Measuring balun common mode impedance – #3

A correspondent having read my series Measuring balun common mode impedance – #1 related difficulties with his Rigexpert AA-230Zoom.

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. Continue reading Measuring balun common mode impedance – #3

Small common mode choke for analyser antenna measurements using 2843000202 (BN43-202)

The project is design, implementation and test of a small common mode choke for use with an analyser for antenna measurements.

The choke must have medium to high Zcm from 1 to 30MHz. It is intended to be used with analysers supporting SOL calibration, so effectively any impedance transformation within the fixture is compensated and the reference plane is the load side terminals of the device.

The candidate core is a low cost #43 binocular ferrite core that is fairly easy to obtain.

Above is a first pass check of the likely Zcm at 1.8MHz using a Fair-rite 2843000202 (BN43-202) binocular core. These chokes have relatively low self resonance frequency so a value for Cs is supplied that delivers self resonance at around 5MHz. Zcm at 1.8MHz needs 8-9t, 8.5t will be used (ie the twisted pair enters one end of the binocular and leaves the other end for convenient layout). (8.5t is not strictly correct, but it is a close approximation in this case.)
Continue reading Small common mode choke for analyser antenna measurements using 2843000202 (BN43-202)

Geometry factors for some common Fair-rite binocular ferrite cores

Designing with some common Fair-rite binocular ferrite cores can be frustrating because different parameters are published for different material types, and some are controlled for different parameters.

An approach is to derive the key geometry parameter from the published impedance curves and published material complex permeability curves.

For example, the above curves for a 2843002402 (also common known as a BN43-2402) were digitised and iteratively Calculate ferrite cored inductor (from Al) used for find the value of Al that gives the observed value for Z at 10MHz on the chart above. Continue reading Geometry factors for some common Fair-rite binocular ferrite cores

A symmetric compensation stub using coax

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #3 discussed compensation of the Insertion VSWR response of a balun which in that case was wound with coax.

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. Continue reading A symmetric compensation stub using coax

Inherently balanced ATUs – part 4

Inherently balanced ATUs reported an experiment to measure the balance of a simulation of Cebik’s “inherently balanced ATU”, and following articles explored balance in some different scenarios, but none of them real antenna scenarios.

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”.
Continue reading Inherently balanced ATUs – part 4

Inherently balanced ATUs – part 3

Inherently balanced ATUs reported an experiment to measure the balance of a simulation of Cebik’s “inherently balanced ATU”.

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

The third experiment

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. Continue reading Inherently balanced ATUs – part 3

Inherently balanced ATUs – part 2

Inherently balanced ATUs reported an experiment to measure the balance of a simulation of Cebik’s “inherently balanced ATU”.

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

The second experiment

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. Continue reading Inherently balanced ATUs – part 2

Inherently balanced ATUs

Hams are taken by fashion and pseudo technical discussion more than objective circuit analysis, experiment, and measurement. Nowhere is this more evident that the current fashion for “True Balanced Tuners”.

LB Cebik in 2005 in his article “10 Frequency (sic) Asked Questions about the All-Band Doublet” wrote

In recent years, interest in antennas that require parallel transmission lines has surged, spurring the development of new inherently balanced tuners.

Open wire lines require current balance to minimise radiation and pick up, the balance objective is current balance at all points on the line.

Cebik goes on to give examples of his “inherently balanced tuners”.

Above, Cebik’s “inherently balanced tuners” all have a common mode choke at the input, and some type of adjustable network to the output terminals. Continue reading Inherently balanced ATUs

Voltage symmetry of practical Ruthroff 4:1 baluns – finding TLT Vout/Vin

I have been asked to expand on the calculation of voltage magnitude and phase set out in Voltage symmetry of practical Ruthroff 4:1 baluns.

Above is Ruthroff’s equivalent circuit, Fig 3 from his paper (Ruthroff 1959). Focusing on the left hand circuit which explains the balun as a transmission line transformer (TLT), and taking the node 1 as the reference, the loaded source voltage appears at the bottom end of the combined 4R load, and transformed by the transmission line  formed by the two wires of the winding, and inverted, at the top end of the combined 4R load.

It is the transformation on this transmission line that gives rise to loss of symmetry.

The complex ratio Vout/Vin is dependent on the complex reflection coefficient Gamma at both ends of the line and the line propagation constant gamma, all of which are frequency dependent complex quantities. Continue reading Voltage symmetry of practical Ruthroff 4:1 baluns – finding TLT Vout/Vin

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #3

This article expands on the detail behind A low Insertion VSWR high Zcm Guanella 1:1 balun for HF with focus on InsertionVSWR and possible compensation schemes.

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #2 discussed the imperfection caused by the quite short pigtails, and although small, it is measurable.

Chris, NX0E, related experience with Dr E M T Jones at TCI where they made, among other things, TCI’s HF baluns. These baluns were compensated using capacitors, and we see that very occasionally in ham grade baluns.

The pigtails can be seen as a short transmission line of higher Zo, and although not uniform, it provides a model for understanding their effect.

Above is a Simsmith model that treats the pigtails as short sections of 300Ω line, the lengths adjusted to calibrate the model to the observed impedance at 30MHz.
Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #3