[TowerTalk] Brainstorming: A homebrew Steppir like beam....

[Richard (Rick) Karlquist (N6RK)]

Before the SteppIR came along, I had thought
about making adjustable height verticals.
If you only need a 2:1 difference in length, it
would be quite easy to use a wire element that
doubles back on itself by wrapping around a moveable pulley,
where the pulley position is controlled by a nonconductive
string that goes around a fixed pulley at the end of the
tube.  This design is homebrewable and completely
eliminates the BeCu tape issue and the graphite brushes
and there is no problem with contact resistance, etc.
You could perhaps use annealed OFHC copper tape to get
lower element resistance (doesn't need to be stiff).
With a 2:1 difference in length, you could get 10 thru
20 meters, but not 6 meters that way.

This homebrew technique might be competitive with the
SteppIR verticals, because you can use tuning
techniques of the type used with mobile screwdriver antennas.
(No stepper motor necessary and you don't need to keep track
of where you are, just tune for a match).  I think when you start to
get into multiband Yagi's, if your time is worth anything
at all, the price of the SteppIR Yagi is actually a bargain.

Rick N6RK

> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com]On Behalf Of Rob Frohne
> Sent: Thursday, April 22, 2004 12:38 PM
> To: towertalk at contesting.com
> Subject: [TowerTalk] Brainstorming: A homebrew Steppir like beam....
> 
> 
> Hi All,
> 
> As Bill (W5WVO) recently put it, "The vast majority of you seem to 
> think the SteppIR yagi is the greatest thing since the discovery of the 
> ionosphere! "  At this time, I haven't been able to find any 
> information on anyone homebrewing anything similar, but there is a 
> need, since the cost of the Steppir is high and there isn't a surplus 
> market for them yet.  I've been thinking about this problem off and on 
> for the last several months, because my crank up tower would hold 
> something the size of a Steppir, but not much more, and I've always 
> wanted a remotely adjustable antenna.
> 
> I'm wondering if the group would humor me and brainstorm on this 
> problem a little.  One of my colleagues suggested I pose this problem 
> to his Mechanical Design class, and so I wrote up a problem statement 
> for them.  I've put it on my web site at:
> http://www.wwc.edu/~frohro/Antennas/Tunable%20Dipole%20Project.pdf
> so you can look at the problem statement too.
> 
> I've had several ideas which I'll throw out below.
> 
> 1)  It would be nice to use readily available consumer or surplus parts 
> in order to keep the cost down, and since this is about the only way to 
> make homebrewing cost effective these days.
> 2)  Hollow fiberglasss poles seem to be readily available at reasonable 
> cost, from Steppir itself and from other sources like Antenna Mart.
> 3)  I did some computer simulations and if pulleys are put on the end 
> of the fiberglass tubes and wires are run down over those pulleys 
> (perhaps with some small weights on the end of the elements), to make a 
> kind of half quad the radiation resistance is still high enough in a 
> yagi so efficient operation can be had.  The gain is almost the same as 
> a regular yagi.  If this configuration is used, it seems more natural 
> to have the pulleys go in a horizontal plane to match those on the end 
> of the elements than the way Steppir did it.
> 4)  You could also use threaded rod to change the size of the elements. 
>   In this case I can only imagine changing the size by a factor of 2 
> easily.
> 5)  If you use BeCu strips like FluidMotion, there is an issue with 
> machining the strips, because the dust is toxic.  You can get the BeCu 
> strips without any holes in them though, and that seems safe to me if 
> you don't machine them.  You could use a combination of strip and wire.
> 6)  Keeping track of the length of the antenna element is a little 
> problematic.  Stepper motors can lose calibration.  For a dipole, an 
> SWR bridge could be used for final adjustments to the length.  A yagi 
> would require more accuracy.  You might use an optical encoder on the 
> spool, or even one of these cheap optical mice for computers.  The chip 
> they use has outputs for that which would make this easy, but I have no 
> idea how they would respond in a high RF environment.
> 
> So what kind of ideas can you contribute to this need for a homebrew 
> adjustable antenna?  Has anyone else tried anything?  What lessons have 
> been learned?
> 
> 73,
> 
> Rob, KL7NA/W7
> --
> Rob Frohne, Ph.D., P.E.
> E.F. Cross School of Engineering
> Walla Walla College
> http://www.wwc.edu/~frohro/
> 
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