[Elecraft] KAT3 antenna tuner

[Wes Stewart]

>From what I calculate, 22 +j8 ought to be relatively easy to match, if the KAT3 schematic is correct.  A low-pass L with series L = 1.48 uH, C = 1967 pF gets it to 1.009:1, assuming no strays and Qc = 1000, Ql = 200. 

If my paper and pencil addition is correct, the available C ~ 2600 pF allows a match to a load of ~16 +j20. (with L ~ 0)

Since I prefer matching at the antenna, I don't have a KAT3, so this is all a paper guess.

Wes  N7WS

--- On Sun, 1/31/10, Kok Chen <chen at mac.com> wrote:

> From: Kok Chen <chen at mac.com>
> Subject: Re: [Elecraft] KAT3 antenna tuner
> To: "Elecraft Reflector" <elecraft at mailman.qth.net>
> Date: Sunday, January 31, 2010, 1:27 PM
> 
> On Jan 31, 2010, at 1/31    10:34 AM, Roger D
> Johnson wrote:
> 
> > I started on this topic because the tuner failed to
> find a good  
> > match at
> > 1800.
> 
> To see the approximate range of the internal K3 antenna
> tuner at 1800  
> kc, take a look here
> 
> http://homepage.mac.com/chen/Technical/Tuner/tuner1.8.pdf
> 
> (warning, it is a very large 9 MB PDF file (lots of
> points), if you  
> download as PDF and view with a PDF reader, you can
> rescale, etc  
> faster than viewing from a web browser).
> 
> Each dot indicates the impedance at the antenna port when
> the radio  
> port of the tuner is set to 50 ohms.  So the tuner
> should tune an  
> antenna that is the conjugate of where a dot lands on the
> Smith Chart.
> 
> The green points are for the case trx > series L : shunt
> C > antenna,  
> and the blue points are when the tuner switches to trx >
> shunt C >  
> series L > antenna.
> 
> > My VNA shows the impedance at the TX connector as
> 22+j8 which should
> > be well within the proper range.
> 
> 
> The conjugate of 22 + j8 ohms appears as 0.44 - j0.16 ohm
> on the  
> normalized Smith Chart in the PDF file.  That is just
> barely inside  
> the coverage region of the tuner.
> 
> You might try manually setting the tuner to the trx >
> shunt C > series  
> L > antenna position, turn C to the max
> (1200+600+300+...) and then  
> manually adjusting around the small values of L for lowest
> SWR.
> 
>  From my fiddling with the values, a 50 ohm source appears
> to prefer  
> large shunt C followed by a small series L on the way to a
> 22 + j8  
> ohms antenna .  It could be possible that other
> circuitry is  
> preventing you from reaching precisely that area of the
> tuner.
> 
> One thing you can do is to move the impedance of what you
> see at the  
> end of your current transmission line.  Since you have
> a VNA, you  
> might want to play with changing the line length of the 50
> ohm  
> transmission line (or use a short length of a different
> impedance line  
> to transform the impedance that you are currently seeing)
> so that its  
> conjugate is more inside the dotted region of the PDF
> file.
> 
> Also, since you have a VNA, why not temporarily disconnect
> the  
> transceiver output from the tuner, replace it with a 50 ohm
> resistor  
> and look at antenna port of the tuner with the VNA? 
> You can then see  
> the conjugate of the impedances which your tuner can
> transform into 50  
> ohms.
> 
> 73
> Chen, W7AY
> 
> P.S.  Anyone who wants to run this program for other
> frequencies just  
> send me a note and I can either send you the app or the
> Xcode project.  
> You need to be running at least Mac OS X 10.5 (also works
> fine on  
> 10.6).  If you want to modify it for the KAT100 L and
> C values, you  
> will need to change the component tables in the source, and
> recompile  
> under the Xcode IDE.
> 
> Some caveats: the chart shows the combinations of C and L
> values  
> without taking into consideration any strays.  It also
> does not take  
> into consideration the effects of the KBPF3.