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<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Hi all;<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>A few months ago, I published to the list a proposal for a
small transmitting loop antenna which would be automatically tuned by snooping
the SDR for frequency information and tuning a resonating capacitor on the antenna
using a PIC processor and servo motor. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>At the time, it was just a proposal and as a proposal,
interest wasn’t terribly high. However, I continued to work on the
project and now have some success to report.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>To quickly recap the project, a PIC18F4321 takes frequency
information from the Atlas backplane that is encoded in 39-bit binary in I2S
format. Once received from the bus, the frequency is decoded to form an
index into a 2K-entry lookup table which translates the frequency information
into a motor position. The PIC drives a stepper motor coupled to a
22-turn vacuum capacitor to a pre-calibrated position which resonates the
antenna at the DDS frequency of the SDR. Voila ! A small loop
antenna that always stays in tune with the SDR.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>To do the project, I had to learn PIC assembly programming
and the IDE. (In retrospect, I wish I had forked over the do-re-mi for a
C compiler and done it in C, but the code is stable and operational, so
it’s going to stay in assembly. (But a word to the wise for any
future projects, do it in C, for gosh sakes!!!)<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Last night, using a 3-foot diameter loop made from
5/8” copper plumbing tubing and a 100pf vacuum cap, I was able to
calibrate across the entire 20-meter band and load the antenna with an SWR of
1.1:1 or less. After calibrating, I was able to tune across the band with
the SDR, while the antenna followed the SDR frequency perfectly, staying always
in resonance. (The SDR antenna tuner was disabled.) Needless to say,
I’m delighted with the results. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I’m at the point now of wanting to lay out the final
PWB for the project, and thought that I would ping the group and see if there
is enough interest in making this a full-fledged project. If so,
I’ll be glad to document the design, code, PWB layout, mechanicals, parts
list, part sources, cost, operation, programming, and anything else interesting
about this project. I would also consider handling a group buy for the
PWBs or even a kit.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I’ve tried to make the project as generic as possible,
so that someone could duplicate it with their own antenna without having to
code from scratch or redesign the mechanicals. I think the design could
easily be adapted to any antenna at all that could be tuned with a stepper
motor and variable cap or variable inductor. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I understand that future projects using the Atlas bus will
also continue to transmit frequency information on the backplane in this same
format. No changes will have to be made to interface future HPSDR
receivers and transmitters. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>So let me know if you guys want me to take this project to
the next stage and lay it all out for you. Personally, it’s been a
really exciting process and I’d be anxious to find others that want to
try the same thing. I am certainly open for suggestions and solicit
comments and ideas.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Very 73,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Ken WR5H<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
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