tfox at knology.net
Sat Aug 12 10:22:00 CDT 2006
While looking at the SteppIR vertical antenna, I began wondering if we could build a similar vertical at a much lower cost. My idea is this:
Take about 35ft of 2 to 3-inch diameter PVC pipe, and paint it green or brown (to match local flora). At the top, just under the cap, put a pulley. At the bottom, put a fishing reel with about 100ft of heavy-duty line, and a Stanley 35ft steel tape measure. Wrap the line up around the pulley and attach to the end of the steel tape. To start off with, manually run the fishing reel and mark the various lengths where the steel tape is resonant at the various frequencies. A clamp is need to keep the tape at the right length, mechanical at frst, then a solenoid.
Then, attach one or more of the following:
1. Optical rotary encoder tied to either the fishing reel (less accurate), or the tape measure holder.
2. Draw black lines ever xx inches along the blank side of the steel measuring tape and use an optical pickup (yellow background vs black lines).
3. Stepper motor with its internal feedback.
Now, use a surplus stepper motor (from an eight-inch or five-inch disk drive??) driven by a PIC micro, to automatically pull just the right length of tape out. Measure the tape length using one or more of the above approaches. Put all the mechanical and electrical stuff inside a box at the base of the PVC pipe.
As an added bonus, a small SWR bridge can be built into the feed and monitored by the PIC to adjust the tape length for the best SWR, as conditions change.
This could cost less than $100 to build, except for radials, concrete, etc... It would take me a little time to program the PIC, but is easily doable.
1. How well would the steel tape measure radiate? I think steel is a poor radiator, if I remember.....
2. What affect would the unrolled tape have on the resonance, and is there a way to defeat that (such as for low-power to install an electrical contact brush that would contact the back side of the tape, which would be stripped of its paint).
3. Any other problems?
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