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This compendium is intended to explain and simplify many of the antenna terms that you
will find in our catalog. It will be helpful to the newer, less experienced radio amateur,
or those who have been away from the hobby for a time, in selecting the proper antenna for
his or her installation. It will also assist with "on the air" discussion about
antennas. There are many books available, through your dealer, which describe antenna
design theory and installation with great detail.
GAIN
is a measure of the power amplification factor of the antenna. In proper Yagi beam
designs, gain is primarily a function of boom length. At VHF and UHF frequencies, very
high gain can be achieved because booms can be physically long with more elements. This
has the effect of increasing the transmitting and receiving power of the antenna.
dB
is an abbreviation for decibel. It is a mathematical expression that shows the
relationship between two values. An increase of 3dB is a doubling of power. Gain figures
have to be related to a reference antenna. For horizontally polarized antennas the symbol
dBd indicates that a half wave dipole was used as the reference. For vertical antennas, a
quarter wavelength vertical over a perfect ground is the standard. When the term dB is
used without a reference, then it is assumed that the reference is a theoretical point
source called an isotropic radiator, referred to as dBi.
FRONT TO BACK RATIO
is a measure of the signal (power) radiated or received from the front of the antenna to
signals from the back of the antenna.
RADIATION PATTERN
is the result of the designer's skill in achieving good forward gain and front to back
ratio. At Cushcraft, we use these skills with computer aided design to combine the right
boom and element lengths with proper spacing between the elements to achieve clean
radiation patterns. Think of the radiation pattern as similar to a flashlight beam. A
narrow beam focuses for greater distance, while a wide beam will not reach as far. Long
boom antennas will have narrower patterns which may be focused more accurately to receive
and transmit distant signals.
VSWR
is voltage standing wave ratio. You will find it listed as SWR in the specification
charts. When properly installed, the antennas in this catalog should show an SWR of 1.2:1
or less at the center of their operating frequency range. The 2:1 SWR bandwidth is the
range over which the antenna will operate without exceeding 2:1 SWR, allowing your
transceiver to operate at its full power output. Mt models may be tuned or optimized for
your favorite operating range CW, phone or in many cases center band.
IMPEDANCE
fifty Ohms is the value RF engineers have agreed to design for in applications requiring
RF connection between equipment. In most radio communications, antenna feed point
impedances between 25 and 100 Ohms are acceptable. Cushcraft engineers use various
techniques depending on the application to achieve the best impedance match of 50 Ohms
between the antenna and connecting cable from your radio.
BANDWIDTH
is the frequency range over which the antenna provides optimum performance. Specific SWR
bandwidths are shown for each model. The antennas in this catalog are designed so that the
best gain and SWR are achieved over the same bandwidth.
POLARIZATION
refers to the plane of the antenna elements in relation to the earth, either horizontal or
vertical. Cross polarization such as trying to contact a vertically polarized mobile
station antenna with a horizontally polarized beam will reduce the efficiency of the link
and shorten the range of communications. This theory also applies to fixed or base station
antennas. Most antennas used for FM communications are mounted with their elements in the
vertical plane. For OSCAR satellite communications, antennas are specially designed to
give circular polarization to compensate for satellite rotation. At HF frequencies below
30 MHz polarization is less of a factor because radio signals are subject to change as
they reflect from the ionosphere back to earth.
RADIATION ANGLE
is used to describe the angle which the signal radiates to or from the antenna in relation
to the earth's horizon. It is applied primarily to vertical antennas. Angles of radiation
as close to the horizon as possible are preferred for good communications. At HF
frequencies, this is accomplished by using half wave antennas, like the R5 and R7. At VHF
and UHF frequencies, multiple vertical elements, one above the other, are used to lower
the radiation angle and provide gain over a quarter wave vertical.
ELEMENTS
All antennas have elements. The basic element is a half wave dipole. Yagi beam antennas
use more than one element to achieve a desired gain and pattern. Vertical antennas are
also a configuration of the half wave dipole. Many verticals are one quarter wavelength
long, which requires some type of ground radial system for proper impedance matching.
Designs like the Ringo Rangers and UHF mobile antennas use multiple half wave elements for
greater range.
BEAMS
can be described as high performance antennas. They have a center support boom and
multiple elements. Beams incorporate many of the techniques described above to improve
both the transmission and reception of ham radio signals. Beams are often called Yagi's
after their inventors, H. Yagi and S. Uda. At VHF and UHF frequencies beams are usually
designed to give maximum performance on one band. They are usually called monoband beams.
Space and size may be a problem at HF frequencies so engineers have developed multiband
trapped beams. Antennas like the A3S and A4S operate on 10, 15, and 20 meters. With the
addition of specially designed add-on kits they will also operate on 40 meters. The
tri-band beam requires less space and a smaller support structure than separate monoband
beams.
HF VERTICALS
are usually designed to cover many ham bands. They radiate and receive in all directions,
eliminating the need for rotators. Verticals are relatively small in size, making them a
convenient and cost effective way to operate your station. HF verticals are most often
ground or roof mounted. Most verticals need extensive ground radial systems. While they
are excellent starter or novice antennas, HF verticals are also used successfully by many
experienced ham operators.
TRAPS
are L/C circuits used in multiband vertical or beam antennas to divide the antenna into
sections for operation on separate bands. Multiband trapped antennas electronically select
the proper antenna sections as you change band on your transceiver.
STACKING
of antennas is used to increase performance. Two or more antennas may be mounted on a
support mast or tower at designed distances. Special coaxial cables, sometimes called
harnesses, and power dividers are used. They provide the correct impedance matching and
allow the use of a single 50 Ohm coaxial cable from the transceiver. Stacking is used to
increase forward gain by compressing the pattern. Theoretically, two stacked antennas have
twice the radiated power output (a 3 dB increase) of a single antenna. To increase the
gain another 3 dB, two more antennas must be added making a total of four and sometimes
called an array. Cushcraft offers many stacking kits.
LOCATION
is very important. Put your antenna where it will be safe. Antennas are electrical
conductors. When they are used with transceivers and amplifiers, high voltages can be
developed on many parts of the antenna. Pick a spot where people and pets will not come in
contact with your antenna when it is operating. Ground mounted verticals and their radials
should be guarded. Place your antennas in locations where there is no chance of coming in
contact with power lines during installation or removal.
WIND SURVIVAL
is a measure of how well慯he antenna will survive in a wind storm. Cushcraft antennas are
designed to survive steady 80 mph winds with a safety margin. Actual survival will depend
upon the quality of your installation. Wind surface area shown in each specification chart
will help you to determine if the tower or support structure which you have selected is
adequate to handle your antennas.
LIGHTNING PROTECTION
The buildup of static electricity and lightning charges can be very harmful to amateur
radio equipment. The degree of exposure to this kind of damage can be reduced by several
methods. One is the addition of a lightning arrester in the coaxial feed cable. These
arresters may be placed just outside the building at the point where the cable enters the
building. Cushcraft offers two types, the gas discharge which is very fast acting, offers
the most protection. The air gap type reduces static and is more economical.
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