|
 The
word is the name of one of the Japanese inventors of these directional
antennas. The other inventor's name was Uda so in technical
circles these are Uda-Yagi antennas in honor of the inventor
team. The invention is from the early 1900s.
While there is a multitude of directional antenna designs in
use today, the Yagi is the most popular, well known and most
seen. Some people call these "beam" antennas, namely CBers and
Amateur Radio Operators and cellular.
Note:
the rooftop TV antennas seen all over the US are a cousin of
the Yagi design but are called Log-Periodic because they are
quite different in serving a huge number or width of frequencies.
How Do These
Antennas Achieve So Much "Gain"?
Basically they are directing a major or almost all of their signal
in just one direction instead of dispersing all around a 360 degree
circle. All those single element whips and rods on cars, boats etc
are omnidireetional, doing the 360 degree job. Zero (unity), 3,
and 5db gain are the typical numbers for those. Usually the taller
they are (for a given frequency) the more gain is laid out on the
horizon, where it is needed.
What Is "Gain"?
This is the common term used when measuring decibels (dB) to determine
the effectiveness of an antenna in a given direction or plane. Simply
put, it is the measure of the intensified signal where it is wanted
or needed because antennas cannot create gain. The antenna is designed
to manage the power in the wanted direction and thus reduce it in
unwanted directions.
MECHANICAL
BASICS OF YAGI ANTENNAS
The "Boom" is the long horizontal bar you would see in attached
drawings or photos here.
The "Elements" are the small thin rods supported by the boom.
These elements are usually vertically positioned, as seen in the
pictures. The front of the antenna has the shorter elements and
the rear has the longer ones. The difference in element dimension
is small compared to those Log-Periodic TV antennas mentioned. Starting
from the rear the boom has the reflector then always the "driven"
element then from 1 to any number of "directors" usually 1 to 13
or so.
The signal output is off the front end of the boom. The signal "beam"
is anywhere from 20 to 90 degrees wide with the longest or most
elemented design having the narrowest width and highest gain or
reach.
What Is Polarization?
Radio signals travel polarized or oriented vertically, horizontally,
circularly or combinations of these. In cellular and most commercial
applications signals are vertically polarized. Your cellular mobile
antenna on the car is vertically polarized, for example. Your Yagi
must be installed with its elements vertically polarized too.
Where Can I Mount The Antenna?
The available length of your feeder cable may determine your mounting
spot and cable length considerations will address later. Otherwise
any vertical or horizontal pipe, roof edge, wall, window sill, balcony
railing or fence post can be used. Also, inside-attic hidden install
is acceptable as long as there is no metal in the roof that signal
has to penetrate. Wood and concrete are normally invisible to RF
signals but at these frequencies (824-894mHz) gain may be reduced.
Where Do I Point The Antenna?
The front of the antenna has to be directed at the target, the cell
site or repeater tower or other user. It is best to use a map and
compass and knowledge of target location but usually you can locate
the target if it has a signal being sent to you. By simply rotating
your Yagi until best signal is found then centering on the estimated
location, satisfactory results may be had. Slowly rotate the antenna
and stop every 10 degrees. Record signal levels. Note the peak and
decline positions and soon a center can be figured out then tighten
the mounting bolts. Be sure to stand at least a couple of feet away
from the antenna and and to the back or sides. Never in front of
the antenna. Your body can greatly alter signal patterns.
What Was That About Cable?
At cellular frequencies of operation, cable can be quite lossy.
Signal loss or attenuation happens all along the cable. The rule
is, the shorter the better. Many consumer-use Yagis come with 18-22
Feet of RG-58 coaxial with 96% braid shielding. This setup has about
a 3.5 dB loss in signal. (16dB Loss/100 Feet/900mHz) If your installation
requires a longer feedline you must upgrade to a double shield RG-58
(11dB loss/100FT) or a double shield RG-8X for a 7.7dB loss /100Ft).
All of the above is fine up to a 50 foot run. From 50-100 feet,
RG-8 or 9913-Equivalent double shield (3.8dB loss/100Ft) type must be used
but it's harder to use and much more expensive than other cables
mentioned here.
ANTENNA
HEIGHT/CABLE LENGTH
The antenna needs to be mounted as high and in the clear as possible.
But there is a point at which increasing height to gain 1dB can
cause an extra 3dB in cable losses. So good thinking must be utilized
when deciding when more height is needed and how much longer the
cable will be. On the other hand, if excess cable length is present,
either remove it or run a long path to the antenna, never coil or
fold up the cable.
Yagi Antennas for Cellular
SAFETY PRECAUTIONS
These, like most antennas, are made of
aluminum, a great conductor of electric power besides radio
frequency energy. You can be instantly killed if the antenna
or support structure being worked with touches an electric power
line. Never even think of installing an antenna during rain
or wet conditions.
Finally, don't stand less than 5 feet in front of the transmitting
antenna when using a regular 3 watt phone. Don't install the
antenna where the signal has to cross where people will be walking
in front of or touch your antenna. While radiation dangers at
low power are not proven, don't take chances with RF emissions.
Shop Yagi Cellular Antennas |
Help me improve my cell signal 
