[Elecraft] Balanced solution for KAT500 tuner?

[Don Wilhelm]

Don,

You must think "system" rather than individual components.  Also with 
antennas and feedlines, the "where is it connected" is a required parameter.

A halfwave dipole in infinite space has a feedpoint impedance of 70 
ohms, but brought down to practical heights, the feedpoint impedance is 
closer to 50 ohms, which is a good match for 50 ohm coax.  Put a common 
mode choke at the antenna feedpoint and connect the coax to it - job done.

Now for the consideration of the loop - actually, a full wavelength loop 
oriented vertically has an impedance of about 120 ohms, so that needs to 
be transformed before connecting to 50 ohm coax.  Typically a 1/4 
wavelength of 75 ohm coax is used for that impedance transformation.  
Yes, put a common mode choke at the antenna feedpoint in all cases.

I encourage you to NOT make assumptions, but measure your antenna 
feedpoint impedance.
An electrical halfwave of feedline (any characteristic impedance) can be 
used to do that.  Connect the halfwave feedline to the antenna, hoist 
the antenna to the operating position and measure the impedance at the 
end of the half wave feedline.  For higher frequencies, the feedline may 
be multiples of 1/2 wavelength.

OK, so how do you find that 1/2 wavelength (or multiple) when the 
feedline velocity factor is a consideration?  You cut the feedline 
initially a little long considering the frequency and the published 
velocity factors for the feedline, then short the far end of the 
feedline while measuring the near end with your antenna analyzer. Move 
the shorting position closer to the near end until the antenna analyzer 
tells you the impedance is 0 +/-j0.  The reactance term is more critical 
than the resistive term.

Equipped with that halfwave transmission line, you can stand at a 
convenient distance from the antenna and measure its actual feedpoint 
impedance - no guessing.


On 9/30/2012 8:37 AM, goldtr8 at charter.net wrote:
> As I am not sure of this stuff I need to ask the following question.
>
> The example below is based on a doublet antenn which has a feed point of
> close to 50 ohms.   So the ladder line is not what you want to transform
> so I understand that a 1:1 is correct.
>
> However if you have an antenna such as a loop which has a feedpoint
> impedance of closer to 200 ohms then a 4:1 would make sense to me.  The
> reason is I need to reduce the feedpoint resistance of the antenna as I
> do not change the resistance based on the feedline.
>
> The rational for the balun is to transform the feedline from balanced
> line to unbalanced line which is the 50 ohm coax.   Thus you want the
> currents matched in the coax thus the balun.    Conclusion balun to
> transform balance to unbalanced and ratio for feedpoint impedance
> matching if required by antenna type.
>