[hpsdr] Small Loop Antenna

[Ken Klein]

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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