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Current Baluns
Current baluns (also known as choke baluns) allow the output voltage, with respect to "ground" or
outside world, to float to any value required to provide equal currents to each feedline conductor. In
essence, current baluns are a universal device which will be used to drive either balanced or unbalanced
lines or loads equally well. Current baluns isolate the device connected at one end from the device
connected at the other end. The balanced terminals on a current balun can be used to feed unbalanced or
balanced loads or lines equally well. They can also be used as broadband phase-invertors, baluns or
ununs.
Voltage Baluns
Voltage baluns always try to force the output terminals to equal voltages which means currents can be
far from even! A voltage balun almost certainly guarantees some feedline radiation (or reception),
because there are very few "perfectly balanced" loads.
We recommend current baluns, rather than voltage baluns, whenever possible. Current baluns provide
better balance, power handling, often have lower loss, and tolerate load impedance and balance
variations much better than voltage baluns.
Systems Requiring Antenna Tuners
Antenna systems requiring use of an antenna tuner for matching often have very high voltages or currents on
the transmission lines and baluns, even at modest power.
In some cases, coax is used to connect the tuner directly to the antenna. The feedline is connected to a
coaxial connector on the tuner, and the tuner "matches" the poor SWR of the antenna system to the station
equipment. The feedline beyond the tuner still has very high voltage, current and loss, even if the tuner input
is matched to 50 Ω.
In other cases, the antenna feedline is balanced, and the tuner has an internal or external balun.
Unfortunately, most of the internal tuner baluns are 4:1 voltage baluns, which we will see is a poor choice.
Less often, balanced tuners are used. Such tuners, while often better at power handling than the more
common but less robust "T" network tuners, may still not provide the best transmission-line balance. They
would have to be ground independent; otherwise, they are the resonant equivalent of a voltage balun.
There are four areas of concern in such systems:
1. In a multi-band system, the antenna almost never presents a uniform load to the balun. As the
operating frequency changes, the balun load impedance can change from several thousand ohms
to a few ohms.
2. Most antenna tuners work best into high impedances, rather than low impedances. Most baluns
inside antenna tuners step the antenna impedance down. The tuner would actually work better if
the balun simply passed the line impedance through without stepping it down.
3. 4:1 Baluns inside antenna tuners, which are usually voltage-type baluns, are very poor performers
when presented with mismatched loads. 1:1 baluns are generally much more efficient and have a
wider operating impedance range.
4. Voltage baluns have restricted frequency response. The "good performance" frequency range is
much narrower in voltage baluns than in equivalent current-type baluns.
Based on the above facts, a 4:1 balun or any voltage-type balun is the wrong choice for use with antenna
tuners in multi-band systems. Most tuners use them because they are cheap, easy to build, and because
almost everyone else uses them.
