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" <w...@w8ji.com> >Subject: Re: Topband: Chokes, et. al. - what is the attenuation ? >To: "Richard (Rick) Karlquist" <rich...@karlquist.com>, ><donov...@starpower.net> >Cc: topband@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 _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK