[hpsdr] Small Loop Antenna

[Jeroen Bastemeijer]

Hi Ken,

Congratulations!!! Sounds like a fun add-on to the HPSDR!

Dpending on the interest, you might consider another strategy:
- Put the source code of the PIC on a website (HPSDR)
- Design your PCB with the Express-pcb and put the design-file on the 
website
(There are other similar possibillities, e.g. posting the PCB in 
gerber-format....)

Just a thought...

Furthermore: Would it be possible to miniaturize the loop to, let say 
e.g. 1 foot? This would make a nice in-house antenna :-)

Good luck, 73 Jeroen PE1RGE

Ken Klein wrote:

>***** High Performance Software Defined Radio Discussion List *****
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> Hi all;
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> 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. 
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> 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.
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> 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.
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> 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!!!)
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> 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. 
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> 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.
>
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> 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.
>
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> 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.  
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> 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.
>
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> Very 73,
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> Ken WR5H
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-- 
Ing. Jeroen Bastemeijer

Delft University of Technology
Department of Electrical Engineering
Electronic Instrumentation Laboratory
Mekelweg 4, Room 13.090
2628 CD Delft
The Netherlands

Phone: +31.15.27.86542
Fax: +31.15.27.85755
E-mail: J.Bastemeijer at TUDelft.nl
GPS: Lat N52.00002 Lon E4.37157 Alt 46.2m

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