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.

]]>These are often sold without specifications, but where specifications are given, VSWR is given as 1.5, though not stated as maximum so should perhaps be read as typical.

This article looks at 2m performance alone.

Above is a VSWR sweep around the 2m band.

VSWR at 146MHz if more than specification, but less than 2.0 below 147MHz.

Above is a VSWR sweep around the 2m band.

VSWR at 146MHz if way more than specification, but less than 3.5 over the whole 2m band.

The purchase price was refunded due to failure to meet specification.

Two antennas were purchased for $24 incl post.

Above is a VSWR sweep around the 2m band. Two antenna were tested with almost identical results.

VSWR at 146MHz if way more than specification, but less than 3.5 over the whole 2m band.

These antennas did show VSWR minimum around 1.5 at 165+MHz.

The purchase price was refunded due to failure to meet specification.

Above is a section through the loading coil unit. The centre conductor of the coax connector is connected to a steel spring (weakly magnetic, so probably austenitic stainless steel), then to the antenna element (which is 8 weakly magnetic wires wound in a helix).

The three antenna vintages tested above had different shaped coil enclosures and different markings, all claimed to be Nagoya genuine product by Revex of Taiwan.

Purchasing the Nagoya NA-771 on eBay is high risk, experience is that recently listed products are unlikely to meed specification at 2m and that is reason enough to reject them.

The antennas can be purchased for as little at $3 each incl post, and there appears to be three kinds of packaging in item descriptions. They are probably all dysfunctional fake copies and worthless cheap Chines junk.

]]>A correspondent wrote of his project with a Guanella 4:1 balun where each pair was wound with a pair of insulated wires, and importantly the output terminals are free to float as the load demands. A Guanella 1:1 balun wound in the same way has the same characteristic.

To preserve balun choking impedance, it is best to preserve balun symmetry, and the use of a short open circuit coaxial stub across the output terminals for InsertionVSWR compensation introduces some asymmetry.

An alternative construction with coaxial cable that is more symmetric is shown above.

It is two pieces of RG58 with the shields fanned out and twisted into pigtails which will be the two terminals of the device. The inner conductors are connected across to the other cable’s shield. The cross connects are kept separated, as are the shield ends, and the space if filled with hot melt adhesive (which is electrically similar to polyethylene). Likewise, I have made sure the cut ends are clean of whiskers and used hot melt adhesive to insulate the parts from each other.

The prototype withstood 2.2kVp and the weak point was the open ends of the RG58. RG58 is strong internally, flashover occurs across the ends… so whilst the neatest smallest pigtails are good for RF, they are not conducive to high voltage withstand. By stripping the braid back 5mm and separating the braid ends similarly, it withstands 4.5kVp. Most ham ATUs cannot withstand even 2kVp, even though the capacitors may be rated for higher voltage, they tend flash over at coax connectors.

There are three open circuit transmission line stubs in parallel.

The first is formed by the outer surfaces of the two pieces of parallel RG58 with PVC jacket insulation. This turns out to have Zo close to 50Ω and VF around 0.6-0.7.

The second and third stubs are formed by the inner of each of the two pieces of RG58.

Because the three TL sections are close to Z0-6=50 and VF=0.66, we can treat them as a single stub with Zo=16.7 and VF=0.66. It is an approximation, but a good one considering the relative magnitude of error due to the pigtails when used on very short stubs as in this case.

In my correspondent’s case, his 70mm single RG58 stub could be replaced with the triple stub of length 20-25mm, enhancing symmetry by both the symmetric structure and smaller size.

Above is a measurement of the prototype. The observed capacitive reactance reconciles with 55mm of Zo=16.7 and VF=0.66 TL.

When the stub is very short electrically (<6°), its impedance is well approximated as the line’s capacitance per unit length of 1/(Co*vf*Ro). In this case, C=1/(299792458*0.66*16.7)=302pF/m, and the 55mm length should be about 302*0.055=16.6pF which reconciles with the chart above.

]]>

Above is Fig 1, a diagram from the Rigexpert AA35Zoom manual showing at the left a link (to be connected the analyser) and the trap (here made with coaxial cable).

Above is the trap measured, the wires were connected as a bootstrap trap as in Fig 1. The coupling link is a 60mm diameter coil of 2mm copper directly mounted on the AA-600 connector, and it is located coaxially with the trap and about 10mm from the end of the trap.

Above is the ReturnLoss plot of the trap very loosely coupled to the AA-600.

Of course this technique will not work on a trap that is substantially enclosed in a shield that prevents magnetic coupling. Note also that many traps used in ham antennas are simply a coil wound on an insulating rod and each end connected to the adjacent tubing, possibly with an overall aluminium tube that may or may not be bonded to the element tube at one end. The latter really become part of the element and measurement separate to the element is not simply translated to in-situ.

The inductor has previously been carefully measured to be 3.4µH. We can calibrate a model of the coupled coils to the observed resonant frequency and ReturnLoss.

Above, the equivalent circuit. We can calculate the flux coupling factor k from the model, it is 2.3% so this is very loosely coupled to avoid pulling the resonant frequency high.

Above is the simulated ReturnLoss response over the same frequency range as measured.

It is practical to measure the resonant frequency of a trap by loosely inductively coupling an antenna analyser, depending on the structure of the trap and the capability of the analyser.

Practical measurements can be explained with a theoretical model of the measurement setup.

]]>A modern solution is an antenna analyser or one port VNA, it provides both the source and the response measurement from one coax connector.

Above is a diagram from the Rigexpert AA35Zoom manual showing at the left a link (to be connected the analyser) and the trap (here made with coaxial cable.

The advantage of this method is that no wire attachments are needed on the device under test, and that coupling of the test instrument is usually easily optimised.

So, what is happening here? Lets create an equivalent circuit of a similar 1t coil and a solenoid with resonating capacitor.

The two coupled coils can be represented by an equivalent circuit that is derived from the two inductances and their mutual inductance. The circuit above represents a 1µH coil and a 10µH coil that are coupled such that 3% of the flux of 5% of the flux of one coil cuts the other (they are quite loosely coupled, as in the pic above.

The resonant frequency of the 10µH coil and 100pF capacitor can be calculated to be 5.033MHz… and this is the value we want to find from our measurement.

Above is a plot of the magnitude of S11. You can see that the cursor set to the theoretical (ie known) resonant frequency coincides almost exactly with the minimum |S11|, and therefore almost exactly with the theoretical (ie known) resonant frequency.

Lets increase the coupling.

Above, the equivalent circuit with the same coils but 9% flux coupling (the coils have been moved closer together).

Above, we have a deeper response, but note the minimum |S11| is now further away from the cursor which is at the theoretical (ie known) resonant frequency.

Too much coupling causes interaction with the test object.

One approach is to simply couple up tightly and find the response, and loosen the coupling until the frequency for minimum response stops moving.

Your instrument may display S11 labelled as the complex reflection coefficient, or it may display the magnitude of the complex reflection coefficient, or it may display Return Loss (which is -|S11|).

VSWR is related to |S11|, minimising VSWR is akin to minimising |S11| (or maximising Return Loss).

Use whatever feature your analyser offers.

Some analysers will not show a useful response for very loose coupling, eg they may not indicate VSWR greater than say 10. You really need to explore the instrument and manual to find if there is a way to display extreme VSWR, even if only at one frequency.

There is good reason why some analysers might not show extreme VSWR. If the inherent resolution of the instrument is poor (eg analysers with 8 bit ADCs), then it may not have sufficient accuracy to usefully display extreme VSWR.

Sometimes it is just that the designer didn’t really understand the instrument applications in the real world.

Of course this technique will not work on a trap that is substantially enclosed in a shield that prevents magnetic coupling.

Here is a measurement made of a parallel resonant circuit at 1.8MHz using a 60mm diameter 1t coil of 2mm copper wire connected directly to an AA-600.

Above is a ReturnLoss scan. It is not possible to expand the scale any more… did I mention that designers often do not understand real world applications. Nevertheless we can see that the middle of the peak in the response is at 1.813MHz where ReturnLoss is 0.34dB (equates to 51.6).

]]>As pointed out in the articles, the solutions cannot be simply extended to real antenna scenarios. Nevertheless, it might provoke thinking about the performance of some types of so-called balanced ATUs, indeed the naive nonsense of an “inherently balanced ATU”.

(Witt 2003) goes to some length to calculate his IMB figure of merit based on a similar load of two not necessarily equal series resistors with the mid point grounded to the ATU chassis. Witt’s IMB is equivalent to the factor |2Ic/Id|that was calculated in earlier articles in this series, and equally useless in inferring behavior in a real antenna system.

(Duffy 2010) gives an explanation of the behavior of baluns in an antenna system, and it becomes apparent that simple linear circuit solutions of a couple of resistors does not give insight into the behavior in real antenna systems.

The bottom line though is that while NEC models might be informing, there is no substitute for direct measurement of common mode current (Duffy 2011)… and it is so easy.

- Duffy, O. Dec 2010. Baluns in antenna systems. https://owenduffy.net/balun/concept/cm/index.htm (accessed 21/02/12).
- ———. May 2011. Measuring common mode current. https://owenduffy.net/measurement/icm/index.htm (accessed 21/02/12).
- ———. Feb 2012. Balanced ATUs and common mode current. https://owenduffy.net/balun/concept/BalancedAtu.htm (accessed 18/03/2019).
- Witt, Frank. Apr 1995. How to evaluate your antenna tuner In QST May 1995. Newington: ARRL.
- ———. May 1995b. How to evaluate your antenna tuner In QST May 1995. Newington: ARRL.
- ———. Sep 2003. Evaluation of Antenna Tuners and Baluns–An Update In QEX Sep 2003. Newington: ARRL.

This article reports the same asymmetric load using the MFJ-949E internal voltage balun.

The test circuit is an MFJ-949E T match ATU jumpered to use the internal balun and resistors of 50Ω and 100Ω connected from those terminals to provide a slightly asymmetric load.

The voltage between ground and each of the output terminals was measured with a scope, and currents calculated.

Above are the measured output voltage waveforms at 14MHz.

Lets work out the current amplitudes. Above, V1 (yellow) is 5.9divpp, V2 (cyan) is 7.2divpp. I1=V1/50=5.9*0.2/50=23.6mApp. I2=V2/100=7.2*0.2/100=14.4mApp.

Expanding the timebase allows better measurement of the phase difference.

V2 lags by a half cycle less 8.25µs, so V2 phase is -180+8.25e-9*14e6*360=-180+42=-138°.

Lets calculate the common mode and differential component of current in each load resistor. We will use Python as it handles complex numbers.

>>> i1=0.0236

>>> i2=0.0144*(math.cos(-138/180*math.pi)+1j*math.sin(-138/180*math.pi))

>>> ic=(i1+i2)/2

>>> abs(2*ic)

0.016100289594275147

>>> id=(i1-i2)/2

>>> abs(id)

0.01781446515461856

>>> abs(2*ic)/abs(id)

0.903776198417105

>>> 20*math.log(abs(2*ic)/abs(id))/math.log(10)

-0.8787820061070818

So, the differential component of current is 17.8mApp, and the total common mode current is 16.1mApp, the total common mode current is 90% of the differential current or 0.9dB less than differential current.

By any standard, this is appalling balance, and demonstrates why voltage baluns are unsuited to the application.

The fact that the “inherently balanced” topology is only 1.8dB better that this voltage balun experiment speaks volumes for the failure of the “inherently balanced” topology.

The measurements reported here are for a specific scenario (components, frequency and load), and should not be simply extrapolated to other scenarios.

The calculated imbalance if you like applies to the specific test circuit, and cannot really be extended to use of this balun in an antenna system scenario.

]]>This article reports the same equipment reversed so that the common mode choke is connected to the output of the MFJ-949E.

The test circuit is an MFJ-949E T match ATU followed by A low Insertion VSWR high Zcm Guanella 1:1 balun for HF. A banana jack adapter is connected to the balun output jack, and resistors of 50Ω and 100Ω connected from those terminals to provide a slightly asymmetric load.

The voltage between ground and each of the output terminals was measured with a scope, and currents calculated.

Above are the measured output voltage waveforms at 14MHz.

Lets work out the current amplitudes. Above, V1 (yellow) is 4.0divpp, V2 (cyan) is 8.0divpp. I1=V1/50=4.0*0.2/50=16.0mApp. I2=V2/100=8.0*0.2/100=16.0mApp.

Expanding the timebase allows better measurement of the phase difference.

V2 lags by a half cycle and 1.0µs, so V2 phase is -180-1.0e-9*14e6*360=-180-5=-185°.

Lets calculate the common mode and differential component of current in each load resistor. We will use Python as it handles complex numbers.

>>> i1=0.016

>>> i2=0.016*(math.cos(-185/180*math.pi)+1j*math.sin(-185/180*math.pi))

>>> ic=(i1+i2)/2

>>> abs(2*ic)

0.0013958203956907485

>>> id=(i1-i2)/2

>>> abs(id)

0.015984771545309726

>>> abs(2*ic)/abs(id)

0.0873218858170239

>>> 20*math.log(abs(2*ic)/abs(id))/math.log(10)

-21.177537875409207

So, the differential component of current is 16.0mApp, and the total common mode current is 1.40mApp, the total common mode current is 9% of the differential current or 21.2dB less than differential current.

Calculation of the common mode component of current involves the addition of two almost equal and almost opposite phase currents and is very sensitive to uncertainty in each of the measurements using this measurement method. This balun should achieve |2Ic/Id|>35dB in this scenario, but it would take a higher accuracy measurement system to measure it.

The fact that the “inherently balanced” topology measures 18dB worse that this experiment speaks volumes for the failure of the “inherently balanced” topology.

The measurements reported here are for the specific scenario (components, frequency and load), and should not be simply extrapolated to other scenarios.

The calculated imbalance if you like applies to the specific test circuit, and cannot really be extended to use of this balun in an antenna system scenario.

Continued at Inherently balanced ATUs – part 3 .

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