Topband: Chokes, et. al. - what is the attenuation ?

[donovanf at starpower.net]

Hi Tom,

I'm sure you're correct that a current forcing balun performs poorly when driving an unbalanced load.  This shouldn't be an issue when using it at the balanced feedpoint of a properly constructed monoband Yagi.

tks

73
Frank
W3LPL

---- Original message ----
>Date: Sat, 14 Jul 2012 22:00:54 -0400
>From: "Tom W8JI" <w8ji at w8ji.com>  
>Subject: Re: Topband: Chokes, et. al. - what is the attenuation ?  
>To: "Richard (Rick) Karlquist" <richard at karlquist.com>, <donovanf at starpower.net>
>Cc: topband at contesting.com
>
>> On low bands, the Q of resonant stubs is fairly low.
>> Therefore, the effective common mode impedance of these
>> baluns will not be extremely high.
>
>
>I see a problem now!!!!
>
>The only common mode issue would be if the center point of the elements, 
>which is the common shields, does not sum to zero volts.
>
>Look at a 16.67 load at one element side, and 33.33 on the other, with 50 
>watts. The lines each force 1 amp into each side 180 out of phase, so shield 
>current from that is zero amperes. No problem.
>
>Here's where the problem is. We have 16.67 volts across one cable (16.67 
>watts), and 33.33 volts across the other (33.33 watts). This places the 
>shield junction at 8.33 volts, in the phase direction of the low-Z element.
>
>That means we have 8.33 volts driving the common mode impedance of the 
>shields, which will make the shields radiate and force the system out of 
>balance.
>
>Correct me if I am wrong, but that is how I see it now at second look. It is 
>really only a balun when the load is balanced, or something makes CM 
>impedance high.
>
>So to me, now at second look, it does not appear to be such a good balun. 
>Unless I'm missing something, it is only a good balun when the elements are 
>balanced.
>
>73 Tom 
>
>_______________________________________________
>UR RST IS ... ... ..9 QSB QSB - hw? BK