On Sun, 2008-07-06 at 20:28 -0700, Alfred Lorona wrote:
> Does coax radiate? Here is another point of confusion. Assume a properly
> terminated coax fed dipole. The RF travels along the surface of the inner
> conductor and along the inside of the shield. When the RF on the inside of
> the shield reaches the dipole, it sees two metallic conductors; one half of
> the dipole and the coax shield. RF is dumb. It does not know that we want it
> to travel only along one half of the dipole. It sees TWO metallic paths and
> so travels on both. The Rf traveling down along the outside of the shield
> radiates as from a vertical antenna. This malicious current is called an
> 'Antenna Current' or a 'Common Mode Current'.
>
> One way to eliminate it is to use a quarter wavelength long feedline with a
> good earth ground at the station end. The impedance at the ground end is
> ideally 0 ohms and the far end of the feedline shield looks like, ideally,
> an open circuit. The Rf current does not flow into an infinite impedance of
> millions of ohms! Worse case scenario is a feedline one half wavelength
> long. The impedance of the shield to the Rf at the dipole center is now zero
> ohms. The RF will love that!
>
> It is extremely difficult to avoid a common mode current unless the antenna
> is perfectly symmetrical in every respect to it's surroundings; situation
> hardly every achieved in a practical installation. Symmetry includes coming
> away from the antenna at a perfect right angle extending the entire length
> and physical symmetry between the two halves of the dipole with respect to
> structures, poles, trees, hills and so on. It has nothing to do with the
> line SWR. And did I hear someone mention a balun?
>
> Paradoxically, some hams do not mind an antenna current and some degree of
> vertically polarized radiation as they think it enhances their radiation
> pattern both on Tx and Rx! To each his own.
>
> 73, AL
>
Sometimes that vertical polarized radiation at a low radiation angle
makes working DX possible that wouldn't be heard on the dipole.
Sometimes that vertical polarized reception with its lack of directivity
allows hearing local noise to the point few wanted signals are heard.
Sometimes it helps, sometimes it hurts. And sometimes the noise comes
from the ham shack computer and wireless network hardware as well as the
family TV and entertainment equipment.
73, Jerry, K0CQ
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