A It is true that stacking antennas produces additional directivity and gain. You can stack antennas vertically (Figure 1) or horizontally, although vertical stacking is most common.
The “secret” is in the fact that gain can only come from taking power that would otherwise be radiated in other direction(s) and concentrating that power into the main, desired lobe(s).
The most easily understood physical demonstration is one that I’ve used for years at radio club meetings (see Figure 2). I take a balloon, blow it up so that it is roughly circular in shape and then declare that this is a radiation pattern from an isotropic radiator. Next, I blow up another balloon to the same size and shape and tell the audience that this will be my “reference” antenna.
Then I squeeze the first balloon in the middle to form a sort of figure-8 shape and declare that I’ve now created a dipole and compare the maximum size to that of my reference “antenna.” The dipole can be seen to have some “gain” over the reference isotropic. Next, I squeeze the end of the first balloon to come up with a sausage-like shape to demonstrate the sort of pattern a beam antenna would have, again comparing the gain to the reference isotropic antenna, er, balloon.
By combining antennas in a stack, you can accentuate this gain and directivity even further. In the end, you have created much more total gain in the antenna system than would be lost in the power dividers or phasing harnesses. Stacking isn’t easy or inexpensive, but the performance gain can be substantial.
Figure 1—An example of vertical antenna stacking.
Figure 2—Demonstrating antenna pattern gain with balloons. Take a balloon, blow it up so that it is roughly circular in shape and then declare that this is a radiation pattern from an isotropic radiator. Next, blow up another balloon to the same size and shape and tell the audience that this will be the “reference” antenna (A). Then, squeeze the first balloon in the middle to form a sort of figure-8 shape and declare that this is a dipole and compare the maximum size to that of the reference “antenna” (B). The dipole can be seen to have some “gain” over the reference isotropic. Next, squeeze the end of the first balloon to come up with a sausage-like shape to demonstrate the sort of pattern a beam antenna creates (C).
From QST February 2001
