[hpsdr] Proposal for Loop Antenna Project

[Ken Klein]

Hi Tony;

Well, I agree that a lot of this will just have to be seen once this is all
put together.  

I don't think the mechanicals are going to present that much of a problem,
but again, I have to build it first!  

I'm doing the first one with a vacuum cap, but I think the same process will
work well or even better with a mechanical cap.  I paid nearly $200 for the
cap I just bought, and I'm sure that's going to out of the range of a lot of
folks.  I see some suitable vacuum caps on Ebay for $60 or so, but they are
Russian surplus and sent from the Ukraine.  Not just everyone's cup of tea,
that.  My source here for caps and motors is Max-Gain Systems, at
http://www.mgs4u.com/index.html.  As far as the mechanical "breadslicers",
there are many on eBay and other surplus houses from $10 and up.

Tuning and aging and environment:  The 3db bandwidths for these antennas are
usually several tens of KHz wide, depending upon band, size of antenna, and
all sorts of stuff.  Playing around with the spreadsheet I mentioned earlier
can give you a feel for how critical or not this all is for your design.
With lots of points in the lookup table and a fairly wide bandwidth notch
for the antenna might allow the system to stay reasonably useful for the
longer haul.  Again, this remains to be seen, but I think it looks
promising.

If another solution has to be found, I'm sure a reasonable automatic
calibration routine could be implemented.  We could try an SWR check at the
top and the bottom of the band and shift the lookup table one way or the
other to minimize the aging drift.  That could be automated in code, and
most of the parts are already there.  We could run an SWR reading into one
of the PIC A/D inputs and tune from there, or at least flag the operator if
it goes out of some predetermined limit.    And the good part is the copper
tube is easily bent back into shape after the hurricane!

So bear with me Tony, if you want, and let's see where this goes; I haven't
done this before, which is why I'm asking for opinions and help.  The
research and design so far has been a lot of fun.

73s and thanks again for the input!

Ken WR5H  


  






-----Original Message-----
From: Tony Langdon [mailto:vk3jed at gmail.com] 
Sent: Sunday, July 01, 2007 7:41 PM
To: Ken Klein; hpsdr at hpsdr.org
Subject: Re: [hpsdr] Proposal for Loop Antenna Project

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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