Netherlands,    
 



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.

LDG AT-100Pro 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.


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.

 
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.

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 !
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