A 4:1 Air-wound Balun
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Since some time I have been using the Diamond CP6 multiband vertical for my HF activities.
While this antenna performs well on 6m, 10m, 15m and 20m and also does a reasonable job
(for it's length) on 40m, it does not work at all - for me anyway - on 30m and 17m. The
latter is one of my more favourite bands to listen on, so it was time to do something
about it. Even though my LDG AT-100Pro automatic tuner keeps the IC-7000 happy while
transmitting on 17m, the signal does not really get out and reception is poor.
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My home has an attic and that was big enough to string some wire up. I decided to have a
play around with a 17m long deltaloop fed with openwire feeder. The feeder was salvaged
of an old G5RV given to me by a friend (thanks Hennie).
Theoretical calculations show the impedance of a deltaloop cut at its fundamental
frequency to be 150 Ohms when fed from the side. So using this setup, I was in need
of a 4:1 balun to present the AT-100Pro with a more reasonable load and at the same time
transform the balanced line back to unbalanced.
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Definition :
A balun is a device that joins a balanced line (one that has two conductors, with equal
currents in opposite directions, such as a twisted pair cable) to an unbalanced line
(one that has just one conductor and a ground, such as a coaxial cable).
A balun is a type of transformer: it's used to convert an unbalanced signal to a balanced
one or vice versa. Baluns isolate a transmission line and provide a balanced output.
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Quite often toroids are used to make baluns with. Even a ferrite rod from an old AM
radio can be used. I however have something against using ferrite materials in baluns.
If you are not carefull, the ferrite can get saturated and heat up. You won't be the
first blowing up a balun when using QRO. So instead of ferrite, I went with an air-wound
balun.
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This air-core balun is wound using the following components :
- 40mm diameter grey PVC pipe with a length of 9.5cm
- 2 endcaps 40mm PVC
- Simple zip cord (multi-stranded core insulated wire)
- 8 turns bifilar wound
- 2 banana type chassis connectors
- 1 SO239 socket
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When fed with the 50 Ohm coax from the AT-100, the balun transforms this impedance to
200 Ohms which is close to matching 150 Ohms. The image below shows how to wire the balun :
Terminals B and C are wired to the banana chassis connectors for hooking up the feedline.
The SO239 terminal center pin is wired to C and it's shield to D. Make sure to connect
D and A together. As can be seen on the image at the top of this page, the wiring enters
the PVC pipe and all connections are made internally. In my case - since I am using the
balun inside - I did not glue the endcaps to the pipe but used a couple little
screws instead. This way the balun is easy to service should the need arise.
Good luck constructing yours !
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