I have found that a lot of details regarding balun construction that can be found on the internet are flawed, especially those relating to the use of Iron Powder cores. My extensive research on this subject can be found here.
Common designs use either a few turns of coax, or bifilar windings on an air former, ferrite rod or ring core. These can work well over a certain frequency range, but there are so many variables in the construction that it is unlikely you will get consistently good results over a wide bandwidth. In this application you need to get the cable size, spacing and coupling between cable cores just right, so that you form a correctly balanced transmission line with constant impedance. I have seen all sorts of twin cable, twisted pairs and mains cable being suggested for this purpose, but when I have constructed them and measured the bandwidth, loss or impedance match, they did not work at all well.
The main problems are:-
In addition to these problems you also need to be aware of the ‘ferrite’ cores being sold at radio rallies or on e-Bay. A large number of the ones I have seen for sale are actually a strange mix of materials, and have been designed for use in switched mode power supplies where high inductances or large currents are required at frequencies below 1MHz . In either case they may not be suitable for use as baluns or transformers on the LF and HF bands unless the design is optimised, and you really don't know what sort of performance you are obtaining unless you make measurements.
These days I take a small impedance bridge to rallies and measure values. In many cases the folks selling them continue to insist they are ferrite despite any evidence to the contrary.
To illustrate this point here are some loss measurements made on ring cores recovered from switched mode power supplies.
In each case the windings were 5 turns of 1mm wire bifilar wound as a 1:1 transformer. As you can see there is great deal of variation between the results depending upon the type of core material.
Powdered iron cores are popular for high power baluns, but they don’t offer much inductance per turn of wire, so their effectiveness when used as baluns can be limited. Ferrite materials provide a much higher impedance value per turn of wire and are much more effective over a wider frequency range, but they can be very lossy when connected to a mismatched load, and heat up to a point where irreversable damage occurs.
Many constructors simply measure the input VSWR when a matched load is connected to the output, but this doesn’t tell the full story. Many people also assume that the iron powder core works as a conventional low frequency transformer, and that the core presents a closed magnetic loop. This is not the case, if windings are placed on the opposite sides of a reasonably sized core, the losses can be considerable.
As a result of observing effects such as this, I would strongly suggest measuring the balun or transformer insertion loss. The easiest way to do this by constructing two baluns and connecting them back to back. Connect a low power transmitter and VSWR bridge at the input and a suitable test load at the output. Transmit at the lowest frequency of operation and set the
Another method suggest by Iain, VK5ZDB, is to use an ATU and power meter as shown below. Note that the balun is reversed in the second configuration, so that the high impedance port is connected to the atu. Make a power measurement in the first configuration and then tune the atu for maximum measured power in the second configuration. Perfoming the calculation will give you a good indication of loss though the balun.
As a further example of how important this can be, when I first bought my current HF transceiver, I wanted to get on air quickly. So I looked in my junkbox and found two baluns. One was a commercial unit I had bought as surplus. This was intended for use with a marine HF radio, using the backstay as the antenna. The other was one that I had made (and used on 80m) about 10 years ago, which was wound on 2” diameter 'ferrite' ring I had bought at a radio rally.
I tried the first balun with a 10m vertical length of wire and the second with a 1/2 wave dipole cut for 40m. Both antennas gave good VSWR readings and I had a few reasonable contacts, with reasonable signal reports being exchanged both ways.
At a later stage when I started constructing more baluns, I finally got around to measuring the performance of these two. The commercial unit which I initially thought had either a 4:1 or 9:1 ratio turned out to be a 1:1 ratio with a loss of 3dB at 3.5MHz and about 10dB at 30MHz, my home built balun also had a loss of about 10dB but that was from 1.8Mhz up to and beyond 30MHz. So both baluns were effectively just working as high power attenuators. In the worst case only a 10th of my transmitter output power was reaching the antenna wire.
Other tests can be made to measure the isolation and balance, but the match and insertion loss will usually indicate other problems first.
For some good information see:-
http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf
1:1 balun
The only foolproof method I have found is to wind coax on a suitable ferrite ring or pass it through a large number (at least 30) of ferrite sleeves. I also suggest that for the balun to be effective you should aim to provide at least 20dB isolation (common mode rejection) between the input and output of the balun.
As an example, if you wish to use the balun with 50 ohm coax at 1.8MHz in order to meet the 20dB design figure, you need the series impedance to be greater than 1K Ohm, which equates to approximately 88uH. Medium size ferrite sleeves (such as CPC part number CBBR6942) will add about 1uH inductance per sleeve, when threaded over coax, so you would need 88 ferrite sleeves to obtain the required impedance. It is more cost effective to pass or wrap multiple turns of coax through ferrite sleeves as 10 turns through a suitable sized ring or core, which would provide a similar value of series impedance. Beware of different core types, many large ring cores are actually made from powdered iron, and they do not provide as high a value of inductance or as wide an operating bandwidth as ferrite. You can calculate inductance values for other frequencies, but I would always recommend as much ferrite as possible, as any additional isolation between input and output only helps to further improve performance.
As another example, I built a G5RV antenna and used 10 turns of coax on a 4” former as the 1:1 balun at the base of the open wire feeder section. I thought that the antenna was working perfectly well, until I replaced the 10 turns of coax balun with two ring cores. This reduced the noise level on 80m from S6 to S0 and on 160m from S8 to S0. The difference in the reception of weak signals was staggering, and I have now added additional ferrite sleeves along the coax in order to further reduce the background noise level. This experience suggested to me that many amateurs who have tried adding a balun to a G5RV and have not noticed a difference, may in fact have not have been using a suitable design.
4:1 balun
As before I tried constructing many different baluns before I realised that they didn’t work correctly. Many ‘voltage’ baluns or auto-transformer designs don’t work at all well, unless you get the materials or construction exactly right. The simplest and most reliable method I have found is to use the 1:1 coax balun with modification.
The basic principle is that two 1:1 baluns are used. The inputs are connected in parallel and the balanced outputs are connected in series, which provides a 4:1 impedance transformation. Ideally transmission line impedance should be half the output impedance. Unfortunately it's not easy to obtain 100 ohm coax, the nearest value you are likely to be able to find is 93 ohm network cable. However I have successfully used 75 ohm coax for 4:1 baluns, without observing any problems.
In my version I used three ferrite cores. The most cost effective ones I found were the large ‘snap on’ interference suppression ferrite cores which are generally made from type 31 or 43 material, offering good performance across the LF and HF range of frequencies. As a rule of thumb you need to acheive an impedance of 4 times the design input and output impedance values. Remove the plastic outer covering and superglue the core halves together. Wind six turns of thin PTFE insulated 75 ohm coax through each of two cores and thin 50 ohm coax through the other. The two cores with 75 ohm cable are connected in parallel at the input, and in series at the output. This forms a basic 4:1 balun. If the input has 50 ohms input impedance, then the output will be 200 ohms. The remaining core with the 50 ohm coax is used as a 1:1 balun on the input side of the 4:1 balun. This is in order to provide further isolation between the input and output ports.
Note that when I described the 1:1 balun and quoted a design figure of 20dB isolation between the input and output, originally that was for a 50 ohm system. In the 4:1 balun design, the increased output impedance means that we should really be seeking to achieve a series impedance of 4K Ohms or more in order to ensure correct operation. Adding additional series impedance in the 50 ohm section is easier than trying to do the same at higher impedance points.
The isolation figure becomes worse if the 4:1 balun is being used to match impedances outside of the design range. For example, if it is being used in conjunction with an ATU to match to a random length of wire, the input and output impedances are likely to be much higher than 50 and 200 Ohms. A ½ wavelength of wire can have input impedance as high as 5 K ohms, which means that a balun which has a series impedance of less than this will not work particularly well. See my notes relating to baluns and auto-tuners.
Martin Ehrenfried – G8JNJ – 23/06/2008 – V1.8
