Stacking Antennas

DX on VHF is usually worked with horizontally polarized beams, so the following description is based on this model.

It is quite common to combine more than one antenna to achieve more directional gain in an array. Usually antennas are grouped in even multiples i.e. from one antenna 2,4,8 etc. although it is possible to have odd numbers like three or some times six, but this gives added complications of matching and feeding.

Generally stacking refers to placing one antenna vertically above another; baying usually refers to having the antennas spaced horizontally apart (side by side) and of course a combination of these as in a box of four.

The gain achieved by doubling the array is usually just less than 3dB (2.6-2.8 dB) at optimum separation. You might ask what is optimum separation. Well this is the minimum distance the antennas must be separated so that their capture areas are not overlapping, shown diagrammatically below.

The capture area of an antenna is it's ability to collect or intercept incoming energy from free space and is related to gain. i.e.

A=Gi/4p

A is the capture area (in square wavelengths),

Gi is the gain over isotropic (as a ratio).

So a dipole with gain of 2.15 dB over isotropic has an effective capture area of 0.13 square wavelengths.

A Yagi of 10dBi (a factor of 10) has a capture area of about 0.8 square wavelengths.

It has been shown that optimum spacing of Yagi antennas is a function of the half-power beamwidth (Q) in the stacking plane.

Dopt=l /(2sin(Q /2))

Dopt is the spacing in l, at which the stacking gain is maximum, and (Q) is in degrees.

The beamwidths in the E and H plane are very often different so the stacking and baying spacing will be different for optimum gain.

It must be remembered that there are other losses to take into consideration such as the power splitters and feeders.

Stacking antennas too far apart can increase the sidelobes of the array. It is therefor better to have slightly closer stacking without too much noticeable loss in overall gain.

For a short Yagi of say 5 elements it may have a beamwidth of 50^o in the E-plane and say 70^o in the H-plane. It would have a horizontal separation of 1.18l and a vertical separation of 0.87l centre to centre.

Stacking antennas reduces the beamwidth of the array in the vertical or H-plane by approximately half. Baying reduces the horizontal E-plane beamwidth by approx. half. So in the case of the 5 element Yagi, the 5X5 array stacked would be 50^o in the E-plane, and 35^o in the H-plane. If the array was bayed the E-plane would be reduced and the H-plane would remain the same.

Depending on what you are wanting to use the array for, the beamwidth can be adjusted by selecting a stacked or bayed arrangement. For normal troposheric scatter operation a stacked arrangement is preferable. This would yield a wider horizontal beamwidth while reducing the vertical beamwidth, thus reducing the power that is wasted radiating up into space or heating up the ground in front of the array.

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