Of course this is one reason for applying a choke to the outside of the
feedline, at the antenna.
73 de Gary, AA2IZ
----- Original Message -----
From: "Alfred Lorona" <w6wqc@dslextreme.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Sunday, July 06, 2008 11:28 PM
Subject: Re: [TenTec] OT SWR VS Power Loss
> 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.
>
> 73, AL
>
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