1:1 50 Ohm Voltage Balun for HF bands

Here’s my small project of an easy to make 1:1 HF Voltage Balun.

I have installed this balun between the input of my HF fan dipoles and the RG213 coaxial cable going down to the transceiver. Below there are photos that show how I’ve constructed this voltage balun. Note that this is not a current balun. The balun presented here has also a DC isolation protection between the input and the output, similar to how a standard transformer behaves. So, any static charges generated on the antenna side cannot be discharged down to our expensive HF transceiver. Also, note that since this is a 1:1 voltage balun, it works best when the antenna is symmetrical, such as dipole antennas. The balun does not modify the impedance level between the input and the output, and it is designed to accept 50 Ohm terminations on either end. The design is symmetrical, so there is no worry which part of the balun will be used as input or output.

The core element of this balun is an aperture ferrite by Amidon. This geometry of the ferrite provides better and ‘stiffer’ coupling between the input and the output sides, hence less losses due to leakage are presented. The high magnetic factor (AL) of the ferrite means that few turns are needed to achieve a desired impedance level, even on the HF band.

I don’t want to dig deep into the design complexities behind the calculation used to estimate the number of turns needed on the primary and the secondary of this 1:1 balun. I have spent dozen of hours experimenting with materials of various shapes from many different manufacturers when deciding to pick up a material for this purpose. For the interested reader, the main idea goes like this: Based on the manufacturer’s datasheet for the calculation of the impedance vs. number of turns graph (some manufacturers supply design equations instead), calculate the number of turns on each side (since this is a 1:1 balun) of the balun, so that the impedance at the lowest operating frequency is not less than about 4 times the impedance value, i.e. 4 x 50 = 200 Ohm. This value will ensure that there is a strong coupling between the input and the output, hence a good power transfer. (the design concept applies for baluns designed on the ‘classic’ toroid ‘daunaghts’ style geometries).

Summary of the specifications for the 1:1 HF Voltage balun:

In any case, my purpose was to design the balun so that it can cover all bands from 80m upwards. The maximum operating band is dictated from the limitations of the ferrite material itself and not much can be done about it. So, whenever you aim to design a balun, aim for a proper much on the lowest possible band first and let the material itself define the maximum operating frequency. Of course there is a tradeoff, as the balun has a finite bandwidth it can cover before the ferrite material changes its characteristics: The lowest the ‘low-frequency’, the lowest the higher frequency as well. So, take care on this!

Measurement of the 1:1 HF balun with Mini VNA Tiny

In order to double check the performance of my 1:1 HF voltage balun I’ve performed a careful measurement with MiniVNA Tiny. The setup is shown below. The VNA is connected to one side of the balun and a 50 Ohm (Diamond) load is connected at the output to emulate the effect of a 50 Ohm antenna.


The measured results of the SWR and the resistance (R) (not Impedance, Z) up to 52MHz, are shown below:

VNA_160830_132901_1 to 1 HF voltage balun - Material BN43-3312

It can be seen that the balun  shows a very good much up to 30MHz (10m), with SWR remaining below 1.6:1 all across the HF spectrum. Additionally, with some small tuning it can be seen that the 50MHz band can also be used.

Another important aspect is that of the insertion loss. In other words, how much power is it transferred from the input to the output port? Some power loss is to be expected due to the presence of the ferrite core and other losses in the connectors, wiring etc. Below are the results of the transmission loss measurement using MiniVNA Tiny.



VNA_160831_083702_1 to 1 HF voltage balun Insertion Loss - Material BN43-3312

From the above measurement, it can be clearly seen that the balun presents extremely low loss characteristic all across the HF band. Even at the 6m band, the insertion loss is only 0.5dB.

Similarly, I’ve constructed (results will be shown on future article) voltage baluns for different impedance levels, e.g. 1:4 (Zprimary=50 Ohms, Zsecondary=400 Ohms loads). On this case, a common practice is that you first calculate the number of turns on the lowest (50 Ohm) side of the balun and then you calculate the number of turns on the other side using the basic formula: (Np/Ns)^2 = Zp/Zs, where N is the number of turns p=primary, s=secondary and Z is the impedance.

Hope this article gave you an understanding of how a simple voltage balun can be designed and constructed.