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


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 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 6-29MHz and an ideal “all rounder”.  For the lower bands, 8 turns on an FT240-31 will be resistive 3.7-30MHz 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.


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…

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 products as Completely Valueless Objects!

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 my Ham Goodies store.  RG-174 is good for up to 100w, but RG-316 is readily available from my Ham Goodies shop 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 condifent 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 (from Ham Goodies) will suffice.

Also made were some simple chokes – The RG58 one has been used on the back of the FT-857D to connect the YT-100 auto-ATU and the larger RG213 (using 8x 31-mix ferrite sleeves) connects my TS-590SG to the MFJ-948 ATU.

Simple RG58 Choke using 2 Ferrite Rings with 3 loops

RG213 Choke using 8 Ferrite Rings with heat-shrink

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.  My own Ham Goodies shop offers the large 31-mix “Problem Solver” as recommended by GM3SEK’s In Practice article.  This is great for mains filters and ad-hoc aerial chokes – the snap-on ferrite offers significant choking and can be fitted to cables quickly for test/evaluation purposes.

 

 


The Sorbie Balun : Original tub, and the complete item
The Sorbie Balun got it’s name thanks to the plastic pot used to make it – As I’m still a young slip of a lad, I’m blessed with a full head of hair so using gel/moulding cream is a daily chore – These little screw-tubs are usually thrown into the recycling or used to store odds ‘n’ sods in the Shack.  I then had the thought that they’d make a good Balun/Dipole Centre – So the Sorbie Balun was created!

Balun/Choke Example

The balun itself consists of 4+4 turns of mini coaxial cable over a large ferrite core – see the image below for a wiring layout.  These kind of baluns are well-documented online so I won’t go into further detail.  As a guide, 8 turns on an FT240-43 using RG316 would be sufficient for full UK legal power (400w).  For lower power, say 100w max – RG174 (or RG316) on a single FT140-43 is acceptable.

     

The lid of the pot is used to mount the SO-239 connector which is then soldered to one end of the coax.  The other end has a solder tag on each – These are then attached to the screws on the side – Add a couple of solder-tags for your dipole wire and off you go.  As a finishing touch, you could mount a small screw-hook at the top if you wanted to hang the balun from a tower or supporting pole.  Some PVC tape is used to secure the lid and provide basic weather-proofing – Bear in mind that this isn’t a fully weather-proof unit!  However, it’s a lightweight item, great for using portable and a fun “coffee-break” project.  Any other small’ish project/potting box will suffice – If you do not wish to use screw terminals, you could add another SO239 and that gives you a 1:1 Line Isolator – Perfect for connecting to the base of a vertical or at the ladder-line/coax junction of a G5RV.

Download the Sorbie Balun and Bottle Choke PDF document

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 be most reactive and rather than 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!