Six 20 meter Sterba curtains 120' high 12 directions
- skylarkcolo
- Apr 27, 2024
- 8 min read
Updated: 9 hours ago
W7YRV antenna in the 1990's on the AZ desert
The Sterba Curtain Array (invented by Ernest J. Sterba in the 1930s for Bell Labs) is one of the most powerful, high-gain directional wire antennas ever conceived for high-frequency (HF) communication.
Unlike the rhombic antenna—which projects its signal along its horizontal length—a Sterba curtain is a broadside array. It hangs vertically like a massive curtain between two or more masts and beams its energy perpendicular to the plane of the wires.
Schematic diagram of six 20 meter sterba curtains at 120' high.
NOW THAT IS A BIG WIRE ANTENNA System.
The Sterba Curtain Array (invented by Ernest J. Sterba in the 1930s for Bell Labs) is one of the most powerful, high-gain directional wire antennas ever conceived for high-frequency (HF) communication.
Unlike the rhombic antenna—which projects its signal along its horizontal length—a Sterba curtain is a broadside array. It hangs vertically like a massive curtain between two or more masts and beams its energy perpendicular to the plane of the wires.
Here is a breakdown of how the Sterba curtain functions, its unique design mechanics, and why it became a staple of early international broadcasting and military operations.
1. How It Works: The Continuous Loop
The defining characteristic of a classic Sterba curtain is that it is constructed from a single, continuous loop of wire folded back on itself in a specific geometric pattern.
The Elements: It consists of multiple vertical and horizontal sections, usually spaced one-half wavelength ($\lambda/2$) apart.
The Phasing: The genius of Sterba's design is the folding mechanism. The current traveling through the wire reverses direction at every fold. Because of the spatial positioning of the wires, the currents in all the vertical elements flow in the exact same direction at any given instant (in-phase).
Cancellation of Horizontal Radiation: The currents in the horizontal connecting elements flow in opposite directions, causing their radiation to cancel each other out. This leaves only the massive, synchronized RF energy of the vertical elements to radiate.
2. High Gain and Sharp Directivity
Because a Sterba curtain can feature many vertical elements linked together, it acts as a massive RF "lens."
Bi-directional Beam: In its native, free-hanging form, a Sterba curtain shoots a highly concentrated beam of energy out of both the front and back of the curtain.
Unidirectional Modification: To focus all the power in one direction, engineers frequently hung a second, identical curtain exactly one-quarter wavelength behind the first one to act as a parasitic reflector. This reflects the rear-facing beam forward, yielding a single, incredibly tight, high-power beam.
Gain Performance: A large Sterba curtain with a reflector can easily achieve 12 to 15 dB of gain. This is comparable to or better than a massive rhombic array, but it achieves this performance in a fraction of the horizontal land footprint.
Footprint: A Sterba curtain achieves its 12 dB gain hanging vertically between two masts over a thin strip of land. A Rhombic requires acres of land to stretch its wires out horizontally to get the same exact signal strength.
3. Sterba Curtain vs. Rhombic Antenna
During the golden era of shortwave, engineering teams (including Soviet teams under designers like Ayzenberg) frequently chose between Rhombics and Sterba Curtains depending on their resources:
Feature | Sterba Curtain | Rhombic Antenna |
Real Estate | High vertical footprint, small horizontal footprint. | Enormous horizontal footprint (acres of land). |
Bandwidth | Narrowband. Strictly resonant; only works well on one or two harmonically related frequencies. | Extremely Broadband. Can cover an entire chunk of the HF spectrum without retuning. |
Efficiency | Very High up to 90%. No termination resistor; virtually all input power is radiated. | Lower 50% (K0UO RePhasing- 90%). Significant power is wasted as heat in the far-end termination resistor, if not using the K0UO RePhasing System |
Construction | Complex wire-rigging and phasing lines; requires tall, heavy masts. | Relatively simple wire geometry across standard poles. |
4. Feedline and Impedance Characteristics
A classic Sterba curtain presents a relatively high input impedance (often around 600 ohms), making it perfectly suited to be fed with standard open-wire parallel transmission lines (ladder line). The feed point is typically located at the bottom center or top center apex of the curtain where the loop breaks to accept the line.
5. Why They Faded from Common Use
While the Sterba curtain was a masterpiece of analog phasing, it had one critical flaw: it is fixed in frequency and direction. If a shortwave station needed to change frequencies to adapt to changing ionospheric conditions throughout the day, a single Sterba curtain couldn't do it.
When comparing the raw forward gain of a classic Sterba Curtain Array against a Rhombic Antenna, the comparison reveals a fascinating clash of antenna philosophies: raw efficiency versus operational bandwidth.
By the late mid-century, the classic Sterba was largely replaced by the HR (Horizontal Dipole) Curtain Array, which uses independently fed dipole elements over a reflecting screen. These modern curtains allow engineers to electronically steer the beam horizontally and vertically, and change frequencies across multiple shortwave bands using a single physical structure.
Still, for point-to-point "pipe-line" communication circuits where a station only needed to blast a massive signal to a single target area on a single frequency, the Sterba curtain is still a Pro.
Why the Sterba Curtain Wins on "Gain-per-Acre"
If a massive, multi-acre Rhombic and a vertically hung Sterba Curtain both score around 12 to 16 dB, why is the curtain considered superior in gain efficiency?
The Termination Loss: A unidirectional Rhombic antenna requires a termination resistor at its far end. This resistor literally burns up 30% the transmitter's total power as pure heat to enforce its one-way directionality. Note the K0UO Re-Phasing does not
The Curtain's Efficiency: The Sterba curtain has no termination resistor. It uses a continuous, resonant loop where virtually 905 or more of the applied RF energy is forced into the air. It achieves its directivity purely through phase synchronization.
Footprint: A Sterba curtain achieves its 12 dB gain hanging vertically between two masts over a thin strip of land. A Rhombic requires acres of land to stretch its wires out horizontally to get the same exact signal strength.
3. The Catch: Bandwidth vs. Directivity
If the Sterba curtain is so much more efficient and compact, why did engineers like Grigory Ayzenberg still deploy giant steel-wired Rhombics across the USSR?
The Rhombic's Superpower: The Rhombic is an extremely broadband (traveling-wave) antenna. A single Rhombic can maintain high gain and a stable input impedance across a 2:1 or even 4:1 frequency range (e.g., performing flawlessly from 7 MHz all the way up to 28 MHz). If the ionosphere shifts during the day, you just change the transmitter frequency.
The Sterba's Critical Weakness: The Sterba curtain is a highly resonant (narrowband) antenna. Due to the necessity for its elements to remain precisely in-phase, it functions effectively only at one particular design frequency (with a very limited 5% to 10% bandwidth range). If the frequency is altered, the phase coherence breaks down, the elements interfere with each other, and your gain drops to zero.
Many hams try to use their Sterba on other bands by using an antenna tuner, it may load up, but it will be out of phase, it may be below zero gain by doing that.
Summary
If you are building an array for a single, fixed-frequency, point-to-point pipeline, a Sterba Curtain will blast a stronger, more efficient signal using a tiny fraction of the land. But if your communication circuit requires jumping across multiple HF ham or shortwave bands throughout the day, the brute-force, wideband footprint of the Rhombic takes the crown.
SEE
Distributed-Feed VOA Curtain Arrays
Distributed-Feed VOA-type Curtain Array (often referred to strictly as an HRS-type curtain or USIA array) is structurally and electrically far superior to a classic Sterba or Bobtail Curtain.
While Ernest Sterba’s 1930s design was a stroke of genius for its time, it has fundamental electrical flaws that modern distributed-feed (branched-feed) systems completely eliminate.
If you compare a distributed-feed broadside array against a classic Sterba, the VOA-type curtain wins across every performance metric:
The Sterba curtain represented a significant advancement in the early days of radio physics; however, it remains an analog compromise. The Distributed-Feed VOA Curtain Array refined the concept of broadside radiation through meticulous parallel network engineering. This innovation resulted in substantially increased gain, a robust radiation pattern, wideband frequency adaptability, and the capability for electronic beam steering.
1. The Critical Difference: Series vs. Parallel Feeding
The root of why the VOA distributed curtain destroys the Sterba comes down to how RF current is routed to the individual radiating elements.
The Sterba Curtain is Series-Fed: A Sterba is a single, continuous, long zig-zag loop of wire. The RF energy enters at one point and must travel through all the conductors in a long series path to reach the outer elements.
The Flaw: Because the wire itself has inherent resistance and a high velocity factor error, phase errors accumulate the further the energy travels from the feed point. By the time the RF current reaches the outer elements, it is slightly out of phase and weaker, causing the antenna's pristine theoretical radiation pattern to distort and its gain to drop.
The VOA Curtain is Parallel/Distributed-Fed: A distributed-feed curtain treats every single half-wave dipole element as an independent unit. A main feedline branches out into multiple, perfectly equal-length parallel transmission lines (a branching tree network) that connect to the center of every single dipole simultaneously.
The Victory: Because every element is fed from a common point via identical paths, phase error is effectively zero. All elements receive identical current and identical phase, forcing the array to hit its absolute maximum theoretical forward gain.
2. Bandwidth: The Octave Jump
Sterba Curtain: Because it relies on precise, absolute physical lengths to maintain phase relationships, it is strictly narrowband. It works on one frequency and suffers massive pattern destruction if you move even 5% away from its design center.
Distributed VOA Curtain: Because it is driven by parallel branch lines rather than series-folded loops, the elements don't "fight" each other when the frequency changes. A distributed-feed curtain can maintain a stable input impedance, maximum gain, and a clean pattern across a 2:1 frequency ratio (an entire octave). A single VOA curtain can seamlessly cover multiple shortwave bands (e.g., from 7 MHz up to 14 MHz) without sacrificing performance.
3. Electrical Beam Steering (Slewing)
Because a Sterba curtain is one hard-wired continuous loop, its directional beam is permanently fixed broadside to the wires.
A distributed-feed VOA curtain allows engineers to install delay lines or electronic phase shifters into the branching feed network. By systematically delaying the phase of the RF signal going to the left columns versus the right columns, the forward beam can be electronically steered (slewed) up to 30 degrees left or right in azimuth. Similarly, delaying upper rows relative to lower rows can adjust the elevation (takeoff angle), allowing a single fixed wire structure to target entirely different continents based on time of day.
4. Gain-per-Acre Efficiency
Because the parallel feed system forces uniform current distribution across the entire vertical aperture, a distributed-feed curtain achieves the absolute highest gain-per-acre of any antenna system ever conceived.
A standard 4-element-wide by 4-element-high distributed curtain backed by a wire reflector screen can effortlessly pull 17 to 22 dB of raw forward gain.
A Sterba curtain occupying a similar physical footprint struggles to get anywhere near that, often hitting a hard ceiling around 12 dB due to the cumulative phase and current degradation inherent in its series layout.
Summary
The Sterba curtain was a magnificent step in early radio physics, but it is an analog compromise. The Distributed-Feed VOA Curtain Array took the concept of broadside radiation and perfected it through rigorous parallel network engineering—yielding vastly more gain, a bulletproof radiation pattern, wideband frequency flexibility, and the power of electronic beam steering.
SEE K0UO's Highest Gain Array, a 20dB CURTAIN PAGE https://www.k0uo.com/post/current-distributed-feed-system-as-used-on-sterba-curtains-will-be-utilized-on-the-new-antenna
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