[ARDF] 2M Turnstiles
jay at west.net
Tue Feb 24 13:00:21 CST 2004
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:
> 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.
> 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.
> 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. 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/
More information about the ARDF