Antenna idea

[Robert Bruninga]

Terry, nice idea.

Here are some first thoughts...
1) PVC is not straight and is not rigid, at least until you get
to pretty large sizes but fiberglass could work nicely.

2) Why not just telescoping aluminum poles.  My lightweight
pull-up pole is resonant from 55' down to 20 feet. And if I made
it out of 4 sections instead of 3, so that the shortest length
was 16 feet, then it would tune 80 through 20m...  Here are
photos of my pole:

http://www.ew.usna.edu/~bruninga/Antenna-Ipipe.html

Just a thought... Bob, WB4APR
 

> -----Original Message-----
> From: tacos-bounces+bruninga=nadn.navy.mil at amrad.org 
> [mailto:tacos-bounces+bruninga=nadn.navy.mil at amrad.org] On 
> Behalf Of Terry Fox
> Sent: Saturday, August 12, 2006 11:22 AM
> To: tacos at amrad.org; bruninga at usna.edu
> Subject: Antenna idea
> 
> 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.   Questions: 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?   Terry  
>