The Compassion Antenna -

Making the Bellows:
 

The bellows prevents the weather and dirt from destroying the antenna coil.  As you can see from the overall design, a fixed shield would not work.  A good bellows must also not absorb RF.  When I tied to microwave a shock absorber bellows to see if it absorbed RF, it got hotter than the water I put in for safety!  So, I figured out how to make a bellows using "Plasti Dip".

Plasti Dip is a rubber coating commonly used to coat tools.  I tried dry coatings of brands other than the official Performix brand and they noticeably heated up in the microwave.  The dried Plasti Dip did not feel warm the slightest, showing that it absorbed little if any RF.

Making a mold for the bellows to coat with Plastic Dip was the next challenge.  After trying clay and similar modeling materials I had almost given up.  It then occurred to me that if I could wrap a plastic triangular tube around a pipe as wide as the antenna coil then it would be a good bellows mold.  My final solution approximates this idea by stacking layers of either wire or round cord.

Order of wrapping the pipe to make the triangular ridges as a mold for the flexible bellows:
 
   1       1       1
  2 2     2 2     2 2
 3 3 3   3 3 3   3 3 3
4 4 4 4 4 4 4 4 4 4 4 4
.
Pipe wrapped with cord
.
    4 4 4 4 4 4 4 4
     3 3 3   3 3 3
      2 2     2 2
       1       1

The pipe is wrapped with cord in spiral layers, the ends of the cord taped to the pipe to keep the cords tight  In this example with four lawyers, the bottom layer has 4 cords wrapped in parallel, labeled 4 in the diagram.  The next layer has 3 cords put in the groves above the first four.  Then a layer with 2 cords.  Final a peak with only one cord.  This gives you the approximate triangular shape to forum the bellows.

Next the cords are carefully and tightly wrapped with plastic wrap.  A length of thin nylon fishing line is tightened over the plastic wrap to pull it into the groves, with the ends taped.  Once the plastic wrap is tight it can be coated with Plasti Dip.

I am still experimenting with the best way to coat the mold.  Because the spray seems more expensive than the canned Plasti Dip, I decided to coat with layers of both.  Also, to ensure continuous layers I have alternated colors of Plasti Dip to see that each layer is completely made.

First, a layer of black is sprayed on until the cords and plastic wrap can not be seen.  Then a thin layer of red is painted on with a brush.  (This takes practice.)  Finally a layer of black is sprayed on as a final coat.  It is important to prevent bubbles at each stage and ensure continuous coverage.

To make a thicker bellows simply add more layers.  I am not certain that thicker is better.  It seems that the thinner you can make the bellows and have it work ok is the best.  With more experimentation it should become clear what is best.

Once the bellows is dry, you can slide it off the pipe and pull the cords out from inside!

Note that trying to use clay or something else to form the mold would have presented the problem of getting the dried bellows off without ruining it.  Using cords or wire has a definite advantage.
 
 

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© Copyright 2007 - Michael John Lake - Make copies for personal use only.