QRP Magnetic longwire balun with VHF-splitter

By Guy, de ON6MU

About the MLB (magnetic longwire balun RE-ABU3HF/S)

This Magnetic Longwire Balun (MLB) makes it possible to efficiently use a coaxial lead-in cable with all forms of longwires, T-forms or other types of wire antennas, without the need for an antenna tuner. A very low loss magnetic transfer of energy from the antenna to the receiver is accomplished and static noise is reduced. Your coax is much less susceptible to interference. You can even connect a dipole to it.
It works fine with a heavy duty 41 foot (12.5 meters) wire, some nylon rope and a quality insulator. At the feed-line end the antenna is terminated with the Magnetic Longwire Balun. This balun permits an exceptionally low loss transference of antenna energy to your coax feed line. The result is significantly reduced static noise on long, medium wave and the shortwave bands.

You do not have to Earth/Ground the Green wire sticking out of the top, but it helps minimize interference if you do. Grounding the balun / coax (pin c1) to a good earth made between 3 and 6 dB improvement on noise and QRM, even though the station was well-grounded.

You could add a shoke inside (or outside) the balun housing to prevent even more from coax radiating too. A few feritte beads over the coax or turn a piece of 50 coax a few time around a feritte core. If possible inclose the shoke inside the balun housing or as close as possible to the SO239 connector.
Long-wire antennas are directional, so bend yours to allow both N-S and E-W orientation. Height is dependent on your location and surrounds … experiment!!

you can add an extra output as described in the schematic if needed (1:4)

L2,C1 blocks VHF signals from entering the balun. Ct1 tunes the VHF antenna and limits the lower frequencies from entering the VHF rod. You can calibrate Ct1 by soldering 1 watt 50 Ohm carbon resistor (or dummy load) between Ct1 (VHF pin) and the ground of the connector. Use a low power setting (0.5 watt) and tune Ct1 to 1:1 SWR.


Schematic QRP Longwire balun with VHF-splitter

Parts list






Another related project: Magnetic Longwire Balun (MLB) also usable for dipoles


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