Stacking Yagi Antennas

Capture Area or Effective Aperture

The capture area of an antenna - usually known to professionals as its effective aperture - is roughly defined as the area covered by a planar or aperture array with the same gain and beamwidth characteristics.

For example, if we built a giant horn antenna with the same gain and beamwidth as the yagi that we're viewing head-on in the diagram below, the front aperture of the horn would be the same as the effective aperture of the yagi.

The effective aperture of a yagi is roughly elliptical, with its longer axis along the length of the elements. Note that the effective aperture extends symmetrically above and below the plane of the elements, and also extends symmetrically out beyond the physical length of the elements.

More Capture Area = More Gain

The bigger the capture area of any antenna, the higher is its gain. A longer yagi - if it's well designed - will have more gain and a larger capture area than a shorter yagi; roughly, the gain and capture area of a yagi are both proportional to the boom length (in wavelengths).

The gain of an array of antennas is determined by the capture area of the whole array. The fundamental principle of antenna stacking is to space the antennas so that their capture areas just touch. This maximizes the capture area of the whole array without making the array any larger than it needs to be. If the spacing is too small, the capture areas will overlap and the array will not achieve its maximum potential gain. The first two examples show this principle in action.

However, it's important to remember that "effective aperture" or "capture area" is a fuzzy concept: for yagi antennas, these areas have no hard physical boundaries. The following examples will explain some of the finer points.

History is Bunk! (Henry Ford)

Many older amateur radio books recommend fixed stacking distances like "half-wavelength", "5/8-wavelength", "half the boom length" etc. Some of these 'rules' were little more than guesswork. You can still see them in some modern books - but only because they have been copied from the older books without thinking!

These old 'rules' will be correct for certain types of yagis (the ones that were popular when the books were written) but they will be wrong for many modern yagi designs. My recommendation is to forget them, and start again with modern ideas.

Vertical Stacking for Two Identical Yagis

This example shows how two identical yagis can be stacked so that their capture areas just touch. That distance will give close to the maximum achievable gain.

Points to note

• Stacking two identical antennas can result in 2.5-2.9dB more forward gain - but only if it is done correctly!

• Vertical stacking will reduce the vertical beamwidth and also will introduce extra sidelobes in the vertical (elevation) radiation pattern. When antennas are stacked only vertically, the horizontal radiation pattern of the array will be the same as the individual yagis.

• Stacking too far apart will increase the vertical sidelobe levels, and make the vertical pattern narrower as the sidelobes eat into the main lobe. Wider stacking will also make the antenna bigger and less strong. There are no practical advantages here, so avoid excessive stacking distance!

• Closer stacking will make the vertical pattern wider, and decrease the vertical sidelobe levels. Although this will result in serious loss of gain if taken too far, it may be a valid trade-off to obtain a cleaner vertical pattern.

• For long yagis (more than 10 elements, boom length greater than about 2 wavelengths) DL6WU has developed a useful formula just below based on the beamwidth. This gives a very good compromise between extra gain and a clean array pattern.

• These are all approximations! The only way to understand exactly what trade-offs you are making is to run a computer model of the array above reflecting ground.

 DL6WU Stacking Formula for Long Yagis

This formula only applies for yagis of more than about 10 elements, boom length greater than about 2 wavelengths.

D = W / (2 * sin(B/2))

where
D = stacking distance, vertical or horizontal
W = wavelength, in same units as D
B = beamwidth between -3dB points.
Use vertical beamwidth for vertical stacking (as above);
use horizontal beamwidth for horizontal stacking.

Horizontal and Vertical Stacking

Points to note

• The capture areas just touch in both the vertical and the horizontal directions.
• The horizontal spacing is greater than the vertical spacing, because the capture areas are elliptical.
• The horizontal cross-arm of the H-frame is in the same plane as the yagi elements, but interaction is minimized because the cross-arm is outside of the capture area of the antennas.
• If the top of the tower extends up to the cross-arm (for mechanical strength) it will be inside the capture areas of the two lower antennas and there may be some interaction. 
• If there are metal guys coming up to the top of the tower, expect some interaction.

Antennas for Different Bands... the Principle

Points to note

• The capture area of the lower-band antenna is physically much larger than the capture area of the higher-band antenna.
• To avoid any interaction between antennas, the capture areas should not overlap.
• To avoid interaction with the tower, the top of the tower should be outside the capture area of the lower-band antenna.
• This configuration is very poor mechanically - it puts large wind-loads on the mast, which may bend or break where it enters the tower.

Antennas for Different Bands... In Practice

Points to note

• The higher-band antenna is inside the capture area of the lower-band antenna, and so is the tower. This is a compromise, but maybe not so serious because both objects are much smaller (horizontally) than a half-wavelength on the lower band.
• The lower-band antenna looks like a large object to the higher-band antenna - "large" in terms of the wavelength, that is - and will seriously disturb its performance. That's why we always aim to keep lower-band antennas completely outside of the capture area of any higher-band antenna. Breaking this rule is likely to result in serious loss of performance for the higher-band antenna!
• This example has shown all the compromises. Try to make as few of them as possible!
• If you can, try to make your system much taller, more like the previous example.

How Far Apart?    A Survival Guide

If you don't have much information, check the vertical stacking distance that the manufacturer recommends for two of the same antennas. This distance is the height of the capture area!

The minimum clearance distance (for antennas on lower bands, same polarization) is one-half of the stacking distance for two of the same antennas.

Example: The manufacturer recommends that you stack two identical 144MHz yagis 10 feet apart. That means that you shouldn't mount one of these yagis any closer than (10 / 2) = 5 feet above a lower-band antenna such as a 50MHz yagi or HF tribander.

If you don't even have that much information, for long yagis you can look at the performance table by VE7BQH. That gives recommended stacking distances for a comprehensive range of 144MHz long yagis. If you can't find your exact yagi design, you should be able to find something comparable as a guide.