Baluns

Baluns (and UnUns) are not complicated devices once you know the basics. Sure, they can be tricky to build as they generally require the dexterity of 3 hands when winding+holding everything in place… In my original experiments, I wanted to learn a bit more about Baluns, UnUns and how best (ie: effectively and cheaply) to address common-mode current issues to ensure that the aerials+feeder perform as they should. In short, I didn’t want to spend £30 on another off-the-shelf box that, perhaps, didn’t do the exact job I wanted it to do – I’d rather spend the money on specific parts in order to build baluns that are suited to my own requirements.

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Firstly: There is no such thing as a Balum – Unless the “um” is you wondering just what they are, when/where to use them and what it needs to made from.

When and where would you use a Balun or UnUn?

• 1:1 Current “Choke” Balun – SO239 to Terminals
  At the centre of a conventional Dipole – To interface the Balanced Aerial to Unbalanced Coax. Also at the junction between Balanced Feeder (ladder-line) and Coax – eg: G5RV. Many G5RVs simply offer a plastic SO239 adaptor which the ladder-line directly connects to. It is strongly suggest that you put a 1:1 Current Balun at this point. Why not a 4:1? Well, the short answer is “depends” – but, in most cases, a 1:1 will be preferable.
• 1:1 Line Isolator – SO239 to SO239
  At the base of a resonant vertical to reduce common-mode current on the feeder – Helps to prevent the outer of the coax from radiating. Can also be placed in the Shack behind your ATU/Radio to reduce noise pick-up from your coax cable. You may have seen/used an “ugly balun” which is simply many turns of coax about 4-6″ diameter – See the “Bottle Choke” section near the bottom of this page for details (and a note of caution).

What Ferrite Core? – for 1:1 baluns/chokes/isolators
For a simple 100w choke, 8 turns of RG174/RG316 on an FT140-43 (1.4″ dia). For UK legal-limit (400w), 8 turns on FT240-31 (2.4″ dia) will be resistive 3.5-30MHz while 12 turns will be resistive 2-15MHz. 75-mix is better for use <10MHz but its performance trails-off further up the HF band.  Cores can be stacked for increased power-handling and choking impedance.

The chart below is based on the late G3TXQ’s ferrite vs frequency table and shows where a particular combination is resistive on the HF bands:

The line-graph below was built using data from Fair-Rite to show the resistance offered by a single ferrite bead across a range of frequencies by ferrite type:

The above chart gives you an idea of how different mixes/types of ferrite work at different frequencies – Although this is quite a basic plot, a longer look will show that a 31-mix offers the best “all-round” solution unless you have a specific frequency range in mind.  Bear in mind that with 2.4″ ferrites, the Resistance will (hopefully) be >1500-ohms and differences between ferrite types will be more than the few Ohms you see here!

• 9:1 UnUn
  Feeding a random/long wire or at the base of a multi-band vertical. Generally, a bit of “random” wire presents a high impedance, so the 9:1 transformation makes life easier on whatever antenna matching unit you have.

• 4:1 Voltage Balun
  At the feedpoint of an off-centre-fed Dipole, eg: Windom. Can also be used with a Delta Loop as these typically present 150-200ohms at the feedpoint. These types of balun may still require a Current Balun on the feeder – a Windom deliberately encourages a short section of coax to radiate before it is choked-off with a 1:1 Current Balun. You can read about my 4:1 Current Balun build here.

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Size vs Capacity/Handling
The main issue when you start building baluns with significant “head-room” is they tend to get heavy. Adequate strain-relief for the wires is needed and possibly a centre-support if it’s a 160m-40m dipole. A top-mounted eye-bolt for use with a pulley may help – That’s a good reason to use good quality enclosures and to cover any holes/connections with potting compound – It may be a messy job and the stuff takes 3-4hrs to fully cure, but can make all the difference.

When using reputable (and adequately rated) components – you can understand why places charge around £50 their products. If you only ever want to run QRP or your licence does not permit more than 100w, you can build simple baluns for very little cost – For example:

You can buy FT140-43 , FT240-31 , FT240-43 and FT240-75 toroids from Ham Goodies. RG-174 is good for up to 100w, but RG-316 is readily available and easy to solder as it’s made from PTFE. This won’t melt under a soldering iron so is ideal to work with if you’re not a confident solderer. It’ll easily take 400w at HF frequencies whilst 174 is rated at 350w (at 10MHz). If you’re just looking for 100-watt devices, RG-174 will suffice. For QRO (over 1kW), RG-142 is the stuff to use – 5mm (like RG58), but PTFE.

Here are 2 examples of my patch-lead chokes – There’s a 100w(ish!) version made from RG58 plus a larger one made from RG-213. Cylindrical type 31 ferrites were used here.

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Bottle Choke, aka: Ugly Choke/Balun
The Bottle Choke is simple-to-make and you can find countless example online – However, after tests, I don’t like them very much.

• Further Reading : Ugly Choke – Ham Universe

This type of choking is not advised as the choke will most likely be reactive and rather than be an effective “choke”, what you actually get is a parallel resonant circuit. In fact, under the right/wrong conditions, these types of choke can actually increase the common-mode current. I strongly advise the use of a good toroid-based choke, especially if used below 10MHz.

On the right, you can see my “bottle choke” made from a plastic water bottle and length of RG58. In the past I have used it at the base of my HF vertical (40m+30m) but it’s now stored under the desk as an example of how not to reduce common-mode current!