It seems that the Holy Grail of many ham HF enthusiasts is a “true balanced ATU.”
The word “true” in there bodes poorly!
It seems that while there are plenty of online experts who have very strong opinions on common mode current, baluns and ATUs, it is very rare that we see quantitative evidence of their assertions, measurements even.
Less commonly does a “true balanced ATU” description include valid measurement of common mode current as evidence of operation.
A “true balanced ATU” project by LY1O in unusual in that it contains a probe purported to measure and display current balance.
Above is a schematic of LY1O’s measurement system, it has a pair of current transformers each with half wave diode detectors in each leg of ATU output. It is important to note that the detectors convert the RF AC wave into a DC value close to the peak value of the AC wave… so they respond to the magnitude only of the current in each leg. Continue reading Balanced ATUs – the Holy Grail?
Last update: 21st January, 2023, 5:09 PM
A correspondent asked whether I had a tool similar to An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03) to assist in the design of a ferrite cored Ruthroff 1:4 balun for HF.
In fact, the problem is the same as the one discussed in the article above, and the model is suited to application to the ferrite cored HF Ruthroff 1:4 balun case.
This analysis applies to a Fair-rite 2843009902 but may not apply to other manufacturer’s BN43-7051.
Above is a screenshot of the model calibrated against measurement. The magenta curve is the prediction and the blue curve is the measurement. Note that very small differences in measured value result in apparently large changes in InsertionVSWR, these two curves reconcile very well, especially considering the tolerances of ferrite material. Continue reading A Simsmith model of a Ruthroff 1:4 voltage balun – 2843009902 (BN43-7051)
Last update: 24th November, 2022, 3:32 PM
The article A simple Simsmith model for exploration of a common EFHW transformer design – 2t:14t proposed a simple model.
The previous proposal
Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side.
The model works quite well for low leakage inductance / low ratio transformers but falls down for the higher leakage inductance / higher ratio transformers.
An improved model
The improved model is similar, but Cse in the model above is distributed to the outer sides of the lumped constant model.
Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side. Continue reading An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03)
Last update: 1st June, 2023, 3:46 AM
in the article A simple transformer model of the Guanella 1:4 balun – some further observations I stated:
Note that a two terminal impedance is a naive representation of many if not most antennas, popular, but a naive over simplification that does not facilitate evaluation of current balance.
An example was a recent posting above that used the model to make assertions about the behaviour of a Guanella 1:4 balun.
This article reports results of two experiments with NEC to model an ‘imperfect’ half wave dipole. It is not exactly resonant, but the main issue is that it is tilted from one end to the other, it is not parallel to the ground surface. Continue reading Modelling an antenna as a simple two terminal resistance is often naive
Last update: 12th November, 2022, 7:52 AM
A simple transformer model of the Guanella 1:4 balun discussed a simple model for the operation of the device, but a model that is too simple for most RF baluns. Notwithstanding that, it does expose some interesting issues that are not only valid at lower frequencies, but will also be manifest in an RF balun.
Consider the effect of breaking the connection at the red X, so that we now have what is often referred to as an “isolated load”. Continue reading A simple transformer model of the Guanella 1:4 balun – some further observations
Last update: 8th November, 2022, 4:50 AM
(Guanella 1944) described a 1:4 balun, of a type often known as a current balun.
From Definition: Current Balun, Voltage Balun:
An ideal current balun delivers currents that are equal in magnitude and opposite in phase.
A good current balun will approach the ideal condition. It will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance (including symmetry).
Common mode current will be small.
If the load impedance is not symmetric, then the voltages at each output terminal will not be equal in magnitude and opposite in phase. (Note that for a truly ‘isolated’ load, one well represented as a two terminal load, the currents MUST be equal in magnitude and opposite in phase, but the voltages may not be equal in magnitude and opposite in phase.)
A simplified model
Above is a schematic of the Guanella 1:4 balun as often presented, this is an edited graphic from the ARRL manual, so may be familiar to readers. Continue reading A simple transformer model of the Guanella 1:4 balun
Last update: 7th November, 2022, 1:31 PM
The assembled experts on QRZ are commenting / analysing a balun shown on Wikipedia as I write this.
The red strike through is mine, for this to work as a balun, the right hand section of coax MUST be an odd number of half waves electrically (yes, Wikipedia got it wrong). Continue reading What kind of balun is this? Is it any good?
Last update: 13th September, 2022, 6:06 AM
A reader of my recent article Sontheimer coupler – transformer issues and the follow up articles giving better transformer designs asked whether the coupler use in the popular AT-100 ATU wouldn’t be a good solution.
A quick Google search did not turn up any published design rationale or measurement data for the AT-100 coupler design.
The above circuit is from (Grebenkemper 1987) and is an embodiment of (Sontheimer 1966). In their various forms, this family of couplers have one or sometimes two transformers with their primary in shunt with the through line, and another which is in series with the through line to sense current. To achieve good Directivity, these transformers must be symmetric, nearly ideal, and they must be independent, ie no significant coupling between the transformers by magnetic or electric fields.
The AT-100 uses a Sontheimer coupler, they are very popular with ham users for perceived better performance, notably better Directivity over a wide frequency range. Continue reading AT-100 ATU Sontheimer coupler using a single binocular ferrite core
Last update: 6th June, 2022, 12:36 PM
Reviewing consistency of measured and model data, the first posting was based on an incorrect model parameter (aol), the article is now revised for the correct value, apologies.
End Fed Half Wave matching transformer – 80-20m described a EFHW transformer design with taps for nominal 1:36, 49, and 64 impedance ratios.
Keep in mind that this is a desk design of a transformer to come close to ideal broadband performance on a nominal 2400Ω load with low loss. Real antennas don’t offer an idealised load, but this is the first step in designing and applying a practical transformer.
The transformer comprises a 32t of 0.65mm enamelled copper winding on a Fair-rite 5943003801 core (FT240-43) ferrite core (the information is not applicable to an Amidon core), to be used as an autotransformer to step down a EFHW load impedance to around 50Ω. The winding layout is unconventional, most articles describing a similar transformer seem to have their root in a single flawed design, and they are usually published without meaningful credible measurement. Continue reading End Fed Half Wave matching transformer – 80-20m – model and measurement
Last update: 24th May, 2022, 9:43 AM
A recent discussion online on the use of this facility in some model or other NanoVNA/firmware combination quickly ran to over 100 posts, and you might think it is really difficult, or plain does not work.
Let me say I am leery of built in features that invite users to perform something they do not understand, and may misinterpret the outcome.
Lots of the discussion ran to explaining why measurement of a sample of coax would be out by 5% or more, lots of pseudo tech discussion about age related, contamination related, quality related explanations for the measurement, things which might cause the measurer to condemn the sample, to discard it.
Well, you would want to be pretty confident in yourself to make that call, given that the explanation might well be measurement error.
I don’t use this facility, so I am quite unfamiliar with it, and there is no documentation, so one make make an informed guess as to how to use it.
Let’s measure… Continue reading NanoVNA – trying the DiSlord built in cable length measurement feature
Last update: 26th April, 2022, 10:13 AM