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Best Plastics, Composites, Rubber for structural antenna parts

90301 Views 212 Replies 43 Participants Last post by  300ohm
Just a warning note about using ABS for structural parts that I've mentioned many times before - hopefully what you're planning to use is the thick walled stuff that has little or no flex. If you go to Rona or Lowe's or Home Depot and shake a 10 foot ABS pipe like a sword it flexes a lot. ;) It seems to be rigid, but its not. If you shake a length of PVC like that it will seem like a wet noodle, so avoid using PVC for structural use either, except when recommended in build plans.

From experience I'm clarifying that ABS should never be used as a pole or mast, and it should be carefully considered if you're thinking of using it as a horizontal boom of any length more than a meter or a few feet.
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PEX tubing

I tried PEX tubing ( used for water supply lines ), purchased from a home center. PEX comes in different variations, some are advertised as UV resistant but you would probably have to look that up on the web. I had 1/4 inch aluminum rod, so I got the appropriate sized tubing, but they also sell it in larger sizes. It isn't near as stiff as the gutter ferrules, but it was good enough for my design. I ran into the same problem of only finding the ferrules sold along with the spikes for an outrageous price. The PEX was about $2 for a five foot long section.

It was a tight fit getting the rod inside the PEX, so I cut some pieces of .155 inch weed trimmer line (which is pretty stiff) to the length of the gaps and then put it inside the pex tubing and then used a dead blow hammer to force the aluminum rods in from each end until they were snug against the trimmer line. It made it much easier to be sure the reflector rod gaps were close to right. If you are using metal tubing, the trimmer line won't work too well, but something like pieces of fiberglass driveway markers might work as spacers.

Have fun!
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Making HDPE parts

For those of us that can't justify spending $1200 on the Thing-O-Matic (see post #140), it is possible to mould HDPE (High Density Polyethylene) into useful shapes. The best source for HDPE that I have found is old blue or black plastic barrels that industrial fluids or foods are shipped in. The additives put in the plastic when these are dyed provide excellent UV protection for the plastic.

HDPE is a thermoplastic, so it softens as it is heated. When it cools, it holds whatever shape it was formed into. The important temperatures are as follows (in degrees Fahrenheit):
260 It becomes noticeably more flexible
280 It is easy to bend, but retains the dimensions it was cut into
300 It changes in appearance, becoming wet and translucent looking. At this point it loses dimensional stability and begins to shrink up on its self, so the piece becomes thicker and smaller.
Above 300 degrees, it does not melt to a liquid, but rather it has a consistency that reminds me of taffy (the candy). It can still be formed by squeezing it in a mould until it cools.

The trick in heating HDPE is that the temperature range from flexible to dimensionally unstable is pretty narrow. It also absorbs a good deal of heat as it warms, so it helps to warm it slowly so that the entire thickness of the material softens evenly all the way through.

I tried using an ordinary oven to heat it, and had some success, but the temperature swings during the heating cycles make it difficult to get just right. It helps to block the direct radiant heat from the heating elements with a cookie sheet on the lower oven rack and use an oven thermometer to get the temperature right. The HDPE can get sticky if it gets too hot, so I put it on parchment paper in the oven.

I've also used a hot air gun to heat it. This works well when adjusting bends you've made. It is possible to heat just the area that needs to be bent and watch the appearance of the plastic to make sure it is not getting too hot.

Below is a picture of some clips I made to hold the elements on an extended gray-hoverman build.

The one on the left turned out as I wanted, but the one on the right got too hot and started to shrink up on its self and changed dimensions a little.

Below is a picture of the build with the clips holding the driven elements
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Hi unclesam

I looked at some of the plastic welding tools on line a while back, but couldn't justify getting one. It looks like there are two basic kinds, one using hot air, and the other electrically heating a metal tube with a flat foot on it. They try to create a hot spot just ahead of the filler plastic rod that is melted into the crack between the two plastic pieces being welded., so that the plastic pieces and rod are all melting together at that point.

I ran across a web page where someone tried to put a funnel on the front of a hot air gun and make his own hot air welder. He wound up melting the hot air gun because it didn't have enough air flow to keep its case cool. With some modification, that idea might work.

HDPE will keep on burning once it is lit, so be sure to have some safety equipment nearby if you try putting a torch to it. If you figure out a method that works on the cheap, that would be great.
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Hi skipticum,

I recently build one of Jed's extended models. I chose the XGH-v002 from the forum post here. This version still has about 18 dbi gain and is shorter than his first model. There is a picture of my build above in this thread in post #145. I don't think that Jed has added this one to his website yet.

As you can see, for the vertical supports, I chose to use some square boom from a couple of old Radio Shack antennas that didn't survive some winter storms. From my preliminary testing, it appears to be working as designed. The important thing is to keep the reflector elements and the driven elements insulated from the metal, and don't use metal parts for any horizontal supports.

If you get a chance, post a picture of your build for us to enjoy. I don't see too many people attempting to building the extended models.
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