At 7:26 AM -0700 7/31/03, Richard Karlquist wrote:
>I have been kicking around the idea of making my own air dielectric
>coax using 2 or 3 inch irrigation tubing for the outer conductor. I
>had just about exactly the same idea with the balls. I had been looking
>at plastic playground balls (like they have at McDonalds). There
>are also balls made to float on tanks of liquid to prevent evaporation.
In a web search I found several manufacturers of hollow plastic
balls, in a range of sizes.
An attractive possibility would be cylindrical "plugs" an inch or two
long, of polyethylene or polystyrene foam. Cut them out of a sheet
like cookies, quickly and easily with a hole saw. Sheets of foam are
available inexpensively as thermal insulation for basement
(foundation) walls.
>For the center conductor, I was going to use PVC pipe coated with
>the aluminum foil tape you mentioned.
PVC pipe will sag. Use aluminum or copper thin-wall "vent" pipe. I
prefer copper because it's so easy to join with airtight and reliable
electrical connections by soldering with a propane torch. Both
straight and angled joints are available.
>Originally, the idea is that
>this would cost very little since I have lots of used irrigation tubing
>laying around. But then I realized that it is too dirty on the inside
>and too banged up to use for coax, so I would have to buy new tubing
>for a buck or two a foot....
It still beats Heliax, in loss, in power-handling, and in price.
You could make a low-loss, high-power, shielded _balanced_ line using
a large-diameter copper vent pipe for the shield and small-diameter
pipes/tubes, rods, or wires, depending on the characteristic
impedance you wanted, with cylindrical-plug plastic-foam spacers.
-Chuck, W1HIS
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