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.
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!
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 – which sounds about right given the size of the toroid. However, the color of it concerns me – it looks like an iron core which is less than ideal for the intended purpose.
Here’s an example of why I hate buying “black boxes” – 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. The screw terminals also need to be looked after because, if they seize, you may take the head of the screw off!
It’s hard to say for sure whether this will 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 100w CW/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/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.
Simple RG58 Choke using 2 Ferrite Rings with 2x 3 passes through the ferrites. This has often been used between my radio+ATU when operating portable.
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.
When I’m portably operating – especially when using a frequency-agile Doublet, I make use of this 1:1 Choke which comprises 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.
- 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!