Practical_Antenna_Handbook_0071639586

C h a p t e r 1 2 : T h e Y a g i - U d a B e a m A n t e n n a 279
• Since the main forward lobe is relatively broad and the null off the sides quite
sharp, small changes in beam heading to put interfering signals in the deepest
part of the null may provide as much as 30 or 40 dB rejection of an unwanted
signal—often with less than 1 dB reduction in the strength of the desired
signal—a characteristic extremely useful on today’s crowded HF bands.
All in all, if you are considering an upgrade to your station and your funds are insufficient
to provide both an amplifier and a better antenna system, a good rule of
thumb is to first spend your money on antennas—not on the amp—because the antennas
will help you with both receiving and transmitting.
Theory of Operation
Figure 12.1 shows the free-space pattern (viewed from above) typical of the Yagi antenna
at low takeoff and arrival angles. The antenna, located at point P, fires signals in all
directions but the beam-forming pattern created by the physical configuration of its elements
causes the relative strength of the signal to be as shown by the solid line envelope
in the figure, with the direction of maximum signal strength indicated by the arrow at
the top. The beamwidth of the antenna is defined as the angle a between the points on
the main lobe that are 3 dB down from the peak at point C.
A perfect beam antenna will have only the main lobe or “beam”, but that situation
occurs only in dreams. All real antennas have both side lobes and back lobes, also shown
in Fig. 12.1. These lobes represent not only wasted power transmitted in the wrong
direction during transmission but also opportunities for interference while receiving.
The goal of the antenna designer is to increase the amplitude of the main lobe while
decreasing that of the side lobes and back lobes. Fortunately, it is often possible to
effect a substantial reduction in the side lobes and back lobes with only a slight
reduction in main lobe (forward) gain or
other desirable parameters such as wide
bandwidth or matching the transmission
0 dB = 6.1 dBd
Back lobe
Side lobe
-3 dB
-40 -30 -20 -16 -13 -10 -8 -6 -4 -2
-3 dB
C
line impedance.
Figure 12.2 shows in schematic form the
basic three-element Yagi-Uda antenna. With
rare exception, all elements of a Yagi lie in the
same plane, are parallel to each other, and
have their centers all in a straight line. In a
rotatable Yagi, these constraints are best met
by attaching the centers of all elements to a
common boom. Depending on the specific design
approach taken, the boom may serve as
an electrical connection, as well.
Main lobe
Figure 12.1 Pattern of a typical beam antenna.