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 infinate 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 comming
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.
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