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

[Rob Frohne]

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/