-- not really a balun
John Doty (email@example.com)
Date: August 24, 1995
Original source: Usenet's rec.radio.shortwave
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
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
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)
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).
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.