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

[Rob Frohne]

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/