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By Joseph H. Reisert
Astron Antenna's Yagi antennas incorporate our unique "Antenna
Feed" system.
This feed system has the following innovative attributes:
• EverSealed, fully epoxy encapsulated to
prohibit moisture incursion;
• Balanced center-fed system for a clean
symmetrical radiation pattern;
• A true 50 Ohm impedance match over a wide
frequency bandwidth;
• An integral ferrite bead current balun
to minimize re-radiation from the outside shield
of the transmission
line;
• Isolated from ground.
Many commercial Yagis have exposed feed systems.
The widely used Gamma match feed system is particularly
prone to moisture incursion via the tubing used
in the series matching capacitor. Furthermore,
in some cases an unsealed coax connector is mounted
right at or near the antenna feed point. This often
facilitates the entry of moisture into the connector
(and eventually into the transmission line!) The
expoxy used in the AntennaFeed system seals the
feed point from moisture incursion and, via a coax
cable pigtail, places a sealed connector interface
outside of the RF field.
The Gamma match is an unbalanced feed system since
power is only connected to one side of the driven
dipole. This often causes radiation pattern distortion.
Our Yagis feature balanced center-feed systems.
This system is more complex than a Gamma match
but, it is inherently more symmetrical because
power is fed to both sides of the driven dipole.
Furthermore, the AntennaFeed system is less affected
by the placement of the transmission line. The
result is a clean and distortion free symmetrical
radiation pattern.
Most forms of unbalanced impedance matching systems
have losses due to the tuning components employed.
Furthermore, matching components inherently decrease
the bandwidth of the antenna. The end result is
narrower VSWR bandwidth and lower than expected
gain. The balanced AntennaFeed system has no bandwidth
limiting components! As a result, it has a low
VSWR over a wide bandwidth and never requires tuning!
No matter what you do to your installation, some
of the radiated power from the antenna will be
coupled back to the outside of your transmission
line. This may result in pattern distortion, VSWR
variations and lower than expected gain. The way
to minimize this problem is to use a balun, an
impedance matching transformer designed to couple
an unbalanced transmission line (such as a coax
cable) to a balanced feed system (or vice versa),
at a feed point.
The half-wave 4:1 type of impedance matching balun
is sometimes used. It works well but has losses
due to its associated half-wave line and is difficult
to seal from moisture incursion. The AntennaFeed
system is epoxy sealed and uses a simple 1:1 ferrite
bead current balun. The ferrite beads act like
a high resistance to any RF currents that may try
to flow on the outside of the coax shield. This
type of balun is simple and broadband yet very
effective. It also has a low wind profile.
When using a Gamma match, the center conductor
of the transmission line is usually coupled to
the driven dipole through a small in-line capacitor.
Therefore, only the shield is grounded! Both sides
of the AntennaFeed system are completely isolated
from ground. Some skeptics will say that an isolated
ground is more prone to lightning strikes since
the feed is not grounded. This is simply not true
since the boom and the other antenna elements are
always grounded on a Yagi. Also, lightning strikes
in a random fashion and always searches out the
shortest and lowest resistance path to ground.
Regardless of the antenna feed system, a good
ground and impulse arrestor are highly recommended
on every installation. To minimize the potential
from impulse damage to a radio, first ground the
shield of the feedline to the tower/mast at the
interference between the antenna and the transmission
line as well as the point where the transmission
line leaves the tower/mast. Also ground the base
of every tower/mast through a large diameter (#4
AWG or larger) copper wire connected with the shortest
possible run to at least one (and preferably three)
eight foot ground rod(s) spread out at the base
of the tower/mast. Finally, install an impulse
protector on the outside of the point where a transmission
line enters a building. Do not forget to add another
short ground wire (see above) from the impulse
protector to an eight foot grounding rod mounted
on the outside of the building.
Summary
The balanced Eversealed AntennaFeed system is superior
to unbalanced feed systems such as the Gamma
match. It is sealed from moisture incursion and
has a low VSWR over a wide bandwidth that never
requires tuning. The built-in current balun insures
that re-radiation from the antenna feedline is
minimized and the radiation pattern is symmetrical.
Properly installed, an AntennaFeed system is
no more prone to impulses than other feed system.
Astron
Wireless Technologies, Inc. and the author retain the rights
to all intellectual
property
contained herein.
This information should be used as a guideline
only to help you in the appropriate selection of an antenna.
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