[hpsdr] Proposal for Loop Antenna Project

[Tony Langdon]

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


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