SIGNAL LAB

Current Balun Vs Voltage Balun: Why The Feedpoint Gets A Choke

Short answer: a current balun forces equal and opposite currents; a voltage balun forces equal and opposite voltages. On any antenna whose two halves are not perfectly identical -- which is nearly every antenna ever installed -- only the current type keeps the feedline out of the antenna's business. That is why the Greyline feedpoint specifies a 1:1 current choke. But the voltage balun is not a scam; it has a real, narrow job, and knowing what it is completes the teach.

The Distinction, From The Man Who Wrote EZNEC

Roy Lewallen, W7EL, laid this out in his paper Baluns: What They Do And How They Do It, and decades of measurement since have only confirmed it. The two devices share a name and a package. They do opposite jobs.

A current balun is a common-mode choke: it puts a high impedance in the path of any current trying to flow equally on both conductors -- the current that belongs to the feedline's outside, not the antenna. Differential current, the kind that does the radiating, passes untouched. The device does not care what the antenna looks like. Equal and opposite currents, enforced, period. Walter Maxwell, W2DU, built the classic version -- ferrite beads over coax -- and Jim Brown, K9YC, has published the definitive measured data on which cores and windings actually deliver the impedance at HF.

A voltage balun is a transformer. It forces the two output terminals to swing with equal and opposite voltage relative to ground. Read that carefully: it guarantees the voltages. It says nothing about the currents.

Why Equal Voltage Is Not Equal Current

Put two lamps of different resistance across the same voltage and they draw different currents -- the bright one and the dim one on the same shelf. An antenna's two halves are exactly those two lamps. Force equal voltages onto halves with unequal impedances and you get unequal currents by Ohm's law. The difference between those currents has to flow somewhere, and the somewhere is the outside of your feedline.

That manufactured current is the feedline radiating: pattern skew, RF on the microphone, SWR that changes when you re-route the coax, noise pickup on receive. Every one of those symptoms traces to antenna current on the shield.

And No Real Antenna Is Symmetric

The off-center-fed vertical dipole is asymmetric on purpose -- the halves differ by design, which is why the Greyline feedpoint specifies a 1:1 CURRENT choke and nothing else. But this is not just an OCF rule. A center-fed dipole with one end near the house and the other over open yard is electrically asymmetric. A vertical with the feedline running down beside one half is asymmetric. The ground under one end differs from the ground under the other. Symmetry on paper almost never survives installation -- which is why the current balun is the right default on every antenna, not a special accommodation for ours.

When A Voltage Balun Is Actually Appropriate

Fair is fair -- here is the voltage balun's legitimate territory. It wants a genuinely symmetric load, where the antenna itself supplies the current balance and the transformer merely delivers balanced drive. On that load it works acceptably. It also provides something a series choke does not: a DC path to ground through its winding, draining static charge from the antenna -- a real virtue in dry, windy climates. And it lives, mostly unlabeled, inside a generation of antenna tuners whose balanced outputs are a voltage-type 4:1 -- which is why some tuner balanced terminals misbehave on some antennas, a pattern K9YC has documented at length.

So: symmetric antenna, static drain wanted, legacy equipment -- that is the voltage balun's legitimate niche. Here is the completing truth, though. A current balun serves the symmetric antenna just as well, tolerates the asymmetric one infinitely better, and a static-drain resistor or choke handles the DC question separately. The voltage balun persists in catalogs on history and price, not physics. Given the choice on new work, the measured literature all points one way.

The Bottom Line

Current balun: enforces the thing that radiates. Voltage balun: enforces the thing that does not. On the Greyline system, the 1:1 current choke at the feedpoint keeps the antenna's current in the antenna and the feedline invisible -- exactly as W7EL's distinction predicts and as decades of W2DU bead baluns have proven in the field. And if your next question is about the 4:1 sitting in your parts drawer, we wrote that one up with real analyzer plots: Can I Use A 4:1 Balun On A Vertical Dipole? ->

See the Greyline feed system -- the choke where it belongs ->