Baluns

Baluns (and UnUns) and 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 – I’d rather spend the money on good/known parts in order to build several that are to my own specification.

I’ve since applied what I’ve learnt to my Ham Goodies web-store – So, for a range of good quality (yet affordable) balun solutions – Please see my Ham Goodies website.

The Short Answer : 8 turns of coax around an FT240-31 ferrite toroid  🙂


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.

Firstly, 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.  For UK legal-limit (400w), 8 turns on FT240-43 will be resistive 6-30MHz and an ideal “all rounder”.  For the lower bands, 8 turns on an FT240-31 will be resistive 3.5-30MHz while 12 turns will be resistive 2-15MHz.  Cores can be stacked for increased power-handling and choking impedance.

  • 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.


Why Not Just Buy Them?
You could just visit your local radio emporium (or online dealer) and buy something off-the-shelf.  But, building your own equipment has always been part of the Ham Radio hobby and does offer several advantages:

  • It’s educational – You’ll learn from your mistakes, what works, what doesn’t and be able to help others in the future

  • It’s cheaper – Us radio hams are a penny-pinching bunch and every little helps

  • Quality control – You know how good the item is because you’ve built it to your needs

Shown here is a £37 box of tricks from LDG – It’s a simple 1:1 Balun used with a resonant dipole (or G5RV coax>ladder-line join).

Look how simple the device is: SO-239 connector, 2x 4mm terminals (cheap plastic thread!), some enamel copper wire and small toroid.

About £10 worth of bits.  LDG say that it’s rated for 200w PEP – Sounds about right.  I’m not a fan this (bifilar) method of winding and prefer the coax option, especially with a resonant dipole.  Connect a 1:1: bifilar balun up to an analyzer (along with a 50ohm load), sweep it from 1.8MHz to 30MHz and you’ll see why…  The color of the toroid also concerns me – it looks like an iron core (less than ideal for the intended purpose).

Ah, this example is why I hate buying “black boxes” – This is a Moonraker/Watson model.  There appears to be enamel copper wire wrapped around a short ferrite rod and then sealed with potting compound.  Just as well because the box is plastic with no real waterproofing properties.

I’ll wager that the potting compound is the most expensive part of this balun which happens to be rated at 400w – Without knowing what’s inside the compound, it’s hard to say for sure whether it’ll withstand a thrashing a full UK power – When looking at power-ratings, you must factor in the aerial being used as ratings are (usually) shown for ideal conditions, ie: 1:1 SWR.  Overall, these devices work just fine under what I’d call normal conditions – that’s with a 100w radio on SSB and the odd burst of high-power data modes.

If you do buy from a Ham outlet, and it’s not clear what’s inside the box: Ask.  If they can’t/won’t tell you, or reply “that would be telling”, then treat their product as a Completely Valueless Object.

Size vs Capacity
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 24hrs to cure, but it can make all the difference.

When using reputable (and adequately rated) components – you can understand why people like GWhip 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…

You can buy FT140-43 , FT240-31 and FT240-43 toroids from Ham Goodies.  RG-174 is good for up to 100w, but RG-316 is readily available from Ham Goodies and is easy to solder as it’s made from PTFE so won’t melt under a soldering iron (ideal 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.  Type 31 ferrites were used here.

Simple RG58 Choke using 2 Ferrite Rings with 2x 3 passes through the ferrites.

RG213 Choke using 8 Ferrite Rings held in place with heat-shrink tubing.

For serious choking of problematic cables, I suggest a 31-mix snap-on ferrite – G4ZTR Aerial Parts of Colchester has sizes from RG58 to RG213.  These offer a better performance than the “domestic” generic/TDK versions.  The larger “Problem-Solver” can be useful in taming noisy mains cables.

For my Portable operating – particularly when using a frequency-agile Doublet, I made this 1:1 Choke made from a PL259, some RG58, an FT240-31 ferrite toroid and this Dipole Centre.  It’s good practice to have a Balun where BAL(anced) meets UN(balanced) feeder and this serves nicely.  The PL259 plugs into a radio or ATU and the balanced feeder attaches to the M4 bolts on the yellow centre.  This also saves using a boxed version which introduces another PL259/SO239 combo – the RG58 coax is kept deliberately short as this is, essentially, balanced line to the Shack!

M0PZT 1:1 Choke Balun using FT240-31


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.

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!