[Elecraft] [KAT500 and OT] Is a tuner using VNA feasible ?

[Fred Jensen]

On 12/23/2012 9:56 AM, Phil Hystad wrote:

> But, the KAT500 discovers an L & C match by a search process, I think
> it is measuring reflected voltage but not quite sure about that.

I believe you're likely correct.  That's how the KXAT1 operates.  My 
KXAT1 tunes a lot faster than the KAT500 but then, it only has 3 L's and 
3 C's plus choosing one of two configurations.  The KAT500 has a lot more.

   Can
> a vector network analyzer type circuit be used in a tuner to actually
> measure the complex impedance R + jX values and then use that to
> compute the required L & C values of the matching circuits (L-network
> ?).  Thus, given an input frequency would it not be possible to
> instantaneously (almost) determine the match?

I would think that, knowing the complex impedance presented to the 
output port of the tuner, one could compute the values of L and C and 
the L-net configuration and just "go there."  I can do it on my HP48GX 
calculator.  Measuring the actual complex impedance does not take a full 
VNA either.

In years past ... that would be *many* years ... I and others 
experimented with what I'll call "analog autotuners" for lack of a real 
name.  Mine was in the trunk of Dad's car along with the 75m radio.  The 
matching network was an L-net.  I used a stepper switch to manually 
select taps on the inductor controlled from the front seat, and an RF 
bridge produced a DC voltage proportional to the phase difference 
between the voltage and current in the short line to the antenna.  This 
voltage drove a DC motor on the capacitor shaft which drove the cap to 
zero the phase angle.  It worked, with some "wrinkles" ... it wasn't 
real reliable due mainly to my construction skills, and I had to detune 
the antenna far enough so that the polarity of the DC voltage remained 
the same over the range of frequencies I wanted to tune, or it got 
"lost."  I know of at least one ham who tried to make it work with 
motors on both the L and C.  I don't recall ever seeing that one 
actually work.

> P.S.  Or, am I completely off my rocker.

I think that you and your rocker are still firmly intact. :-))

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2013 Cal QSO Party 5-6 Oct 2013
- www.cqp.org