Funny you should ask. That is almost exactly what I want except I want mine
to handle at least 500 watts (with *my* antenna anyway) and fit in a K2
case.
I am designing and building my own. So far I have a fairly neat breadboard
operating and when I am ready all of the assemblies of the breadboard will
fit right into the K2 case. I am about to order the components for the
controller and should have that built up and installed in two or three
weeks. The controller will be a Basic Stamp BS-2sx PIC, some octal latches,
an A/D converter and miscellaneous frequency measuring components,
optoisolators, relay drivers, etc. The breadboard is for verifying that the
capacitors, inductors, and relays can handle the stress. For testing only
it is controlled by a diode matrix switched by a single pole rotary switch.
My design uses Form A reed switches (the *big* ones) with about 500 turns of
#38 magnet wire wrapped on them to form the relays I need. There are 32 of
them because each of the capacitors are switched in by a pair of relays, one
at each end of each capacitor to maintain balance by decoupling *both* ends
of each capacitor which is not in use. The little latching relays as used
by Elecraft would be a whole lot quicker and easier but they would not
handle the voltages I expect at higher power levels.
My design has a homemade balun at the input, two strings of inductors, and a
single set of capacitors to form a 'balanced L-network' ... at least that
is what I call it. There are 8 capacitors with the smallest 5 pF stepping
up binarily (X2 at each step) so that the PIC can switch in any value from 0
to 1275 pF in 5 pF increments. Each bank of inductors works in the same way
with 0.25 microHenry as the smallest increment.
I have a forward and reverse current monitor at the input identical to the
one in the K2 100W amp but I have not decided if I will go to the trouble of
putting in a LCD display for SWR, power, band/frequency, etc. None of that
is really needed but it would be kind of nice to duplicate the general
appearance of the K2 with an LCD display.
If anyone is interested in following my progress (or lack thereof) let me
know and I will provide occassional updates.
Don K7FJ
Here is what I would like in an auto tuner.
1. Balanced Output for ladder line - two sets of inductors and one set of
capacitors (like the Rich Measures roller L design). The inductors can
step
along in parallel. No control software changes are needed - just another
set of inductors and relays. The capacitors can switch between the input
and
output depending on the load impedance.
2. A current balun on the input side of the tuner driving the 2 sections
of
inductors.
3. 160m to 10m operation.
4. Matching range somewhat better than the Johnson Matchbox.
5. A rating for 150W max. power output.
6. A switch for bypassing the tuner and connecting the ladder line
directly
to the output of the balun.
7. A peak-reading, illuminated power-output meter.
8. A 4-position input (rig) selection switch.
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