I was hoping someone else would answer first and make the logical argument.
Leif's explanation makes sense.
Anyone that has done much antenna modeling has practical knowledge of these
trade-offs. A yagi is not perfect so it is simply not practical to produce
a single lobe antenna. There are several trade-offs that can be made. Quite
often gain is maximized without a lot of attention to other factors. Often
when a little gain is sacrificed other good factors can be improved on like
side-lobes. The factors that can be traded are gain, side-lobes, impedance
(real low is not practical),SWR bandwidth, and design frequency ( Best to
design a bit high in frequency because rain and other yagis in a stack
reduce the tuning frequency). A yagi designed for absolute maximum gain
would not be a practical design. Usually it would have high side-lobes,
very low impedance (perhaps 2 ohms), very narrow SWR bandwidth and would be
a narrow design that is easily detuned by rain. In all likely-hood the gain
would not be realized due to inefficiencies in the elements and the
matching network trying to work into a 2 ohm load. The model would likely
to be in error as well. Many of us have seen a yagi with gain that is 10 or
20 db too high and shows a 6db front to back ratio. These are not real
designs and show flaws in the modeling.
There is an optimum number of elements per boom length depending on design
objectives. There are also optimum boom lengths where front to back and
maximum gain converge better. A yagi designed for absolute minimum
side-lobes will tend to sacrifice 1 to 2 db of gain. It is possible to
reduce side-lobes significantly when only .5db of gain is sacrificed. This
is practical when you have plenty of power and have a high noise level.
73, Jim Shaffer, WB9UWA.
On Tue, Apr 29, 2014 at 8:22 PM, Leif Asbrink <leif(a)sm5bsz.com> wrote:
I can not give a stringent proof, but to me it is obvious.
Maximum gain from a circular aperture is obtained when the
amplitude and phase are the same over the entire aperture.
(think telescope with lenses) The radiation pattern is then
a Bessel function and it has sidelobes. To reduce them one
would have to make the amplitude fall off gradually towards
the edges and obviously maximum gain is no longer obtained.
When you stack yagi antennas, maximum gain is obtained when
the side lobes are about -9 dB only.
If you model a yagi antenna as a structure with a somewhat
reduced wave speed (90 degrees phase shift from end to end
vs free space) you get a pattern wit sidelobes similar to
those of a circular aperture. That is the maximum gain for
the length of the structure. You can increase the wave speed
(go to a lower frequency) Then the gain is lower and the
The mathematics is similar to the design of filters.
If you want to design a low pass filter you can make it steeper
by forcing a zero at some point above the corner frequency.
The zero will obviously be followed by a non-zero at a somewhat
Leif / SM5BSZ
On 24 Apr 2014 16:53, "Antonio Matias"
The latest antenna designs are more dedicated to reduce side lobes and
they call low noise.
Obviously this is achieved by sacrificing some
Why is this obviously so?
I believe in practice it tends to be so, but it is not clear to me it
should obviously be so. In fact, logic tells me that power radiated in
sidelobes leaves less in the forward direction so reduces gain.
Moon-Net posting and subscription instructions are at
Earl Shaffer, WB9UWA
My EME array photos
Detailed array photos