Practical_Antenna_Handbook_0071639586

278 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
that could be raised to the top of a tower by a single person. There it could be rotated
through the entire 360-degree azimuthal horizon with not much more than a heavyduty
TV rotor.
What makes the Yagi so popular an antenna? In brief: gain, directivity, simplicity of
construction, single-element feedpoint—and all in a very small space. A three-element
Yagi for 20 m can, in the footprint of a 50-ft-diameter circle, provide a more constant
forward gain rotatable through all compass directions than can a half-dozen fixed
rhombic wire antennas, each 400 ft long on its longer axis. Contrast the real estate requirements
and the construction simplicity of a single tower supporting a single Yagi to
those six rhombics needing as many as 24 supports!
By the mid-1950s, manufacturers such as Hy-Gain, Mosley, and Telrex were offering
two- and three-element Yagis not just for individual high-frequency bands, but for
three bands in one assembly employing a single coaxial feedline! These latter products
were made possible by the use of traps in their elements, as we will discuss later in the
chapter.
Today, common usage often has us referring to the Yagi as a beam. Example: “The
antenna here is a three-element beam” or “The antenna here is a three-element triband
beam”. In general, whenever you hear a physical antenna referred to as a “beam”, you
can almost always interpret that to mean “Yagi”. Whereas terms such as beamwidth can
pertain to the characteristics of any type of antenna, the “beam” antenna is invariably
synonymous with the “Yagi”.
Use of a directional beam antenna provides several advantages over a more omnidirectional
antenna, such as a dipole or random length of wire:
• The beam antenna provides an apparent increase in radiated power because it
focuses the available transmitter power into a single (or limited range of)
direction(s). The doughnut shape that is a dipole’s radiation pattern in free
space has a gain over an isotropic radiator of approximately 2 dB in its best
directions, broadside to the axis of the dipole. Add one additional element and
the focusing becomes nearly unidirectional by increasing the effective radiated
power (ERP) another 3 dB or so in the desired direction. Or add two elements to
the dipole, and see an ERP increase of 6 dB over that same dipole in the desired
direction. That’s equivalent to swapping out a 25W transmitter for a 100W unit!
• Antennas are generally reciprocal devices, so they will work for receiving
applications much as they do for transmitting. When aimed at a desired signal
source, the beam antenna increases the amplitude of the received signal
available at the input of the owner’s receiver relative to the internal noise of the
receiver itself. This is of great importance on the VHF and UHF bands, and not
so important on the HF bands.
• The directivity of the beam antenna is usually marked by one or more sharp
nulls in both the azimuthal and the elevation response patterns. This attribute
forms the basis for some popular VHF and UHF direction-finding techniques.
• The receiving pattern of a Yagi substantially knocks down the strength of
signals and band noise coming in from compass directions to the sides and rear
of the desired signal path, thus effecting an improvement in the received signalto-noise
ratio (SNR). This effect is important and very useful at HF.