[hpsdr] Small Loop Antenna

[Joe - AB1DO]

Hi Ken,

great going!

Actually, it occurs to me that you might be able to appeal to a much wider audience (and make kitting really worth your while) if you can adjust it such that besides using frequency info from the back plane, you use the same info through CAT commands. The PIC code could be set up to regularly poll the radio and respond to its commands (much as a SteppIR control box does to retune its antenna every 50kHz). Enable user selection of various radio types to account for the different flavors of CAT commands and you're good to go.

Just a thought.
73 de Joe - AB1DO
  ----- Original Message ----- 
  From: Ken Klein 
  To: hpsdr at hpsdr.org 
  Sent: Wednesday, September 19, 2007 22:42
  Subject: [hpsdr] Small Loop Antenna


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  Hi all;

   

  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.  

   

  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.

   

  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.

   

  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!!!)

   

  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.  

   

  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.

   

  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. 

   

  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.  

   

  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.

   

  Very 73,

   

  Ken WR5H

   

   



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