[Elecraft] Baluns and 450 ohm line

Tue Jun 4 18:14:00 2002

On Tue, 4 Jun 2002 [email protected] wrote:

> In a message dated 6/4/02 2:53:58 AM Eastern Daylight Time, 
> [email protected] 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.

>   
> >  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.

73 de John - KC4KGU
K2/100 #2490