Understanding Yagis with Isolated Ground
Antenna Technologies Limited Company 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.
Antenna Technologies Limited Company 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.