Build
an All Band HF Air Core
1:1 Choke Balun
THE "UGLY BALUN"
A
balun's purpose is to allow connecting a BALanced
load (e.g., a dipole or driven element) to an UNbalanced
line such as coax, thus the name, Balun.
In transmitting antennas, this is accomplished by presenting a high impedance (resistance),
to RF currents flowing outside the coax shield. This forces currents in each
side of a driven elements to be equal. This is especially important in beam
antennas because it prevents distortion of the beam's pattern caused by unequal
currents in the driver(s). In a simple dipole, the balun assures
that
the dipole, and not the feed line, is doing the radiating!
When you connect center fed antennas, like
dipoles, V's, triangles, yagis, rhombics, loops and so on, to coaxial cable,
unless care is taken, it is not difficult to end up with feeder radiation. Not
only can the loss in power be quite significant, but the radiation
characteristics of the antenna system will also be seriously compromised.
In laymen's terms, it won't be what you are expecting from the pattern of your
antenna.
As the feedline becomes part of the antenna, currents can flow from the line
into the mains and on TV cables, metal masts and yagi booms, causing a variety
of EMI problems that can be very difficult to trace. Frequently these
problems are simply due to unbalance - and the solution is the humble balun!
If an antenna system is fed at center with a parallel
conductor line (provided that correct installation procedures are followed)
balance will be maintained, USING A BALUN, with currents in equal and opposite
phase cancelling each other out.
When the connection is to a coaxial cable,
WITHOUT A BALUN, this cannot occur because currents flowing inside the cable
from the connection to the inner conductor are separated from those flowing on
the outside from the connection to the shield, and the result is unbalance
causing feeder radiation. However, if the two electrical circuit elements
(antenna and coaxial cable) are coupled using a balan, balance will be
maintained.
Enter.....The
Ugly Balun!.....N4UJW
An Inexpensive,
High-Performance, Ugly
50ohm Balun
"Building a no-grief 1.8MHz
to 30MHz 50ohm-balun is easy.!"
"No costly ferrite-cores are needed, just a
short length of 3 to 5 inch size plastic pipe, about 25 feet of 50ohm coax plus
some nylon cable ties.
Solid-dielectric coax is best for this application because foam-dielectric has a
tendency to allow a change in the conductor to conductor spacing over a period
of time if it is bent into a tight circle. This can eventually result in voltage
breakdown of the internal insulation.
The required length of the plastic pipe depends on the diameter and length of
the coax used and the diameter of the pipe. For RG-213/U coax, about one foot of
5 inch size pipe is needed for a 1.8MHz to 30MHz balun. For 3.5MHz to 30MHz
coverage, about 18 to 20 feet of coax is needed. This length of coax is also
adequate for most applications on 1.8MHz.
18 to 21 feet should cover all of 160 through 10 meters.
The number of turns is not critical
because the inductance depends more on the length of the wire (coax) than on the
number of turns, which will vary depending on the diameter of the plastic pipe
that is used.
The coax is single-layer close-wound on the
plastic pipe.
The first and last turns of the coax are secured to the
plastic pipe with nylon cable ties passed through small holes drilled in the
plastic pipe.
The coil winding must not be placed
against a conductor.
The name of this simple but effective device is a
choke-balun.
NOTE: Some
people build choke-baluns, without a plastic coil-form, by scramble-winding the
coax into a coil and taping it together. The problem with scramble-winding is
that the first and last turns of the coax may touch each other. This creates two
complications. The distributed-capacitance of the balun is increased and the
RF-lossy vinyl jacket of the coax is subjected to a high RF-voltage. The
single-layer winding on the plastic coil-form construction method solves these
problems since it divides the RF-voltage and capacitance evenly across each turn
of the balun"....AG6K
Here are examples in the
pictures below using cable ties on
PVC pipe forms which work well also. Your choice!
(Imagine
the coil form is removed). Pictures are showing how to secure the coils together.
Do not let the
first and last coils touch!
Picture above
courtesy of VE7AVV
Picture above courtesy of KC2NXV
shows using
2 pvc couplers joined and glued using about 2 inches of 4 inch PVC pipe, so the
couplers would adhere and be stronger.
Credit for this article goes
to AG6K, Rick Measures and was edited from a Pre-copy version of another article
titled "A BALANCED - BALANCED ANTENNA TUNER"
published in QST,February,1990
Email Concerning the "Ugly
Balun"
From Russ Wilson <ve6vk@telusplanetlnet>
"Don. I built one of your "Ugly Baluns". I was
using it on a dipole for 80/40 meters.
I had some TVI without the balun, so no doubt the feedline was radiating.
With the balun attached, the TVI completely disappeared. I built a second one as
I had the same TVI problem with an R7. The balun cured this as well. I can run
my linear now with no problems as all. So I appreciate your expertise and your
article."
"Thank you.
Best Wishes
Russ, VE6VK"
CHECK OUT RUSS'S ANTENNA
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