"John Smith" wrote in message ...

What keeps the mast below the J pole from radiating?
Dale W4OP


They tell me it's the 1/4-wave matching section which stops the rf from
moving on down the mast. A shorted 1/4 wave piece of transmission line looks
like an open circuit or essentially a choke to the antenna. In fact,
microstrip line construction uses 1/4 wave sections for RF chokes.

73,
John - KD5YI


Yep. It was disscussed on this forum how with certain types of
waveguides, that every so often, there would be a 1/4 wave "insulator"
that would suspend the center conductor from the outer ground.

A short rotated 1/4 wavelength "towards the generator" on a Smith
Chart will tell you that electrically, it's virtually like an open.

But even when you know this, it's still kinda neat to think how it
really doesn't matter how long or how grounded the "handle" of the
J-pole is.

Very cool stuff.

Getting back to my question, i would say that i disagree that the
only limiting factor is the breakdown voltage of the insulators. The
limiting factor would be the current carrying capabilities of the
conductors themselves, and when you include the skin effect, a larger
diameter copper pipe can handle more current naturally.

I think a 3/4" J-pole should be able to handle 500 watts at least.


Dr. Slick