By Joseph H. Reisert
Astron Antenna's Yagi antennas incorporate our unique "Antenna
This feed system has the following innovative attributes:
• EverSealed, fully epoxy encapsulated to prohibit
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.
• 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.
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
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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