Stacked Yagis: Gain, Spacing and Switching for Contest Stations

Walk into the antenna farm of a serious contest station and you will see Yagis arranged in vertical stacks: two, three or even four identical beams on the same tower, each pair connected by a bundle of precisely cut coax. Those stacked Yagis are not a showpiece. They are the most effective way to improve a directional station without moving to a bigger tower, because a stack adds gain, lowers and narrows the elevation pattern and, with the right switching, lets the operator choose between a low-angle and a high-angle beam in a moment. The price is mechanical load, cost and complexity. This guide explains the principle, the spacing trade-offs, how the feed system works and when a single, longer beam is the better answer.

Exact stacking distances and phasing line lengths depend on the antenna design, so use the manufacturer’s recommendations or antenna modelling for your actual beams. The numbers in this article are principles, not a build sheet.

What Stacking Means

In antenna terms, a pair of identical antennas fed together, one above the other, forms a broadside array. As the general reference on antenna arrays explains, arrays fed in phase radiate their main lobe perpendicular to the plane of the elements, and in general, the larger the number of elements, the higher the gain and the narrower the beam. Two Yagis stacked vertically and fed in phase are exactly that: each Yagi already has a main lobe pointing forward, and stacking them squeezes the beam in the vertical plane while the horizontal width stays similar to a single Yagi.

That is why stacking differs from adding elements to a Yagi. A longer boom makes the beam narrower in both planes and moves the weight and wind load along the boom. A stack, in contrast, mostly changes the vertical pattern: the main lobe becomes narrower in elevation, and the energy that would have gone high into the sky is pushed toward the horizon where DX stations live. As a general reference on the Yagi-Uda antenna notes, a Yagi has moderate to high gain depending on the number of elements, and the same page shows stacked Yagis fed in phase on a single mast to increase gain in the horizontal direction by reducing power sent toward the ground and sky.

How Much Gain Does Stacking Add?

Doubling the number of identical antennas can at best double the radiated power in the preferred direction, which is about 3 dB. Real stacks fall a little short of that figure because of feed-line losses, imperfect matching and the compromise in spacing. A second equal beam therefore adds a bit under 3 dB in practice, a third adds a smaller increment and a fourth smaller again. Each additional increment of gain costs more than the previous one in tower space and weight, which is why two-high stacks are by far the most common, three-high stacks appear at serious stations and four-high stacks belong to the most ambitious sites.

The gain figure is not the whole story. The stack also changes the take-off angle. Because the vertical aperture is larger, the main lobe is narrower and sits lower, which concentrates energy at the low angles that favour long-distance paths. A stack placed at a good height can therefore sound much louder than the decibel number suggests on a low-angle path, while on a high-angle path, for example a short hop across a continent, it may do less well than a single beam. That trade-off is the reason stack switching exists, which we will come to below. For the background on how height changes take-off angle, see our guide to antenna height and take-off angle.

Stacking Distance

The spacing between stacked antennas is expressed in wavelengths, and it is the single most important design choice. Three principles apply:

• Wider spacing gives more gain, up to a point. As the separation grows, the vertical pattern narrows and the additional gain increases, until the pattern develops strong secondary lobes.
• Too little spacing wastes the stack. When two antennas sit too close, their near fields interact, the feed-point impedances change and some of the potential gain disappears. This is called mutual coupling.
• The right spacing depends on the antenna. A long-boom Yagi has a narrower beam and its effective aperture is larger, so it can use a larger spacing than a short-boom design. The antenna’s gain and beamwidth determine the best stacking distance, which is why makers publish a recommended figure for their own designs.

On HF, the practical limit is usually the tower. The distance between the booms may be fixed by the height of the tower or by the number of stack positions available, and a perfect spacing might not fit. Many operators choose a spacing that suits the tower and then model the result. On VHF and UHF the structure is smaller, and stacking is easier to do close to the ideal distance. For mechanical reasons, keep the booms parallel and aligned vertically, so that the beams point in the same direction at every rotation angle.

Comparing Stack Configurations

Choosing from the table is about the paths you care about. A station focused on Europe from North America, or on long paths across the Pacific, benefits from a low-angle stack. A station that works many different distances may value the option to switch.

How the Feed System Works

Stacked antennas must be fed with equal power in the same phase, and the usual method is a phasing harness: identical lengths of coax from each antenna to a common junction. Two principles keep the harness honest:

1. Equal length, same type. The cable from each antenna to the junction must have the same electrical length. That means the same cable type, cut from the same roll, with connectors fitted the same way. A difference in length changes the phase and steers or splits the beam.
2. Match the combined impedance. Two 50-ohm antennas connected in parallel present about 25 ohms, which would not match a 50-ohm feed line. A common solution is a quarter-wave transformer in each leg. For example, 75-ohm quarter-wave sections between each antenna and the junction transform each 50-ohm load to a higher impedance, and the two in parallel produce a value close to 50 ohms. Because the electrical length of coax depends on its velocity factor, cut the sections to the length for the cable you actually use, not to a free-space number.

Commercial stacking boxes and harnesses take care of this, and they typically add the switching described next. If you build your own, use quality cable, weatherproof every joint and measure the finished array with an antenna analyser before it goes up the tower, because a mismatch in one leg can be hard to diagnose later.

Stack Switching

A well-designed stack lets you select which antennas are in use. In a two-high stack the options are the top beam alone, the bottom beam alone or both together. A three-high stack offers more combinations. The reason it matters follows from the take-off angle discussion:

• Both antennas. The narrowest, lowest elevation beam, ideal for long-distance, low-angle paths and for maximum gain.
• Lower antenna only. A broader pattern with a higher take-off angle, useful for shorter distances or when the band supports high-angle propagation.
• Upper antenna only. A lower angle than the bottom antenna alone, which is useful when the band is open at low angles but the full stack is too narrow or a path is changing.

Operators routinely flip between combinations while listening, because the best choice changes with the opening and the time of day. It takes a second to switch, and the difference can be several S-units. The switching box also provides redundancy: if one antenna fails, you can still operate on the other. Control cables run from the station to the switch at the tower, so make sure they are shielded and bonded properly to avoid noise or RF getting into the shack.

Mechanical Load and Practical Issues

Stacking doubles or triples the wind load and weight on the tower. Check the tower’s rating with the extra antennas, plus the effect of ice if your climate has it. Remember that the rotator turns all of it, so choose a rotator and mast rated for the combined load. Our antenna rotator guide explains how to size a rotator and why the mast, not just the rotator, is often the limiting component.

Other issues that show up in practice:

• Alignment. If the antennas point in slightly different directions, the vertical lobes will not combine properly. Mark the booms and check the alignment at installation.
• Ice and wind. A stack presents a big area. A stack that is fine in light wind can twist the mast in a storm.
• Feeder weight. A phasing harness adds cable. Support it properly so that the weight does not pull the connectors.
• Climbing and maintenance. A taller, heavier array is harder to lower, work on or replace.
• Cost per decibel. The second antenna costs the same as the first but yields about half the gain increment per dollar. Compare that with the price of a longer boom or a taller tower.

Stack or Single Longer Yagi?

If the choice is between a second antenna and a bigger single antenna, consider the goals. A longer boom raises gain across the whole band and improves front-to-back ratio, but the take-off angle does not change much and the boom is long and heavy. A stack changes the elevation pattern and offers switching, but needs space on the tower and a feed system. Many stations do both over time: start with a good single beam, add a second antenna at a lower height when budget and tower allow, and install the switching later.

If your goal is to hear weak DX stations in a contest, the stack’s lower take-off angle and its ability to hear the right propagation mode are often more valuable than a few extra tenths of a decibel of boom gain. If your priority is simplicity, a smaller directional antenna such as a hex beam may be enough, as our comparison of Yagi versus hex beam explores. And if you are planning for a particular contest category, the guide to contest categories helps decide how much antenna investment makes sense for the class you will enter.

Common Mistakes

• Mixing different antennas. A stack works best with identical antennas, so that each contributes equally. Mixing designs gives uneven currents and unpredictable patterns.
• Cutting phasing lines carelessly. Unequal lengths or mixed cable types spoil the phase relationship.
• Stacking too close. Mutual coupling eats gain and shifts impedance.
• Ignoring the tower rating. A stack that looks fine on paper can overload a tower in a storm.
• Forgetting the switching. A fixed stack is a compromise; the ability to select antennas is much of the stack’s value.
• Skipping the measurement. Always check SWR and bandwidth on the finished array before it goes up.

The Bottom Line

Stacked Yagis are the classic upgrade for a station that already has a good beam and a tower with room to spare. Identical antennas fed in phase give a gain increase just under 3 dB for two, a narrower and lower elevation pattern and, with a switching box, the freedom to match the take-off angle to the opening. Choose the spacing using the manufacturer’s data or modelling, feed the stack through equal phasing lines with proper impedance matching, check the tower and rotator loads and measure the result before it goes up. Do that, and the stack will outperform a single antenna on exactly the long-distance paths that contests reward.

Frequently Asked Questions

How much gain does a second Yagi add?

Up to about 3 dB in theory, and a little less in practice because of spacing compromises, feed-line loss and mismatch. The third and fourth antennas add progressively less.

Do the antennas have to be identical?

For a conventional stack, yes. Identical antennas fed in phase combine predictably. Different designs have different gains and impedances, which spoils the array pattern.

What stacking distance should I use?

Use the manufacturer’s recommendation or model your antennas. The best distance depends on boom length, gain and frequency, and spacing that is too close wastes gain through mutual coupling.

Why would I switch off one antenna in a stack?

The take-off angle changes with the combination. The lower antenna alone favours higher angles and shorter distances, while the full stack suits low-angle, long-distance paths.

Is a stack better than one longer Yagi?

They solve different problems. A longer boom increases gain and front-to-back ratio, while a stack lowers and narrows the elevation pattern and provides switching options.

Can I stack antennas on different bands on the same tower?

Yes, and many stations do. Keep the antennas for adjacent bands well separated and consider filters, as multi-band sites often meet cross-band interference.