[Elecraft] Balun loss
Don Wilhelm
[email protected]
Wed Dec 17 00:26:01 2003
Actually Earl, the sources of the common mode currents are usually 2, and
they are different animals. One is the pickup of radiation from the antenna
itself onto the line - and that is cured only by a symetrical arrangement of
the feedline. The feedline still picks up energy, but the amount from each
side of the antenna is the same but of opposite phase and they cancel if the
arrangement is symetrical - if not symetrical, they add as vectors.
The second source for common mode current is that the current exiting from
the inside of the coax shield sees two conductors - the outside of the coax
and the antenna leg. The current will divide between the two paths unless
there is something to prevent it - that is where the balun comes in.
The first source of current is the one Ron is referring to, and he is
correct that a balun at the feedpooint will do nothing to stop the coupling
onto the line.
73,
Don W3FPR
----- Original Message -----
From: "Earl W Cunningham" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Tuesday, December 16, 2003 11:35 PM
Subject: Re: [Elecraft] Balun loss
> Ron, AC7AC wrote:
>
> "The only way to stop them in a conductor is to place it so the RF
> currents induced in it are exactly opposite and equal to produce a net
> zero current. In a dipole that means running the feedline away at exactly
> right angles."
> ==========
> This can possibly alleviate the common mode current coupling between the
> coax and the antenna, but the recommended method is to use a choke balun,
> per the ARRL Antenna Book.
>
> 73, de Earl, K6SE
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