[Antennas] balanced ant tuner question

[PaulKB8N at aol.com]

To All:
 
W8JI has, I believe, concluded that the balanced tuner design with the  balun 
on the front end is not as efficient and a single-leg design with a good  
balun.  I have tested a lot of designs and have reached that same  conclusion, 
based on antenna current measurements from several tuner  designs.
 
I use a matched pair of 0-1.5 amp RF Ammeters to measure the efficiency of  
the tuners I build.  My conclusion?  Under most circumstances, a  simple L 
network with balun is superior.  The down side of the L is  that it requires large 
amounts of capacitance under certain loads.  These  large values of C can be 
achieved simply by adding parallel fixed capacitors  across the variable 
capacitor.  One must carefully choose these fixed  capacitors, as those with poor 
RF characteristics will drain efficiency from the  design.  Silver mica caps 
with the highest working voltages you can find  are recommended.
 
I have built several balanced designs.  There is a tradeoff between  balance 
and efficiency.  You can "force" balance with the balanced design  by using 
different L and C values on either leg of a supposedly balanced  antenna, 
however, this often results in a less-than-perfect SWR to the  transmitter.  With 
the balun on the output, I always check to see if the  point of perfect SWR is 
also the point of highest antenna current.  Unless  the balun is badly 
designed, that will be the case.
 
I deal with imbalance not at the tuner, but at the antenna.  I'll  extend or 
shorten one of the legs to see how it affects balance.  What is  quite 
interesting is the effect that objects near the antenna have on its  electrical 
length.  On my antenna with 100 foot legs, it often takes  adjustments of a couple 
of feet on one leg to achieve balance.  This  appears to be because of one 
leg's proximity to the house.
 
Will adjustments to leg length to achieve balance work equally on all  bands? 
 Not always.  Different factors come into play on different  bands.  For 
example, when the antenna is close to a house, there may be  gutter along the roof 
that is resonant on 30M that affects balance only on that  band.  It has been 
my experience, however, that lengthening or shortening  of a leg to improve 
balance on one band typically improves balance on the  all the bands.  
Sometimes it is a compromise.
 
Typically, legs of equal length feed with open feeders will  almost always 
result in currents that are balanced within 10% of each  other.  Many antenna 
gurus say that is acceptable.  I try to get  things as equal as possible.  Maybe 
its just psychological, but performance  seems to improve with improved 
balance.  I can usually get a balance of  2-3% on all my antennas through trial and 
error adjustment of the legs.   The balancing act becomes more tricky the 
more asymmetrical the antenna  is.
 
Bottom line:  It is more efficient to "pull" balance by making  adjustments 
at the antenna than to "push" balance at the tuner.  
 
Paul, K5AF