Half-Square and Bobtail Curtain: Feed the Current, Not the Voltage

The half-square and the bobtail curtain are attractive antennas because they behave like compact, self-contained vertical arrays. They can produce useful low-angle, mostly vertically polarized radiation without needing the same buried radial field as a classic ground-mounted vertical.

A half-square is essentially two quarter-wave vertical sections joined at the top by a half-wave horizontal phasing wire. A bobtail curtain extends the idea to three quarter-wave vertical sections, with the top horizontal wires feeding the outer verticals from the center section. In both cases, the horizontal wire is mainly part of the phasing and current-distribution system, while the vertical sections do most of the useful low-angle radiation.

The important practical lesson is simple: do not feed these antennas at the convenient high-voltage bottom point unless you accept a touchy, site-dependent match. For a predictable installation, feed them at the low-voltage, high-current point.

Where the Current and Voltage Are

In a down-hanging half-square, the two bottom ends of the vertical legs are high-voltage, low-current points. The useful low-impedance feedpoint is at one of the top corners. That top corner is a current-rich point, and the vertical wire hanging down from that feedpoint acts as the immediate return side, or counterpoise/radiating leg, for the coax-fed system.

Half-square, current-fed

       low V / high I feed
              ↓
       o----------------------o
       |                      |
       |                      |
       |                      |
 high V / low I          high V / low I

For the bobtail curtain, the equivalent current-feed point is the top of the middle vertical. The bottom of the middle vertical is the traditional high-voltage feed location when using a tank circuit or voltage-feed system, but it is also the most environmentally sensitive point.

Bobtail curtain, current-fed

       o----------o----------o
       |          ↑          |
       |    low V / high I   |
       |                     |
 high V     high V       high V

The distinction matters because voltage points and current points behave very differently in the real world. A current-rich feedpoint tends to have a more ordinary impedance and a lower voltage to the surrounding environment. A voltage-rich feedpoint has high impedance, high electric field, and much greater sensitivity to whatever is nearby.

The Attraction of the High-Voltage Feed

The high-voltage feed has one obvious advantage: convenience. The feedbox or matching network can sit near the bottom of a vertical leg instead of being mounted up in the air. On a bobtail curtain, this is why the traditional high-voltage feed with a parallel-tuned circuit near ground level became popular.

With a suitable tank circuit, the operator can often “make it match” even when the raw impedance is awkward. That does not automatically make the system stable. It only means the matching network can transform the high, often reactive impedance to something the transmitter accepts.

That convenience comes at a price. A high-voltage feed is a current-minimum feed. The impedance is high, the electric field is strong, and the system becomes sensitive to details that would barely matter at a 40–70 Ω current-feed point.

Why the Bottom High-Voltage Point Is Unpredictable

At a high-impedance point, the antenna is looking for a very small current at a very large voltage. Tiny stray capacitances and leakage paths can therefore become important parts of the feed system.

Wet rope, a damp fiberglass pole, a nearby tree branch, the operator’s hand, a metal mast, the coax shield, or a change in soil moisture can all become part of the electrical system. The antenna may still tune, but the tuned condition can move with weather, installation geometry, or even small layout changes.

The scale of the problem is easy to see. At 14 MHz, 1 pF of stray capacitance has a reactance of roughly 11 kΩ. Five pF is roughly 2.3 kΩ. If the feedpoint impedance is several thousand ohms, a few picofarads from wet insulation or nearby objects is no longer a tiny detail. It can become a major shunt path.

At a 50 Ω current feedpoint, the same few picofarads hardly move the match. That is the whole point.

The half-square has an extra annoyance: it has two bottom high-voltage ends. Even if only one bottom end is used as the feedpoint, the other bottom end is still a high-voltage capacitive termination close to ground, moisture, supports, and nearby objects.

The bobtail curtain has three bottom voltage points. When the middle bottom is used as the feedpoint, the tuner and feedline are sitting exactly where the electric field is most eager to couple into the environment.

Why the Top Current Feed Is More Predictable

A current feed does the opposite. It feeds the antenna where the voltage is low and the current is high. In that part of the antenna, the feed impedance is normally in the range of ordinary coaxial systems, or close enough to handle with a simple choke and modest matching adjustment.

The deeper reason is not just “lower SWR.” It is field distribution. At the top current-feed point, the voltage to the surrounding world is relatively low, so accidental capacitance to the environment has much less leverage. The feedpoint current is encouraged to flow in the intended antenna conductors instead of wandering onto the outside of the coax, the mast, wet rope, or station ground.

That makes the antenna less picky about weather, mounting details, and local ground conditions. The radiation pattern still depends on height, ground quality, nearby conductors, and installation symmetry, but the feed impedance becomes much more repeatable.

Feeding the Half-Square

For the half-square, the preferred current feed is at one top corner.

Half-square corner feed

        coax center
            |
            v
       o----------------------o
       |
       |  coax shield
       v
       |
       |

In practice, break the wire at the top corner. Connect the coax center conductor to the horizontal/top-wire side and the coax shield to the vertical downleg. Reversing those connections normally only changes the phase reference, not the usefulness of the antenna. The important point is that the feed is across the top-corner gap, not from the bottom high-voltage end.

The vertical wire below that feedpoint is not “just a support wire.” It is the counterpoise side and one of the radiating vertical elements. Treat it as part of the antenna.

Keep the coax from running parallel to the vertical radiator. Bringing the coax away from the feedpoint at roughly 45 degrees is a good practical choice because it reduces coupling between the feedline and the high-current antenna wires.

A good 1:1 current choke belongs at the feedpoint, or as close as mechanically possible. The choke’s job is to stop the outside of the coax from becoming a third, uncontrolled antenna element. That matters especially with these antennas because the whole design depends on the intended vertical currents and phasing wire doing the work.

Feeding the Bobtail Curtain

For the bobtail curtain, the cleaner current-feed point is the top of the middle vertical.

Bobtail curtain center-top feed

            coax center to top wires
                    |
                    v
       o------------o------------o
                    |
                    | coax shield to center vertical
                    v
                    |
                    |

Again, the middle vertical hanging downward is the counterpoise/radiating side of the feed. The horizontal wires feed the two outer verticals, and the center vertical becomes the central current reference for the system.

If the exact top feedpoint is somewhat below 50 Ω, you can move the feedpoint slightly down the center vertical or use a modest matching method. But this is still very different from feeding the bottom end at a high-voltage point. You are still working in a low-voltage, current-rich part of the antenna, so the system remains much less sensitive to stray capacitance and weather.

The Ferrite Choke and the 45-Degree Coax Drop

A 45-degree coax exit is good practice. The goal is to get the feedline out of the near field of the antenna without letting it lie alongside a vertical radiator or the top phasing wire. The cleaner the feedline exit, the less the coax shield contributes to the pattern and impedance.

A mix 43 ferrite choke can be a reasonable choice around the upper HF range, including 20 meters, provided the actual core size and number of turns produce enough common-mode impedance on the operating band. The key is not simply “use ferrite.” The key is enough choking impedance at the band of operation.

A poor choke only moves the problem. A good choke makes the coax stop participating in the antenna.

Why These Antennas Shine from 20 Meters Through 10 Meters

The half-square and bobtail curtain are especially attractive as monoband arrays on 20, 17, 15, 12, and 10 meters because the physical size is still manageable.

These are practical dimensions for fiberglass poles, trees, portable masts, or lightweight supports. The feedpoint is high, but not absurdly high. The antenna is also naturally monoband: the spacing, vertical length, and phasing-wire length are all chosen for one band.

That is not a disadvantage if the goal is a simple, effective, low-angle monoband DX antenna. It only becomes a problem when we expect a carefully phased vertical array to behave like a random multiband wire.

Why Lower Frequencies Often Have Better Alternatives

On 40 meters and below, the same antennas become large quickly. A 40-meter bobtail is roughly 40–42 meters wide with 10-meter vertical sections. An 80-meter bobtail is roughly 80 meters wide with 20-meter vertical sections.

A half-square is smaller, but still large: roughly 20 meters wide on 40 meters and 40 meters wide on 80 meters, with quarter-wave vertical drops.

They can work well if you have the supports and real estate. But for many stations, the mechanical difficulty, narrow bandwidth, high feedpoint access, and interaction with nearby ground and objects make other options more practical.

On lower bands, a well-built inverted-L or T-vertical with a controlled radial/counterpoise system, a pair of phased verticals, or a dedicated receive antenna plus a simpler transmit vertical may be easier to build, adjust, and maintain.

Lower-band half-squares and bobtails are not wrong. They are simply more demanding. Design choices become tradeoffs: the dimensions that give maximum gain may not produce a convenient 50 Ω feed, while the dimensions that produce a friendlier feed impedance may give up some pattern performance or bandwidth.

The Bottom Line

The bottom high-voltage feed is convenient, but it is also the most environmentally sensitive place to feed a half-square or bobtail curtain. It puts the matching system at a voltage maximum, close to ground, wet supports, stray capacitance, and the coax shield. That is why it can tune well in one location and become unpredictable somewhere else.

The top current feed is less convenient mechanically, but much more predictable electrically. On the half-square, feed a top corner. On the bobtail curtain, feed the top of the middle vertical. Let the vertical wire below the feedpoint act as the counterpoise/radiating side, route the coax away at an angle, and install a proper 1:1 current choke at the feedpoint.

In short: feed the current node, not the voltage node. That is the difference between an antenna that merely matches and an antenna that behaves.

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