Stacking Yagi Antennas

By Joseph H. Reisert

When two (2) Yagi antennas are placed at the optimum stacking distance, they usually have the following characteristics:

The gain increases by about 2.75 –2.9 dB over the single antenna.
The beamwidth in the plane of stacking is approximately one half the beamwidth of the original antenna while the other plane is not affected.
The first side lobes are typically 13-14 dB below the main lobe in the plane of the stacking.
After several years of trial and error, I have come up with a very simple formula to calculate optimum stacking distance for Yagi antennas, namely S = 51/bw where S is the center-to-center spacing of the antennas in wavelengths and BW is the beamwidth in the plane of the stacking.

Let’s use the 918-10 Yagi at 915 MHz as an example. The gain of the single Yagi antenna is 14.15 dBi and the beamwidths are 35 and 38 degrees in the "E" (azimuth) and "H" (elevation) planes respectively.

For example, let’s assume the two Yagis are stacked side by side or in the E plane. For the purpose of this write-up it does not matter whether the antennas are mounted vertically or horizontally polarized.

Using the formula shown above the optimum spacing in the azimuth plane (side-by-side): S = 51/35 or 1.457 wavelengths. We know that wavelength in inches + 11803/F where F is io MHz. Using this formula, a wavelength at 915 MHz is approximately 13". X 1.457 (wavelengths) or approximately 19".

For two 918-10 Yagi antennas stacked or mounted side by side spaced 19" center to center the gain is 16.9 dBi, an increase of approximately 2.75 dB over a single Yagi. The beamwidth in the plane of stacking is 17 degrees, approximately one-half the original value.

Now let’s see what happens if we understack or place the antennas too close together. With the stacking of 13" or 1.0 wavelength the radiation pattern is quite clean but the gain is 16.07 or only 1.92 dB over that of a single Yagi. The beamwidth is reduced to 22 degrees.

Going to the opposite extreme, let’s try wide or 24" stacking. The gain is 17.16, a 3.0 dB gain improvement (not bad) but the side lobes increase to only 9.0 dB down from the main beam and the beamwidth has decreased to only 14 degrees! At this spacing, the antenna beamwidth will be very narrow and difficult to aim while there will be pickup on the large sidelobes.

From these three examples, it can be seen that the simple formulas shown above can be easily used to quickly determine the optimum stacking or spacing distance for a Yagi antenna when the beamwidths are known. Most antenna manufactures include the beamwidth on their data sheets.

The table below shows the recommended stacking distance in the E and H planes for a few Astron Yagis.

Astron Wireless Technologies, Inc. and the author retain the rights to all intellectual property contained herein.
This information should be used as a guideline only to help you in the appropriate selection of an antenna.

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