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Magnetic Longwire Balun
-- not really a balun

From: John Doty (
Date: August 24, 1995
Original source: Usenet's

Would like to hear if someone has tried the Magnetic Longwire Balun by RF Systems. Does it work? Does it work as good as the manufacturer claims?

I haven't tried it, but either its manufacturers are consciously misrepresenting its capabilities or do not understand what it does. It is *not* a balun: the so-called "Magnetic Longwire Balun" (MLB) has only three terminals, while a true balun requires four.
A balun prevents net current flow between a transmission line and an antenna, while allowing energy to pass between them. With only one terminal for the "antenna", the MLB cannot possibly transfer energy from the antenna to the transmission line without also allowing a net current to flow between them.
With a proper four terminal balun connected to a proper two terminal antenna, the antenna current can flow between the antenna terminals, the transmission line current can flow between the transmission line terminals, and energy can flow through the balun without a net current flow from antenna to line.
This is not to say that the MLB is useless: except at a few resonant frequencies a wire antenna is a poor impedance match to a coaxial line. A matching transformer can significantly improve the match over a broad band. I suspect that the MLB is an effective matching transformer.
The point of using coax to feed your antenna is that it keeps currents due to noise pickup in your house separated from the signal currents picked up by the antenna. A proper balun helps enforce this separation, but the MLB not only doesn't enforce it, it subverts it. Unless the MLB is directly and effectively grounded, the second terminal of an MLB-fed antenna is the shield of the coax cable itself! Noise pickup from the house flows on the outside of the coax out to the MLB which efficiently couples it to the inside of the coax and into your receiver.
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).
You can feed this sort of antenna directly, without a matching transformer, by attaching it to the coax center conductor. The resulting antenna will show very high efficiency at wavelengths where it is an odd multiple of 1/4 wavelength long, and very low efficiency at wavelengths where it is a multiple of 1/2 wavelength long. Another disadvantage of this configuration is that it offers no protection against electrostatic damage.
A transformer at the base of the antenna can smooth out the wild efficiency swings and also give static electricity a path to ground. I've posted instructions for winding such a transformer on this group several times. If anyone wants to see them, send me mail.
Alternatively, you could use a commercial transformer. I recommend the ICE Model 180 "Beverage/Longwire Matching Unit". Unlike the MLB folks, ICE seems to actually understand what their product does. In addition to the inherent electrostatic protection offered by the transformer, this unit also has a gas discharge tube and a blocking capacitor to further reduce the danger of electrostatic discharge. It's very solidly constructed. Finally, at $32 it's less expensive than either the Palomar ($40) or RF Systems ($60) MLB's.
ICE products can be ordered on-line at the official ICE web site Here you will find datasheets as well as free publications on grounding and beverage antennas.
The usual disclaimer: I have no interest in ICE, I'm just a satisfied customer (I have three of these transformers: thanks go to Richard Steck for alerting me to the existence of this product).

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