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
feritte core of 15mm, or small Amidon red ring core,
FT50-43 or T50-7
or for higher power: or Philips 4C6 or 4C65 (pink color), Amidon T130-2 red or T200-2 red
3 pieces of insulated wire (Cul) of 0,8mm
CT1=5...30pF capacitor trimmer (green)
C1 = 10pF
L2=0,8mm Cul, 4,5 turns, 5mm diameter
2 x 25mm 3mm diameter weater resistant bolts (innox etc...)
"banana" type plug for VHF antenna (no ground needed)
electrical junktion box (painted after)
Peak Frequency range: 100kc...30Mc (mostly depending on the core) and 144...146Mc
Max. RF power CW: 5...10W (also depending on the mismatch of the antenna and the transmission intervals)
Max. RF power SSB: 10...15W (also depending on the mismatch of the antenna and the transmission intervals)
Output impedance to 50 Ohms coax
1:9 output for longwire
Rod VHF 1/4 (or 3/4)
wave electrical length for 144...146Mc
project: Magnetic Longwire Balun
(MLB) also usable for dipoles
Homemade QRP transmitter with Xtal oscillator for QRP. AM modulation and CW for the 10-meter band and 12-meterband. For radioamateurs using an antenna, schematic, HF, home made QRP transceiver, homebrew for 12meterband 28Mc, 25Mc, 30Mhz, 27Mhz, 28Mhz, 29Mhz