A 4:1 Air-wound Balun
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.
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.
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.
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
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
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 !