" 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.
The mechanical challenges of something like the SteppIR would be a good
problem for a mechanical design class. However, what YOU want is an antenna
that has dipole like performance over a wide frequency range, and a
mechanically adjusted element is but one way to solve the problem. What
about a low loss antenna tuner at the feed point of a fixed dipole? A good
systems analysis might show that it's just as good a solution. Another
alternative (requiring less adjustment range for the tuner) would be a a
multi band dipole, with a few different length wires... the interaction
problem that makes tuning so difficult for the "tunerless" version isn't a
big deal here. All you want is the wire to be "about the right length" for
the band, and the tuner takes care of the reactance for the last bit.
(This is basically what the dual band whips from SGC do.. they combine a
helical element with a straight one, to make a wide band antenna without
requiring too much range from the tuner at the base).
A good analysis might tradeoff the reliability of stepper motors driving
metal tapes/wires vs actuators (relays, motors, etc.) driving some other
form of reactive component (lumped inductor/capacitor).
You'd also need to look at the losses in the lumped elements vs the losses
in the tapes. Both can be made very low, but there's a cost/performance
tradeoff.
The fixed length wire with a tuning network won't necessarily have a
classical "free space dipole" pattern, but then, in a real installation with
feedlines, towers, buildings, trees, and real earth, neither will the
adjustable length dipole. A question which could be answered by modeling,
in a quasi-quantitative way, is whether the difference is "significant". If
you've got a dipole, odds are you aren't looking for tenth of dB performance
changes.
>
> 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.
Unless you use multiple rods, stacked. There are some clever schemes for
doing this.
> 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.
Or, just use flat strip and some other way to measure the length: Some form
of encoder and printed markings? Look at reflected power and adjust to
minimize? For the latter, you could use open loop to slew to "about" the
right length, and then close the loop to fine tune.
One might also be able to order BeCu strips with holes in them.
> 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?
>
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