[Antennas] balanced ant tuner question

Wed Dec 21 09:57:36 EST 2005

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