[TowerTalk] Modern Programmable Antenna Tuners

[Jim Lux]

On 10/28/13 10:08 AM, Jim Brown wrote:
> On 10/27/2013 11:01 AM, Jim Lux wrote:
>> Whether it will take the SWR to 1:1 is a function of the step size in
>> the tuner design (if it's a switched L and C) and the control algorithm.
>>
>> Most autotuners stop when the SWR is below, say, 1.3:1 or 1.2:1.
>
> I was a beta tester for the Elecraft KAT500 tuner, and two of them are
> now a part of my SO2R setup. Officially they are rated for 1kW, but
> unofficially they run fine at 1.5kW with moderate levels of SWR (3:1 or
> so). The tuner is rated for 160M to 6M, and the reflection loss through
> the unit is carefully kept under control all the way to 50 MHz.


The power limits, as with all tuners, are driven by the Q and the 
circulating currents or standing voltages inside the tuner.


>
> The firmware allows the user to select the degree of mismatch at which
> the tuner goes into bypass mode, to view the L and C settings and their
> configuration in use and to force values for the sort of networks you're
> describing.

Yes, but the L and C setting probably doesn't reflect the actual 
impedances in the circuit (for the reasons you describe below).


>

  but the resulting loop
> inductance of the path makes it a real mess on the higher bands and 6M.
> When I pointed this out to a Palstar engineer working the booth, his
> response was, "it's an antenna tuner," meaning that the stray could be
> tuned out using the tuner.

precisely this..

That *is* the challenge in using a tuner as a "programmable matching 
network".  In the tuner application, the strays are "tuned out" by 
twisting the knob (whether under computer or manual control).

Essentially ALL tuners are this way, which makes them kind of tricky to 
use for a "lumped load at the center of the element" kind of 
application. (e.g. a SteppIR with no big moving parts kind of application).




>
> The smart power amp manufacturers are careful to maintain a clean return
> path by running transmission line for all parts of the RF path. In the
> Ten Tec amps, it's coax. In the Elecraft KPA500, it's a short length of
> twisted pair between the input terminals and the circuit board.

Whether this is needed, or even desirable, in an autotuner is sort of an 
open question.