Magnetic longwire baluns 1:9

combined with ground, a 1:4 and 1:1 balun connections
By Guy, de ON6MU

About the MLB (magnetic longwire balun RE-ABU1HF)

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 3) to a good earth made between 3 and 6 dB improvement on noise and QRM, even though the station was well-grounded.
The key to getting good noise rejection from coax used to feed a longwire is grounding the coax shield well. It makes little sense to extend the coax beyond the farthest ground point from your receiver, since beyond that last ground point the coax would pick up signal anyway, despite its shielding. Thus, a low noise coax-fed longwire will typically fall within the spectrum ranging from verticals through tilted wires and inverted L's to Beverages (long, low, horizontal wires). This balun can smooth out the wild efficiency swings and also give static electricity a path to ground.

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!!

Pin 1 = 1:9 ratio for longwire (6...100m wire)

Between Pin 2 and Pin 3 = 1:4 balun

Between Pin 1 and pin 2 = 1:1 balun (dipoles)

You can use one of the bolts to mount your balun too.

If you do have an unknow toroidal core you would like to use, and you do not have the proper equipment to test it, there is a little experiment that can give you at least an indication of the frequency range of the core. So If you want to use other toroidal cores you will have to experiment with the number of windings and test it with a SWR/power-meter, field strength meter and a 450 ohm 1 watt carbon resistor (470 + 10k parallel) soldered between pin 1 and pin 3 (gnd). One SWR/power meter is connected directly to the coax output SO239 and with a field strength meter measure the radiated power at the resistor and check the SWR. Connect your transceiver and test on all bands (@ 0.5 watt).
Second method is using two SWR/power meters and a 50..70 Ohm dummy load. Connect the output of the SWR/Power meter on a dummy load and the output of the meter between pin 1 and pin 2. The second SWR/power meter is connected with a coax to the SO239 of the balun and your transceiver. Check input and output power and the SWR while transmitting on all bands (at low power!). You should see a power drop measured at the balun at non supported frequencies of the core. SWR reading can vary too as the frequency range of the core is lower or higher then the transmitted frequency.
The more turns you manage the better the bandwidth.
I have found that the lower frequencies If you use an Amidon T130-2 red toroid are not to good. To improve the balun on the lower frequency range (1...4Mc) we need to add as much turns as possible. I managed to put 11 turns of 1mm wire (see picture below) and has a very positive influence on the bandwidth, or I simply had a bad T130-2...
Amidon T130-2

Schematic: Magnetic Longwire balun, with 1:1, 1:4 and 1:9 connections


MLB PVC watertight housing and building tips
50 mm PVC tubes with watertight rings
Can be bought in any DIY store. These PVC pipes are very ridgid, weatherproof and water tight! Plastic thickness is approx 2mm.

balun connected to SO239 (for PL259 connector)
This where you connect your 50 Ohm coax to. SO239 connecter is sealed inside with glue and painted with graphite spray/paint. After glueing around the edges of the centre pin, paint a few layers of graphite over the connector and inside the ring which will ensure a good ground contact, prevent corrosion and further seal up the connector for any tiny openings where water could come in. Be sure not to paint the centre pin!!

inside the balun housing
First solder the 3 antenna connections of the balun. There are 3 'outside' antenna connections needed. The middle for the connecting the longwire antenna (output nr 1 1:9), yellow left side wire for output nr 2 (1:2/1:1) and the black wire on the right for the ground output nr 3. Use innox, copper or galvanised bolts. Vernish, paint or glue at the inside of the pipe the bolts to seal them up and protect them against any corrosion.

If all is sealed up perfectly it should be watertight submerged under 1 meter water.

Parts list longewire balun


Specifications RE-ABU1HF

1:4 Balun
bifilar coil nicely spaced over a red-amidon toroid will do fine
Just to make it complete, and if you just happen to need one, here is the principle schematic for a 1:4 balun only:
4:1 balun bifilar coil nicely spaced over a red-amidon toroid will do fine.

1:2 balun
And here is a 1:2 balun:

trifilar coil nicely spaced over a red-amidon toroid will do fine


Picture of my handy little QRP (low power) /A 1:9 balun
QRP portable 1:9 balun
Same schematic as the 1:9 balun, but without GND and 1:1 / 1:4 output pin. Using a smaller core that fits snugly inside a 30mm PVC filmroll.
A core of 20mm should be fine up to approx. 20watt FM (40watt SSB), like the FT68-2 or FT80-2
Calculating the power dissiaption of the core: P = U2 / (Q . XL )

After testing glue the top part.
Seal everything.
For QRP or SWL purposes only you can use a Amidon FT50-2, FT50-43 or a T50-7
click here for details



RF Choke to prevent hf currents on the feedline
(or...1:1 Choke Balun, sometimes called the "UGLY BALUN")

Ferrite beads hence chokes are used (in a way similar to inductors) as a passive low-pass filter. The geometry and electromagnetic properties of coiled wire over the ferrite bead result in a high impedance (resistance) for high-frequency signals, attenuating high frequency EMI/RFI electronic noise. The absorbed energy is converted to heat and dissipated by the ferrite, but only in extreme cases will the heat be noticeable.

Ferrite beads or coax turned over a ferrite bar are one of the simplest and least expensive types of interference filters to install on preexisting electronic cabling. For a simple ferrite ring, the wire is simply wrapped around the core through the center typically 5 or 7 times. Clamp-on cores are also available, which can be attached without wrapping the wire at all.

However, here we are using a toroid. Just turn 4 times on each side and opposite of each side a piece of RG-58 coax, like this:
RF shoke

choke made out of a toroid and rg-58 coax how it looks on an amidon red toroid

homemade choke finishedHF Choke finished

Choke coils are useful in a wide range of prevention of electromagnetic interference (EMI) and radio frequency interference (RFI) from power supply lines and such. Also prevents TVI from radiating feedlines.
You can add this to your longwire balun, antenna, or whatever needs preventing RF currents from the coax feedline...
Some use it as a 1:1 choke balun, called the "ugly balun".

Tip: use this choke after a longwire balun described above, or build it in the same container.


Another related project: Magnetic longwire balun and VHF-splitter


Homemade magnetic longwire balun (MLB) for radioamateurs, shortwave listeners (SWL's). Connect your antenna with this schematic to your receiver and transmit, receive,HF. home made