High impedance Baluns
(published in Electron #4, 2007)
This chapter is the fourth in a series of articles on baluns for antenna applications. The first article is an introduction with some background on balun types. The second article is on wire baluns. The third article is discussing baluns around core materials. It is advisable to read the articles in the above order especially since each next chapter is building on information already explained earlier and referencing to this.
Many types of balun are designed for application in a low impedance environment e.g. for matching to the input and output of (balanced, power) amplifiers, resonant antenna's etc. System impedance will be between a few Ohm to around 100 Ohm which requires the (current) balun to exhibit a sleeve impedance of hundreds of Ohm that is not too difficult to realize over a wide band-width. Low frequency behavior is determined by the sleeve reactance which will quickly reach the required values when being constructed around a ferrite core. High frequency drop-off is mainly determined by the parasitic capacitance across the (sleeve) reactance, this in turn depending on the number of turns and the proximity of the first to last winding.
At antenna systems outside resonance, system impedance may be considerably higher than 100 Ohm, with comparably higher demands to the design and construction of baluns. An interesting analysis of such situation may be found in the article of Kevin Schmidt, W9CF, Putting a balun and a tuner together. It is shown that for high impedance antenna systems, balun (sleeve) impedance should be above 1000 Ohm and preferably even higher.
To ensure these high impedances over a wide bandwidth is not trivial anymore.
A balun to connect to a high impedance, balanced feed-line is not a simple component.
- It should force equal currents in the balanced connections with as low a current as possible to ground. This is always a problem at 'flux'- transformers that usually will not be constructed in perfect symmetry (see e.g. trifilar transformer in previous chapter).
Furthermore, any form of un-balance will make the feed-line radiate / receive in an undesired way as your neighbors will tell about your transmissions and you will discover yourself tapping into the (usually vertically polarized) electro-smog in your environment.
- It should exhibit high transfer efficiency at all
operating frequencies. With comparably low allowable internal power
dissipation (e.g. maximum 4 Watt in a
Especially the first requirement differentiates a balun for a symmetrical feed line from coaxial feed-line applications.
Even to date we may find see proposals for antenna tuners at with a voltage balun in the output circuit to take care of a balanced output (-currents). To this extend the voltage balun is looking something like figure 1.
For this voltage balun, the primary and secondary winding usually have equal turns although step-up ratios may be seen as well. In the 1 : 1 variation, the generator voltage 'U' will also appear across the load and with proper winding techniques the secondary winding may be balanced around the ground connection in order to also balance the parasitic capacitance.
A practical solution to this balancing
problem may be found in figure
All turns have been spaced equally at the inside of the coil-former, to minimize inter-winding capacity.
When designing this balun for a 50 Ohm
system, with a lower operating frequency of 2 MHz., at a
As in previous chapters this transformer has been measured for transmission and reflection. Because of the construction details, a small unbalancing may be expected so the measurements have been performed two times, each time with a different output terminal grounded. For all measurements, the balun was terminated into 50 Ohm. Figure 3 presents the transfer measurements.
In figure 3 it is shown that indeed some transfer difference may be noticed . When measured in counter phase (blue curve) transfer is improved at 20 MHz. by 0,3 dB and at 30 MHz. by 0,8 dB. This different transfer behavior most probably is due to some capacitive unbalance in this voltage transformer
Apart from some unbalancing, transfer characteristics are not really ideal either with transfer loss of 1,7 dB and 2,5 dB at 30 MHz. This will limit application in a high(er) power system at this operating frequency. Never-the-less, this 1 : 1 voltage transformer is doing a better job than the trifilar 1 : 1 transformer we discussed earlier in the chapter on baluns with cores.
We also performed some reflection measurements, identical to earlier transformers. Results may be found in figure 4.
The reflection graph (SWR) is showing some interesting facts:
- SWR never drops below 1,5
- Below 2 MHz. SWR is going up because of diminishing impedance
- Above 10 MHz SWR is going up sharply to take impractical values above 20 MHz.
To investigate this disappointing behavior we measured the real and imaginary impedance values that make-up the reflection curve. In figure 4 we notice reactance (Xseries) to not drop to insignificant values, which accounts for the not too low SWR in the frequency range up to 10 MHz. Above this frequency, the reactance rises further due to loss of inter-winding coupling. This effect is showing up as a leakage induction, making (series) reactance to go up again with frequency.
For 4C65 materials, ferrimagnetic resonance
frequency (fr) is at 45 MHz., meaning permeability and loss to have the same value
(Q-factor is 1). This is showing in figure
Around 65 MHz. rising of Xseries breaks off to fall off sharply and become negative. This is the effect of the parasitic parallel capacitance becoming the dominant impedance.
We might consider applying higher ferrimagnetic resonance materials, e.g. 4D2, with fr at 150 MHz. Unfortunately the higher the ferrimagnetic resonance, the lower the permeability (Snoecks law) so we would need more turns to comply with reactance requirements at the low frequency cut-off. More turns also would mean higher parasitic capacitance and this would lower the high frequencies cut-off.
Because of these effects this voltage balun may not be improved too much beyond current, already not ideal performance.
To operate as a balun for a symmetrical tuner, impedance to ground should be 1500 Ohm or higher. Going back to figure 3 one may notice the inter-winding reactance to be below this value already at 10 MHz. and at 30 MHz. even below 350 Ohm. This again makes this voltage balun less fit for the job.
Let's look again to the earlier current transformer,
this time specifically for application in a symmetrical tuner. All earlier
requirements still apply and so we again arrive at the 4C65 current balun at
The transfer characteristic for this component may be found in figure 6, this time with the graph for the sleeve impedance to ground added.
In figure 6 we find this current balun already better performing than the voltage balun at 2 MHz. with 0,2 dB of damping versus 0,25 dB. In the rest of the HF frequency range, insertion loss is much lower for the current balun again with a damping of 0,1 dB at 30 MHz. as compared to 1,7 - 2,5 dB for the voltage balun. Also beyond this frequency, the current balun is still in full service for quite some time.
Impedance to ground
Using this component in a symmetrical tuner, the impedance to ground is especially important. Although permeability of 4C65 ferrite is going down at higher frequencies, the vector summation of permeability and materials-loss keeps on rising for a very long frequency range so total sleeve impedance will keep going up. This is opposite the impedance in the voltage transformer that is determined by the parasitic inter-winding capacitance, generating a falling impedance graph.
Starting at 10 MHz. the current balun is close to the minimum impedance requirement (Z > 1500 Ohm). At lower frequencies impedance is still too low. This is easily enhanced by adding ferrite to the core. For each additional ferrite ring, impedance is added by the same factor. With three toroides in a stack and the same ten turns of RG58, we easily reach to over 1000 Ohm at 2 MHz. which is close enough for this lower operational frequency.
Stacking cores is the better method over constructing three individual ten turn current baluns and putting these in series. In this latter situation, parasitic capacitance of one sleeve coil will inevitably resonate with the inductance of the next transformer, creating a low impedance instead.
A last but useful feature of the current balun over the voltage balun is the direct DC-path of the first type to ground; static charge may not build up making this component safer to operate and more 'silent' in dry seasons (no discharge noise).
A frequently recurring discussion to put the balun before of behind the tuner has found a definitive answer in the article by Roy Lewallen: 'The 1 : 1 current balun' . He proves that there is no difference in operation and the balun will have to exhibit an equally high 'sleeve impedance' in both situations.
Bob J. van Donselaar,