Measuring trap resonant frequency with an antenna analyser

Finding the resonant frequency of a resonant circuit such as an antenna trap is usually done by coupling a source and power sensor very loosely to the circuit.

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.

Why / how does it work?

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. Continue reading Measuring trap resonant frequency with an antenna analyser

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

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #2

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

Insertion VSWR is the VSWR looking into the balun with a matched load (termination) on its output, it is a measure of imperfection of the balun. It ought to be a specification item for low Insertion VSWR baluns, but it rarely given.

What is not mentioned in the above definition is the symmetry or balance of the load.

Above is a Smith chart plot of input Z of the balun with an isolated load of 50+j0Ω. Isolated to mean that there is no direct path from either load terminal to ground, it could be seen as a symmetric load with extremely high common mode impedance. All of the external connections use N type connectors with Zo=50Ω.
Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #2

De-embedding transmission line to allow remote measurement

Recent articles An interesting case study of measurement of a balun’s Insertion VSWR and Another measurement of a balun’s Insertion VSWR made measurements through a transmission line of the load on a balun.

In the first article, the measurements at the input of around 7m of 50Ω line were adjusted to move the reference plane to the load end of the coax using the add/subtract cable feature of Antscope to de-embed the transmission line.

The second article used a FA-VA5 analyser and VNWA software to make the measurements and to some extent, de-embed the transmission line. In this case the transmission line was quite short at 370mm, and whilst the facility adjusted for propagation time, it did not adjust for attenuation though that was very small in this case and of little consequence. The FA-VA5 analyser and VNWA software combination would not suit the scenario in the first article as will be demonstrated.

This article examines the response to a 6m length of RG58 with O/C load at 30MHz.

We can see that although the phase of Gamma (phase of 0.85+j0.01) is close to zero the magnitude is 0.85 when the magnitude should be 1.00 for an O/C load. Continue reading De-embedding transmission line to allow remote measurement

Another measurement of a balun’s Insertion VSWR

Further to An interesting case study of measurement of a balun’s Insertion VSWR, this article presents similar measurements of a small DIY balun.

Balun404

Above is the top view of the balun, and the test termination comprised two 100Ω 1% resistors clamped between the screw terminals, so pigtails were just 3mm in length.

Balun402

Above is a view of the interior.The coax pigtails are quite short, they exist at the input and output. Continue reading Another measurement of a balun’s Insertion VSWR