[TowerTalk] Shielded balanced line NOT using coax

[Chuck Counselman]

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