, where line losses tend to be 
higher.

But at HF, line loss is seldom a big issue.  So, for practical purposes, 
even if the power bounces up and down the line a few times, it (mostly) 
all gets radiated eventually.  For more on the conjugate match and why 
SWR in and of itself really doesn't matter, as long as the antenna plus 
feed line combination can be matched to the transmitter, see the book 
"Reflections", by Maxwell, W2DU.

It also explains why antennas fed with open wire line, which has 
vanishingly low loss at HF, can operate effectively with very high SWR, 
often 10:1 or more.   As long as the antenna + feed line can be matched 
to 50-ohms nonreactive at the point where the antenna interfaces with 
the final amplifier, all will be well.


- Jim, KL7CC


On 11/9/2012 3:46 AM, Don KA1KU wrote:
> Thanks for your insight on this.  I had no idea the SWR would be so high on
> the G5RV although I was aware of these characteristics  in general.
> I just did some checks across 20 meters and I get a SWR of 4 on the W2.  The
> chart you sent indicates 2+ but not sure if I should expect that.  This is a
> Full size G5RV at 45 feet feed with 31 feet of 450 ohm ladder line and about
> 90 feet of 50 ohm coax to shack.
> With the new W2, first I wondered if it was working correctly.  I think it
> is.  Second, is something wrong somewhere in my antenna system?  Looks like
> the high SWR is to be expected except perhaps on 20M.  Does the SWR of 4
> indicate a problem somewhere?
> Lastly, about the power reading of the W2.  With power set at 5.0 on K2/K3
> the W2 reads 8 and 10 respectively.  With power at 10.0 the W2 reads 15 and
> 17.  This seems like a lot of difference.  Could it be the K's are that much
> out of calibration?  Is the W2 reading wrong?  What else?  Maybe the K's are
> just creating a lot of energy!
>
> Thanks for all your help on this.
>
>
>
> -----
> 73,
> Don KA1KU
> --
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