Hi Jim,
Thanks for the interesting comments....
On Apr 25, 2004, at 2:21 PM, Jim Lux wrote:
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.
Actually, what I want is more like an adjustable yagi, somewhat like
Steppir does, but perhaps using drooping elements to make the size
smaller. The dipole is a first step to that end. You don't want to
bite off more than you can chew at once, and so I figured starting with
a dipole would be a good first bite.
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).
The idea of using a tunable inductor isn't bad. The screwdriver
antennas seem to be based on this idea, and work pretty well at higher
frequencies.
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.
Personally, I'm not looking for tenths of a db change. I don't think
the general audience for an antenna of this type is really looking for
that either.
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.
I've thought about this, but all my ideas seemed kind of complicated.
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.
Yes, lets hear some more ideas like this.
One might also be able to order BeCu strips with holes in them.
I sure would like to be able to do that. The guys at Steppir told me
that they have to machine the holes themselves, but it would be a great
breakthrough to be able to find the strips with holes at a reasonable
price. I did a little checking with some manufacturers, but didn't
have any luck there. Don't let me discourage anyone though, because I
wasn't thorough.
73,
Rob
--
Rob Frohne, Ph.D., P.E.
E.F. Cross School of Engineering
Walla Walla College
http://www.wwc.edu/~frohro/
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