MFJ-918, MFJ-2919, and MFJ-2920
1.5 kW, 1.8-30 MHz Ferrite Bead Balun
Introduction:
Without a balun, the outer surface of your coax shield provides a common-mode
path for transmitted RF to flow from the antenna down to your operating location. Here, it can
disrupt or damage sensitive electronics and, if severe, may even change the antenna's radiation
pattern, distort your transmit audio, or cause a painful RF burn. On receive, the process is
reversed, with stray locally generated carriers and "hash" traveling up the cable to the feedpoint
to raise the noise floor and cover weak signals.
Description:
The MFJ-918, MFJ-2919, and MFJ-2920 are designed to replace the feed-point
insulator on wire antennas and provide balun protection. Rugged stainless steel eyebolts provide
the superior pull-strength needed to support a full-sized 160-Meter dipole, and the top eyebolt
accepts a center-support halyard to reduce wire strain. Two #14 stranded copper-wire pigtails
solder directly to the antenna wire for reliable conductivity. On the inside, high-permeability ferrite
beads are installed over RG-303 Teflon coax to ensure high power handling, maximum
suppression, and minimal signal loss. Thick-wall chemically welded PVC construction guarantees
long life under adverse environmental conditions, and a high-quality Teflon-insulated SO-239 at
the base provides a secure connection for your coax. Three versions are available:
[ ] MFJ-918:
A 1:1 ratio 50-Ohm current balun designed to minimize losses and deliver superior
suppression over the entire HF range.
[ ] MFJ-2919:
A 4:1 ratio transmission-line transformer style balun designed for 200-Ohm loads.
[ ] MFJ-2920:
A 9:1 ratio transmission-line transformer design for 450-Ohm applications.
Installation:
Drip Loop
Antenna Wire
Pigtail 3-4
Turns
4-5 Turns
Solder in Place
Form
[ ] Loop 4" of wire through each eyebolt, then wrap back and twist 4-5 turns for a secure hold.
[ ] Form a drip loop in the pigtail, then wrap 3-4 turns around the antenna wire.
[ ] Solder the pigtail and antenna wire together, making sure solder flows down into the strands.
Be sure to form good drip loops -- these keep water from running into the enclosure, ensure pull
forces are supported only by the eyebolts, and prevent overheating the PVC during soldering.
Also, when possible, attach a nylon halyard to the top eyebolt to provide extra center support.
Orient the base of the balun down so water can drain from the weep hole located next to the
connector. A coat of clear nail polish or lacquer reduces oxidation of solder connections.
Important Warning: To prevent accidental death or serious injury, never install an antenna
where it can contact power lines. Also, to prevent dangerous RF burns, keep energized
antenna surfaces out of reach to humans and animals.