Thanks, Tom.
I should have stated that in the absence of radials, the coax will radiate
instead due to the common mode current on the outside of the shield. That's
a given.
Let's forget that I said a small number of symmetrical radials, and assume
a large number. You said "With four radials things look pretty good", but
lets make it 60 or 120. And while we're at it, let's choke the heck out of
the coax with lots of ferrite beads over the jacket. What percentage of the
radiation comes from the radials then? Probably, it's pretty insignificant.
(Maybe. :-)
73, Mike
www.w0btu.com
On Fri, Aug 24, 2012 at 2:04 PM, Tom W8JI <w8ji@w8ji.com> wrote:
> > It goes without saying that both halves of the antenna (radials and the
> > vertical) must be present in order for the bottom-fed vertical monopole
> to
> > radiate.
>
> A vertical will radiate without any ground, and actually radiate pretty
> well if the common mode feedline currents can be controlled and loading
> losses minimized.
>
...
The worse thing is probably keeping the feedline tame, and it gets worse
> with smaller or fewer radials (no matter what type they are).
>
> > I don't have a problem with understanding that both legs of a 90 degree
> > center-fed dipole (such as with one element vertical and one element
> > horizontal) radiate. But everything I've ever studied seemed to indicate
> > that in the case of a symmetrical counterpoise (balanced, equally spaced
> > wires and with the monopole directly under their apex) parallel with the
> > earth and with a sufficient number of radial, >99% of the far field comes
> > from the vertical monopole.
>
> I'd be careful about that number if you are talking about two radials. I
> don't know what the exact ratio is, but I do know pattern distortion can
> be significant even without a feedline with two radials. With four radials
> things look pretty good, if we keep the feeder out of the radiating system.
>
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|