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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 (choke), 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 air choke. 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 canceling 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!.....
Another Ugly Choke Balun by G4APL Paul, G4APL, has brought together some excellent examples on how to construct and build Choke Balun in the pictures above and the information below to share with us: Paul decided to build two or three of them, depending on what material could be found in the shed. The idea was to add a balun to the HF beam, a Mosley Mustang Mark3 3 element 10,15, 20 metre trapped yagi. This is fed with co-ax and has the Mosley earth strapped at the feed point. The beam has been adjusted for the low end of the HF (High Frequency) bands. Paul had used a commercial balun in the 1970’s before and burnt this one out. It was supposed to be rated to 1 kilowatt pep. (peak envelope power). Using the information material from the instructions for the "Ugly Balun" above, it was time to see what was in the shed. A short length of 4 inch drain pipe was located that was left over from the 90 foot of drain pipe laid underground that carries the RF cables. Also found a 2 inch piece of down pipe. Materials: HF Beam Balun 12 inches length of 4inch diameter PVC drain pile cable ties 18 foot of RG58 two suitable cable plugs Having got all the required tools out, it Paul and hour to build the Ugly Choke Balun as pictured above. The HF Balun is
attached vertically to the Stub Mast at the Mosley Mustang Mark3
beam Paul was very surprised with the results. Testing the aerial and Balun with 250 Watts the standing wave was surprisingly good. Never seen this beam produce these results before! 14.005MHz to 14.150MHz 1.1:1 14.200MHz 1.2:1 14.250MHz 1.4:1 14.300MHz 1.9:1 21.005MHz to 21.300 1.1:1 21.400MHz 1.2:1 28.005MHz – 28:400MHz 1.1:1 28.500MHz 1.2:1 Now to do some dxing and
see how the aerial performs.
Exellent results! =============================================== KO4MAX
deserves the credit just for putting up with
me.
I wanted to
add (in this update), that 21 feet is the length of coax used
in the construction.
We put the
antenna up on a Sunday afternoon, and it was sketchy to say the
least.
The auto
tuner in his Icom 746 Pro kept engaging automatically (even
when it was turned off).
I knew it had
to be coupling with the feed line.
So, I showed
him your Ugly Balun page and then the Hy Gain manual I have, which
calls for 12 turns 6 inches in diameter.
He read the
Cushcraft A3S/40m manual which states 8 turns at 6 inches in
diameter.
After much
debate we went with your design. See photos
below.
Monday, enter
the Ugly Balun.
Now, the
antenna works and it works well.
The second
day on the air (Tuesday) he sent an email informing me that "it
rocks" which is a quote.
He also says
it's a big improvement over the dipoles he had on those
bands.
75/80 meters
and the warc bands is all that is left in his
shack.
We'll rework
those dipoles for those bands.
Thank you for
your contribution and website!
We found the
Ugly Balun to be a huge help when it came time to stop the feed line
coupling.
We think
it gives this big A3S/40m (10-15-20-40m 3 Element
beam) "character".
73'
TL7VE - DX8FW
-
Tim
----------------------------------------------------------------- Balun Pics below submitted by K8CPA. "Ugly Balun" mounted on base of tower.
==================================== Notes
from builders:
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