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Author Topic: Feedline - K1JJ  (Read 6605 times)
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Radio Syracuse
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Syracuse Radio W2INR

« on: June 12, 2005, 11:50:58 AM »

Feedline   -   Open wire

Tom K1JJ


: I was wondering - why should the cheap plastic commercial open wire brown crap be more vulnerable to the weather variations  problem (or any other problem for that matter) than genuine "open wire" line??. What causes losses on feedlines in the first place?

Except for normal I2R losses that any feedline will have based on its wire diameters, a significant loss is caused by the dielectric material that is used to hold the two conductors apart.

Coax is lossy mainly due to the dense insulation material packed between the inner conductor and shield. I use a type of hardline here that uses a plastic "disc" every 3" to support the inner conductor..VERY low loss.

And then, the brown crap uses a bridging plastic about 50% of the path. (the punched out squares reduce this loss).

The good, home made open wire uses very little contact area with thin ceramic spacers to separate the feedline along with a wider path.

A pure air dielectric is the most desirable type, but how do you support and maintain the center conductor spacing?

The ultimate line is no spacers at all... two wires spaces 2' apart pulled taunt down from the tower to the ground. All air except for the end support points..

There are others factors, but generally the less contact and BETTER quality material the dielectric has, the lower the loss the feedline will exhibit per foot. Conductor size helps too, of course.

So, rain and contaminants on the dielectric act to be a loss path across the two conductors and produce "circulating currents" or loss.

Picture it if you poured salty water on the feedline. The more surface area for the salt to contaminate, the worst the problem is.

In contrast, how would you bridge two parallel wires with no insulators? Just air.

G - The INR

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