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)
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
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
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
& H beamwidths in degrees
distance E plane in inches
H plane in inches
Wireless Technologies, Inc. and the author retain the rights
to all intellectual
This information should be used as a guideline
only to help you in the appropriate selection of an antenna.
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