[ARDF] 2M Turnstiles

Tue Feb 24 15:32:02 CST 2004

On Tue, Feb 24, 2004 at 01:00:21PM -0800, Jay Hennigan wrote:
> On Tue, 24 Feb 2004, Kenneth E. Harker wrote:
> 
> >      W5JEN and I have built our first two meter turnstile antenna based
> > on the WB6DRV contruction article:
> >
> >      http://www.west.net/~jay/turnstile.html
> >
> >      We modified the design slightly.  Instead of "4 ea. 8-32 x 3/4"
> > round-head machine screws" we used 4 ea. 8-32 x 1" round-head machine
> > screws, and instead of elements made from hex spacers and welding rod,
> > we are using aluminum arrow shafts.  The arrow shafts have 8-32 threading
> > inside (used to screw on an arrowhead) and are easy to cut to length with
> > a Dremel tool.  You need to use 1" long machine screws instead of 3/4"
> > machine screws because of how deep inside the arrow shafts the 8-32
> > threading is located.
> 
> Cool.  What is the typical cost of these?  Sounds mechanically better and
> likely to be broader bandwidth with the thicker elements.

Depending on volume, anywhere from $3 ea. up.  It probably isn't cheaper
than the welding rod, but (a) you don't need to mechanically bind the rod
to the hex spacer (something I'm not sure I could do easily) and (b) 
it looks cooler (IMHO.)  The big problem is that the 8-32 threading inside 
the arrow shaft only starts 1/4" or more inside the shaft, so you need a 
longer machine screw and a relatively thin nut and lock washer on the
outside of the PVC (we had trouble finding KEPs nuts in some of the local 
hardware stores anyway) to attach the machine screw to the PVC.

> >      Having built one of these, we've had a little trouble achieving a flat
> > SWR.  Cutting the arrow shafts to various lengths between 19" and 17" results
> > in a SWR curve that dips both below and above 145 MHz.  i.e. at 17.5"
> > lengths, there may be a nice wide dip in SWR that goes as low as 1.1:1 from
> > 137 MHz or so to 142 MHz, climbs up to 1.5:1 or 1.6:1 by 145 MHz, and dips
> > again to 1.2:1 around 150 MHz.  Every length we tried seems to have this
> > double dip, with the local maxima between the two dips topping out at 1.6:1
> > or maybe 1.7:1, and usually around 145 or 146 MHz.  So, 1.5:1 isn't too
> > terrible, but I find it odd that the SWR curves (a) don't seem to move much
> > up or down in frequency, and (b) show a double dip.  I have not checked to
> > see exactly how omni-directional it is yet.
> 
> The optimal SWR is about 1.3 to 1.  Trimming for best SWR will distort
> the pattern.  You're feeding two 72-ohm dipoles in parallel so the design
> impedance is around 36 ohms.  The mismatch between this and the 50-ohm
> feed will give you about a 1.3:1 SWR.  If you trim the length for best
> SWR, the pattern will become dogbone-shaped.  See W4RNL's article at
> http://www.cebik.com/turns.html for details.

So, when we first build it and get 1.5:1 or 1.6:1 in the target frequency 
range (144-146 MHz) what should we be doing (if anything) to tune element 
length?  How do you know that the element length is right?

> >     One thing I wasn't sure about when we built this was how long, exactly,
> > the phasing section ought to be when you solder the solder lugs onto it.
> > i.e. should the 16" length be from the center of solder lug to the center of
> > solder lug?  Or from the tip of the solder lug to the tip of the solder lug?
> > Or is it really not that precise to matter?
> 
> I did it overall length end-to-end.  You would need an antenna range and
> lots of trial-and-error to make this exact.  Note that for RG-179 (teflon)
> I used 13 1/2 jacketed and 1 1/4 stripped on each end.  

Right - that I follow just fine, but once I soldered the solder lugs on, 
the overall length is no longer 16" if you include the solder lugs in the 
length.  So, what I am wondering is for each end (if you'll excuse ASCII
artwork):

      A   B    C              ___
       _____                     \\    braid/jacket
      /     \_____        ________\\\_____________
      |  /\    ___|______|        ||||
      |  \/  _____|      |________||||____________
      \_____/
                           dielectric
       Solder lug  center

Should the 16" length be measure from A (end of the solder lug) to A,
from B (center of the solder lug, where the machine screw goes) to B, 
or some point C (on the solder lug) to C?  The way I actually soldered
this first antenna, was more like a 16" C-C distance, but should I really 
be soldering so that the overall B-B length is 16"?  Or does it matter 
that much if it's off by 1/16" or 1/8" total length?

>                                                            The ends wind up
> being part of the dipoles from an electrical standpoint.  Spinning the
> antenna while looking at it on a service monitor showed a reasonable pattern
> with about 2dB dips at 45 degrees.  This varies somewhat with height above
> the ground.  I tried 1/2 inch longer and shorter and the pattern seemed to
> get a bit worse at 2m off the ground which was where I was trying to optimize
> it.  Different dielectric material in the phasing section will require
> different lengths based on the velocity factor.
> 
> -- 
> Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
> WestNet:  Connecting you to the planet.  805 884-6323      WB6RDV
> NetLojix Communications, Inc.  -  http://www.netlojix.com/
> _______________________________________________
> ARDF mailing list
> ARDF at kkn.net
> http://kkn.net/mailman/listinfo/ardf

-- 
----------------------------------------------------------------------------
Kenneth E. Harker      "Vox Clamantis in Deserto"      kharker at cs.utexas.edu
University of Texas at Austin                   Amateur Radio Callsign: WM5R
Department of the Computer Sciences          Central Texas DX & Contest Club
Taylor Hall TAY 2.124                         Maintainer of Linux on Laptops
Austin, TX 78712-1188 USA            http://www.cs.utexas.edu/users/kharker/
----------------------------------------------------------------------------