[hpsdr] Proposal for Loop Antenna Project

[Philip Covington]

On 7/1/07, Tony Langdon <vk3jed at gmail.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> At 10:23 AM 7/2/2007, Ken Klein wrote:
> >***** High Performance Software Defined Radio Discussion List *****
> >
> >
> >Subject: RE: [hpsdr] Proposal for Loop Antenna Project
> >
> >Tony;
> >
> >Thanks a million for the comments.  I'm honestly hoping to get lots and lots
> >from the HPSDR folks for this project.
>
> It's an idea worth merit.
>
> >To address your questions, I think I have found at least a reasonable
> >starting point for your concerns.  There are several companies that have the
> >shafts, pillow blocks, couplings, and gears that should provide the
> >mechanical structure.  I just ordered a handful of parts from a company
> >called Stock Drive Products.  The sell small quantities to guys like us.
>
> OK, would like to see how it goes.  I can worry about shipping
> concerns later, I have ways of working around issues such as US only
> delivery. :)  As long as there's a straightforward way of
> mechanically coupling a motor to the tuning cap that doesn't require
> one to have too much mechanical experience or a degree in mechanical
> engineering... Or a shop full of machinery to make the bits fit! :)
>
> >So I envision a motor driving a straight shaft coupled to the cap.  I'll be
> >using a 22-turn vacuum cap I just bought as well.  The shaft will be
> >threaded for a couple of inches, so that I can trap a nut to the shaft to
> >hit the limit switches.  I'm also going to use a couple of small timing belt
> >sprockets to drive a 10-turn pot for position information.
>
> Suitable caps and options will be another issue as well.  Vacuum caps
> would be nice, but what's the cost?
>
> >To tune, I'm going to deploy the antenna with the mechanicals in place and
> >tune the antenna using a VNA that I built (the Ten-Tec one available from
> >TAPR).  Any antenna analyzer could be used as well as the SDR itself.  I'm
> >going to tune the antenna every couple of kHz throughout the range of each
> >band covered by the antenna and note the position sensor voltage at each
> >point.  From that I'll make a lookup table that will have all these points
> >in code.  Each time the DDS frequency is changed; the servomotor will find
> >the closest resonant point from the table and run the motor to that
> >position.  This is how I'm going to tune the antenna.  I won't be looking
>
> How likely is drift with changes in temperature, ageing of the cap,
> mechanical movement with wear and tear from the wind, etc?  Given
> that the loop is a very high Q antenna, I would feel more comfortable
> if the system was able to recalibrate itself (i.e. closed loop)
> rather than rely on open loop control via a lookup table.
>
> >for receiver noise or SWR; I'll just be retuning the cap to the previously
> >determined resonant point for that frequency.  Of course, frequencies
>
> How repeatable is this after years of service?
>
> >I hope I've explained this well enough.  Let me know if this answers your
> >concerns or if you want more detailed info.  I'm really just starting this,
> >so it might be awhile before I have worked out all the nitty-gritty.
>
> Well, the mechanics, I'll see what you come up with.  Electically, I
> have concerns about your method, because there is no onging feedback
> to compensate for changes in antenna characteristics due to
> environmental considerations.  Open loop adjustment is fine for a QSY
> in the same band on the same day, but may not suffice when going
> between summer and winter, or if the last hurricane turned your
> antenna into something looking more like a pretzel! :-D
>
> 73 de VK3JED
> http://vkradio.com
>

Ken's project can be used for loops and remotely (mechanically) tuned
antenna tuners, etc...   I don't think he is proposing a mechanical
design for the loop capacitor driver... though it will be interesting
to see how he does the mechanical part.

73 Phil N8VB

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