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 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. This operational control is via software written for
Windoze, and I believe also for Mac. The tuner has three antenna ports
that can be switched either from the computer via a serial port, or on
front panel switches, and the tuner remembers settings per port, per
band, and even for segments within a band. The control software can also
lock out any output port(s) for a given band, or make any port the
default for any band with any other ports as switchable options.
I can't say that I've studied auto tuners, but looked at Palstar units
in their booth at Dayton several years ago, and was disappointed by
their failure to understand the significance of the return path for
antenna switching (the same mistake made in the otherwise excellent Ten
Tec 229 and 238 manual tuners). These tuners run single wire s from
antenna outputs to switching and amplifier inputs, using the chassis as
the return. That's OK on the lower bands, 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.
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.
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|