[Elecraft] Baluns and 450 ohm line

[N2EY@aol.com]

In a message dated 6/4/02 6:07:51 PM Eastern Daylight Time, 
kc4kgu@ENTERZONE.NET writes:

> On Tue, 4 Jun 2002 N2EY@aol.com wrote:
>  
>  > In a message dated 6/4/02 2:53:58 AM Eastern Daylight Time, 
>  > kc4kgu@ENTERZONE.NET writes:
>  > 
>  > > I currently feed my
>  > >  75m doublet with twin pieces of RG8X.  This presents a line impedence 
> of
>  > >  about 100 ohms.  Prior to feeding with twin pieces of coax, I cut the
>  > >  doublet for resonance at 3.875Mhz.  It showed a 1:1 match at 50 Ohms
>  > >  according to the MFJ-269 analyzer.
>  > 
>  > I'm a little confused with your setup.
>  > 
>  > Are there two pieces of RG-8X "side by side", with the center conductors 
>  > going to the feedpoint of the dipole? 
>  > 
>  > Is there a balun in the setup somewhere?
>  
>  
>  No.  No balun.  The two center conductors are used for the feedline and
>  the shield floats at the antenna and is grounded at the shack.

What happens to the two center conductors at the shack end?
>  
>  > >  Now, since the MFJ-269 is designed to give readings based on 50Ohms, 
> what
>  > >  should I be looking for when I check one of my antennas that is being 
> fed
>  > >  with twin coax with a 100 Ohm feedline impedence?
>  > 
>  > That depends entirely on the length of the coax. A basic example:
>  > 
>  > If the electrical coax length (don't forget velocity factor) is an 
> integral 
>  > multiple of a half wavelength, the impedance at the feedpoint will be 
>  > "repeated" at the other end - regardless of the transmission line used! 
>  > 
>  > 73 de Jim, N2EY
 
>  1/2 wavelength of coax including velocity factor.

That means the impedance at the shack end of the coaxes (as measured between 
the center conductors) will be a pretty close replica of the impedance at the 
feedpoint. But as you QSY from the freq where the coaxes are exactly an 
electrical  halfwave long, all sorts of things will happen to the apparent 
impedance and SWR because the line will act as a transformer unless the 
feedpoint Z is exactly 100 ohms.

73 de Jim, N2EY