There are common some key properties that are relevant:

- where loss is high, core loss tends to dominate;
- the specific heat of ferrite is typically quite high;
- the capacity to dissipate heat is related to many factors.

Ferrite materials have loss at HF and above that warrants consideration.

Even though the effective RF resistance of conductors is much higher than their DC resistance, the wire lengths are short and conductor loss is usually not very high.

Core loss will commonly be much larger that conductor loss and so dominate.

The specific heat of ferrite is typically towards 800K/kgK, almost as high as aluminium so ferrite absorbs a lot of heat energy to raise its temperature.

When heated by a constant source of power, temperature will rise exponentially as a result of the combination of mass, specific heat, and loss of heat from the core as temperature increases. We can speak of a thermal time constant being the time to reach 63% of the final temperature change, and for large ferrite toroids (eg FT240) that may be over 2000s.

Factors include the temperature difference between the core and ambient and if you like, the thermal resistance between core and ambient. Ambient temperature may be high if the device is installed in a roof space. Incident heat from the sun increases the challenge.

Maximum core temperature depends on maximum operating temperature of the enclosure (PVC), wire insulation maximum temperature, fasteners (eg nylon screws or P clips), and Curie temperature all weigh in.

Thermal resistance is higher where the core is contained in a closed enclosure.

Lets say a EFHW transformer using a FT240-43 is housed in a small sealed PVC box mounted outside in fee air. The transformer uses a 2t primary winding as per a plethora of articles on the ‘net.

Above is a core loss profile for the transformer where the load is such that the impedance looking into the primary is 50+j0Ω. At 3.5MHz, core loss is 34%.

Lets say that the core can dissipate 10W continuously without damage or compromise. In that case, with core loss of 34%, the transformer could be rated for 10/0.35=28.6W continuous or average RF power input. One would confirm this continuous rating with a bench test measuring temperature until it stabilised. Thermographs are a good means of documenting the heat rise.

In applications where the transmitter was active only half the time, an ICAS (Intermittent Amateur and Commercial Service) rating would be appropriate, we would rate it as 28.6/0.5=57.2W ICAS.

Note that as we ‘increase’ the power rating, consideration must be given to voltage breakdown which is an instantaneous mechanism, there is no averaging like heat effects.

Now some modes have average power (ie heating effect) less than the PEP, so we could factor that in. Average power of SSB telephony develops a Pav/PEP factor for compressed SSB telephone of 10%, so we can calculate a SSB telephony (with compression) PEP ICAS rating as 57.2/0.1=572W.

So this is a pretty ordinary ordinary transformer which we have been able to rate at 570W SSB ICAS exploiting the low average power of such a waveform.

Above is a core loss profile for the transformer where the load is such that the impedance looking into the primary is 50+j0Ω. At 3.5MHz, core loss is 8.5%.

Lets say that the core can dissipate 10W continuously without damage or compromise. In that case, with core loss of 8.5%, the transformer could be rated for 10/0.085=118W continuous or average RF power input. Again, one would confirm this continuous rating with a bench test measuring temperature until it stabilised. Thermographs are a good means of documenting the heat rise.

In applications where the transmitter was active only half the time, an ICAS (Intermittent Amateur and Commercial Service) rating would be appropriate, we would rate it as 118/0.5=236W ICAS.

Lets calculate the SSB compressed telephony rating. we can calculate a SSB telephony (with compression) PEP ICAS rating as 236/0.1=2360W.

Even more important at this power level is assessment of the voltage withstand.

So, when you see claims of power rating, read the details carefully to understand whether they are applicable to your scenairo. The last scenario about might be find for 1500W SSB compressed telephony, but not suitable for 500W of FT8.

An exercise for the reader: calculate the power rating for A1 Morse code (assume Pav/PEP=0.44).

]]>The LTSPICE model was of a ‘test bench’ implementation of the balun which comprised an air cored solenoid of two wire transmission line, with a slightly asymmetric lumped load.

This article discusses limitations of SPICE in modelling practical baluns.

Guanella’s 1:1 balun and his explanation – Zcm gave the characteristics of a example ferrite cored balun.

Above is Zcm of a 11t balun wound on a FT240-43 toroid. The ferrite core acts on the common mode choke element and has negligible effect on the differential transmission line mode. The key characteristics are:

- TL Zo=100Ω and electrical length at 3.6MHz is 0.43°; and
- Zcm at 3.6MHz is 1007+j1862.

Let us create a model for a narrow band of frequencies around 3.6MHz where will will consider Zcm approximately constant.

Above is the LTSPICE schematic.

Above is the response. We will focus on the results at 3.6MHz (where Zcm is most accurate).

The cursor 2 value is total common mode current and cursor 1 is differential current. It can be seen that common mode current is very low, 35dB less than differential current so despite the asymmetric load, there is very little common mode current. The balun is effective is reducing common mode current (much more so than the air cored example given that reduce common mode current by less than 4dB at 3.6MHz).

The green plot is Zin, and it is quite close to 50Ω by virtue of the electrically short TL section at 3.6MHz.

The load formed by R2 and R3 could be replace by the frequency dependent three component model as discussed at Equivalent circuit of an antenna system, and the frequency specific balun Zcm used (as above), and a solution obtained for a single frequency.

Essentially, whilst it is possible to insert these single frequency values to drive the computation engine, it is not a good tool for the job when one really wants to capture the frequency dependent characteristic of the balun and the complexity of antenna and feed line coupled conductors, radiation etc.

To some extend, it suffers the problems discussed at Using Ohms law on antenna baluns.

SPICE is a quite capable modelling tool, however:

- SPICE does not really have convenient tools for modeling the frequency dependent complex impedance of ferrite cores often used in HF baluns; and
- SPICE does not really allow for entry of the coupled antenna and feed line conductors (ie modeling the antenna system).

- Duffy, O. Dec 2010. Baluns in antenna systems. https://owenduffy.net/balun/concept/cm/index.htm (accessed 31/05/19).
- ———. May 2011. Measuring common mode current. https://owenduffy.net/measurement/icm/index.htm (accessed 31/05/19).
- Guanella, G. Sep 1944. New methods of impedance matching in radio frequency circuits. The Brown Boveri Review.

The analysis was presented in an LTSPICE model of a ‘test bench’ implementation of the balun, and it showed that on a slightly asymmetric load, common balance was only good in a small region around the choke’s self resonant frequency of 41MHz.

One metric that is useful in indicating the effectiveness of a Guanella 1:1 balun in achieving current balance or reducing common mode current is the choking or common mode impedance Zcm of the stand alone balun.

Modern thinking and experience is that |Zcm| needs to be 1000Ω or higher for effective common mode reduction on many HF wire antennas, and considerably higher for some highly asymmetric antennas.

Above is Zcm for the example balun. It is very low at low frequencies and rises to 133+j914Ω at 30MHz.

The quite low Zcm below 30MHz explains why it was a dismal failure at reduction of common mode current. Adding more turns would improve Zcm, but exacerbate the already poor impedance transformation.

This balun was not at all suitable for HF, and use of a magnetic core was mentioned as a possible solution.

Above is Zcm of a 11t balun wound on a FT240-43 toroid. The ferrite core acts on the common mode choke element and has negligible effect on the differential transmission line mode. The distinct advantages are that:

- it uses 25% of the wire length so TL electrical length at 30MHz is 36°; and
- Zcm is much higher mid band and |Zcm| is not less than 2000Ω from 3.5 to 30MHz.

The resistive element of Zcm, Rcm represents the core loss. If there is sufficient Zcm, common mode current Icm is sufficiently low that little power is lost. For example if at some transmitting frequency Rcm=2000Ω and Icm=0.01A, then P=I^2R=0.01^2*2000=0.2W.

A ferrite core can substantially increase Zcm whilst reducing TL length.

Power lost in the ferrite core can be very low in a well designed system.

- Duffy, O. Dec 2010. Baluns in antenna systems. https://owenduffy.net/balun/concept/cm/index.htm (accessed 31/05/19).
- ———. May 2011. Measuring common mode current. https://owenduffy.net/measurement/icm/index.htm (accessed 31/05/19).
- Guanella, G. Sep 1944. New methods of impedance matching in radio frequency circuits. The Brown Boveri Review.

In fairness, Guanella did not call the thing a “balun”, but if we accept a very general meaning of balun to be any device intended to facilitate or permit a different state of balance to either side of itself, this is a balun.

Above is an extract from (Guanella 1944), and contains an almost complete description of the 1:1 balun. The ideal centre tapped transformers shows are a device for separating the differential and common mode currents so that appropriate elements can be used for those currents.

Let us consider a coil as in Fig 1a with 10 turns of two wires with geometric mean diameter (GMD) of 1.6mm and coil diameter and length of 50mm and 100mm. Note that the spacing between turns is much greater than the spacing between wires, so treating it as a transmission line (TL) wound around a non magnetic tube is valid. The TL length is approximately 3m 13.1ns, and its Zo is around 100Ω.

Above is the LTSPICE schematic. The balun is modeled with as asymmetric load (R2 and R3).

Above are the model results.

Note the upper graph of I(R2) and I(R3). They are almost equal in magnitude and opposite in phase around 41MHz, but hardly so at low frequencies.

Above are the cursor values in the previous graphic. I(R3)-I(R2) is the common mode component of current.

At 41MHz, the differential current is around 10mA, and common mode current of 1.5µA or 0.015% is very low. This is a very effective common mode choke around 41MHz.

At 30MHz, the differential current is around 10mA, and common mode current of 314µA or 3.4% is not very low and becomes much worse as frequency is reduced. This is not a very effective common mode choke below 41MHz.

Returning the the plots, the green curve is input impedance magnitude and phase. The input impedance is hardly equal to 50+j0Ω (the sum of R2 and R3) over the frequency range, and in fact varies cyclically with frequency in the upper frequency range.

At low frequencies, the impedance of the choke disturbs in the impedance transformation. In this case, the impedance below about 30MHz is too low for close to ideal impedance transformation.

The TL contributes to impedance transformation, more so as electrical length increases. When TL length exceeds 15°, it degrades nominal impedance transformation significantly, and that happens at 3MHz in this example and is 141° at 30MHz.

In summary, the example fails as a 1:1 current choke at low frequencies because the choke B is too small, and it fails at high frequencies because the TL length is to large.

Guanella did not really deal with the issue of standing waves on the transmission line, his discussion assumed matched line or intentional transformation.

How to increase the impedance of B and at the same time reduce the TL length?

The solution offered without proof is easy, introduce some higher permeability magnetic core to the choke giving higher choke impedance and with reduced TL length.

A good broadband Guanella 1:1 current balun has:

- high common mode impedance (typically >1kΩ for use in an antenna system); and
- near 1:1 impedance transformation over a broad range of load impedance.

Achieving both is a challenge as measures to improve one tend to degrade the other.

Air cored Guanella 1:1 baluns fall short of these objectives.

Guanella, G. Sep 1944. New methods of impedance matching in radio frequency circuits. The Brown Boveri Review.

]]>Above is a diagram of a Pawsey Balun used with a half wave dipole (ARRL).

Pawsey Balun on an asymmetric load reported model results in an asymetric dipole antenna, and showed very high common mode feed line current.

Pawsey Balun on an asymmetric load – bench load simulation showed that although the Pawsey balun is not of itself an effective voltage balun or current balun, it can be augmented to be one or the other.

So, you might ask what they do, what they are good for, and why they are used.

If you were to construct a quite symmetric half wave dipole and directly connected a coax transmission line to the centre, you would destroy the symmetry of the system as connection of the shield to one dipole leg only effectively connects the common mode conductor (the outer surface of the shield) to one leg of the dipole.

The Pawsey stub or balun is a narrowband device (ie tuned) that adapts the coaxial feed line to a pair of symmetric terminals for attachment to the antenna feed point.

In a perfectly symmetric system (source, feedline type and topology, antenna), current in the radiator will be symmetric and there will be negligible common mode current on the feed line.

Symmetry is easier to achieve with some types of VHF/UHF/SHF antennas than at HF.

Equivalent circuit of an antenna system gives measurements of a fairly symmetric G5RV Inverted V dipole + feed line and in that case, the Z1 and Z2 values are different on the two bands reported, more so on 80m.

On the other hand, a corner reflector with half wave dipole feed for 1296MHz can be constructed with very good symmetry, and fed from behind the reflector, a Pawsey balun should give the necessary feed symmetry to preserve system symmetry and have symmetric dipole currents and negligible common mode current on the feed assembly.

The question of why are they used is more difficult than the other questions. They do have application, but they are also used inappropriately and given that it is most unusual to seem validation of balun performance by measurement, such use highlights the bliss of ignorance.

]]>Pawsey Balun on an asymmetric load reported model results in an asymetric dipole antenna, and showed very high common mode feed line current.

This article looks at two test bench configurations modelled in NEC.

The configurations are of a horizontal Pawsey balun for 7MHz constructed 0.1m over a perfect ground plane. The ‘balanced’ terminals are attached to the ground plan by two short 0.1m vertical conductors which are loaded with 33 and 66Ω resistances. At the other end, the horizontal transmission line is extended by two different lengths and connected to the ground plane using a 0.1m vertical conductor. The two extension lengths are almost zero and a quarter wavelength.

The total horizontal length from the ‘balanced terminals’ to the grounded end of the transmission line is a quarter wavelength for the Pawsey balun and a further 20mm making approximately a quarter wavelength in total.

Above is a plot of current magnitude and phase from 4NEC2. The current on the two vertical conductors containing the 33 and 66Ω loads is quite different, and the product gives load voltages that are approximately equal in magnitude and opposite in phase.

You could be forgiven for thinking that the Pawsey stub itself is a good voltage balun, but in fact the voltage balun behaviour is due to the fact that the transmission line and Pawsey stub conductors in common mode are approximately a half wave electrical length, and being grounded at the far end, the common mode impedance looking into the ‘balanced’ terminals is very low… a hallmark of a good voltage balun.

The total horizontal length from the ‘balanced terminals’ to the grounded end of the transmission line is a quarter wavelength for the Pawsey balun and a further quarter wave making a half wavelength in total.

Above is a plot of current magnitude and phase from 4NEC2. The current on the two vertical conductors containing the 33 and 66Ω loads is approximately equal in magnitude and opposite in phase.

You could be forgiven for thinking that the Pawsey stub itself is a good current balun, but in fact the current balun behaviour is due to the fact that the transmission line and Pawsey stub conductors in common mode are approximately a quarter wave electrical length, and being grounded at the far end, the common mode impedance looking into the ‘balanced’ terminals is very high… a hallmark of a good current balun.

At intermediate lengths, the common mode impedance will range from one extreme to the other, and for the most part, it will be neither a good current balun nor a good voltage balun.

The Pawsey stub or balun is not of itself a good current balun or a good voltage balun, but can be used as part of a more complete solution to act as either a good current balun or a good voltage balun.

Creating that context may be impractical for many antenna topologies.

Without careful implementation of the context, the Pawsey balun or stub is anyone’s guess. Nevertheless they are written up this way in textbooks and find practical application, even though their performance is likely to be unpredictable and unmeasured.

]]>Above is a diagram of a Pawsey Balun used with a half wave dipole (ARRL).

Whilst these have been quite popular with VHF/UHF antennas, the question arises as to how they work, and whether they are effective in reducing common mode current IIcm) for a wide range of load scenarios.

To find an answer, and NEC model was constructed of an OCF half wave dipole a half wave above a perfect earth conductor, vertical feed line to ground, and the current magnitude and phase for all conductors evaluated.

Above is the current plot. Note that horizontal conductors are defined from low X to high X, and vertical conductors from low Z to high Z.

It can be seen that there is relatively high Icm at ground level, and when the currents in both Pawsey Balun conductors are added near the feed point, again relatively high Icm. Also of interest is that the currents in the dipole legs are flowing in opposite directions at the feed point.

This antenna acts more like a top loaded vertical monopole than a horizontal dipole.

Above, the radiation pattern reinforces the view that it is behaving as a top loaded vertical monopole.

In this scenario, the Pawsey Balun has not effectively suppressed common mode current (as would a good current balun), indeed it seems to facilitate it (as a good voltage balun would).

The Pawsey Balun as shown in the diagram is not of itself either a good current balun or a good voltage balun.

]]>- Under what circumstances is the current into one terminal of the secondary significantly different to the current out of the other terminal of the secondary?
- Is this in fact a better current balun than voltage balun under some conditions? What?
- Is it a good voltage balun?

At low frequencies, the current into one terminal of the secondary is identical to the current out of the other terminal of the secondary.

Factors that contribute to departure from that ideal are phase change through a secondary winding that is electrically long (ie more than say 5°), and lower capacitive reactance coupling of secondary to primary winding. Both of these are more likely as frequency is increased.

It is possible to build a ferrite cored transformer of this type that has very high common mode impedance, with almost perfect current balance… but it is challenging to achieve that objective over a very wide frequency range (say 10:1).

The fact is that the winding configuration does not exclude the device being used as an effective current balun, ie to drive equal currents of opposite phase into the load.

Lets deal with this question out of turn.

From Definition: Current Balun, Voltage Balun:

An ideal voltage balun delivers voltages that are equal in magnitude and opposite in phase.

A good voltage balun will approach the ideal condition. It will deliver approximately equal voltages (wrt the input ground) with approximately opposite phase, irrespective of the load impedance (including symmetry).

Consider the transformer driving a load comprising a 100Ω and 200Ω resistor in series, and the junction between them grounded.

A first approximation at low frequencies is that the balun will drive twice as much voltage in magnitude on one terminal as the other, and they will be of opposite phase.

The load voltages are not equal in magnitude and opposite in phase. The subject balun is an abject failure as a voltage balun.

The subject balun is likely to be a good current balun at frequencies where the length of the secondary winding is less than say 5° electrically, and where capacitance from secondary to primary is kept very small (say Xc>2000Ω at the operating frequency).

Since the subject balun is an abject failure as a voltage balun under any circumstance, and it can be a good current balun (albeit over a limited frequency range), it can be a better current balun than voltage balun under the conditions given above. (Note that the characteristics of a good current balun and good voltage balun are mutually exclusive.)

In fact A prototype small 4:1 broadband RF transformer using medium µ ferrite core for receiving use describes such a transformer with high common mode impedance from 1.8 to 18MHz (albeit 1:4 transformation).

(ARRL 2011) gives guidance on the function of current and voltage baluns:

A current balun forces symmetrical current at the balanced terminals. This is of particular importance in feeding antennas, since antenna currents determine the antenna’s radiation pattern.

A voltage balun forces symmetrical voltages at the balanced terminals. Voltage baluns are less effective in causing equal currents at

their balanced terminals, such as at an antenna’s feed point.

A (far) more complete development and definition IMHO is at Definition: Current Balun, Voltage Balun.

- Silver, H Ward ed. 2011. The ARRL handbook for radio communications. 2011 ed. Newington: ARRL.

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It is labelled as a simple 1:1 voltage balun, which is to imply that it is NOT a current balun (they are mutually exclusive).

He goes on to quote:

which begs the question, under what circumstances is the current into one terminal of the secondary significantly different to the current out of the other terminal of the secondary?

Is this in fact a better current balun than voltage balun under some conditions? What?

Is it a good voltage balun?

The correct answers highlight that characterising a balun based on the way it is implemented is potentially flawed.

More at A balun puzzle – discussion .

]]>The articles showed some techniques for measuring common mode impedance of a current balun.

The following examples are of a test choke wound on a BN43-202 binocular core, and the results are quite similar to what might be expected of a broadband HF current balun. The measurements were made with a Rigexpert AA-600.

Above, the measurement result using RigExpert’s newest software Antscope2.

Yes, the graph is pretty much useless, it is grossly off scale and there appears no means of adjusting the vertical scale.

Above is a measurement of the same choke again using the AA-600 but displayed on a back level version (v4.2.57) of the old Antscope software. A much more useful graph, but the old Antscope software may not work with the newer models of analyser.

Above is an Excel graph created from a CSV export of the scan done with Antscope2, a much more useful graph but hardly a productivity tool when after every scan you need to export the data and paste it into the prepared spreadsheet, adjusting cell ranges, axis limits etc to suit the new scan.

Another option is to find a third party tool to plot the data.

A popular tool is ZPLOTS, but as I write, it does not support the csv export from Antscope2.

I do not have an AA-230Zoom, it does initially look an interesting instrument with wide ADC and N connector, but it seems it may not work with the older (retired?) Antscope PC client software, and Antscope2 is severely crippled.

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