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Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Seventh part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

This article describes a measurment of common mode impedance Zcm of the packaged balun.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a PTFE membrane vent was added later).

Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Last update: 9th September, 2023, 2:23 PM

MismatchLoss of severely mismatched EFHW transformer – system response

It is easy to become focused on the behavior of a component, but don’t lose sight of the fact that it is but a component of a system where components interact, and the system response is the bigger / more complete picture.

In the article MismatchLoss of severely mismatched EFHW transformer , a caveat stated and restated was:

Transmitters are not necessarily well represented as a Thevenin source, so measurements using such sources (VNA, SA with TG) and application of linear circuit theory are not necessarily applicable.

So, can we estimate a likely system response to a 30+j0Ω load, good 1:49 transformer and modern HF 100W (20dBW) SSB transceiver designed for a nominal 50Ω load?

The following analysis gives a likely solution and it deals with a common implementation where the source is anything but a Thevenin source.

PA VSWR protection

Most transceivers of this type incorporate several PA protection measures, and one of them is commonly to reduce IF gain so that reflected power measured in a directional coupler near the antenna jack is not more than say 4W. This accommodates VSWR up to 1.5 without power reduction due to VSWR protection.

So, with an extreme mismatch, Pref=4W due to the PA protection system.

30+j0Ω load via an ideal 1:49 transformer

The scenario of 30+j0Ω load if that of the quoted measurement in the previous article, used without comment on its merit.

Lets use the calculation from the previous article, the equivalent case of a 50Ω Thevenin source with load of (30+j0)/49=0.6122+j0Ω.

The quantity 1/(1-|s|^2) is the MismatchLoss, $MismatchLoss=\frac{P_{fwd}}{P_{fwd}-P_{ref}}=\frac1{1-\frac{P_{ref}}{P_{fwd}}}$ and it is 20.92, so we can calculate that if Pref=4w (by virtue of PA protection), that $P_{fwd}=\frac{P_{ref}}{0.952}=\frac4{0.952}=4.202 \text{ W}=6.235 \text{ dBW} $. Continue reading MismatchLoss of severely mismatched EFHW transformer – system response

Last update: 31st August, 2023, 8:00 AM

MismatchLoss of severely mismatched EFHW transformer

In an social media discussion about loss of EFHW transformers under mismatch conditions, one of the gathered experts said:

It doesn’t even have to be highly complex Z. Just presenting an impedance other than 2450 sends the loss through the roof. The back to back transformer test is misleading unless the antenna presents something very close to 2450 on each band for which it is used.

giving this graphic to quote someone else’s work in support.

Interpreting this graphic is fraught with risks, the author obviously does not understand and accept / follow the conventional meaning of term loss. Continue reading MismatchLoss of severely mismatched EFHW transformer

Last update: 30th August, 2023, 3:02 PM

Where is the best place to measure feed point VSWR – error in Z0

At Where is the best place to measure feed point VSWR I discussed location of the VSWR meter and projection of its reading to another point on a known transmission line.

One of the conclusions drawn in that article is:

Feed point VSWR can be estimated from measurements made at another place if the transmission line parameters are known. It, like all measurements, is subject to error but it may be a manageable error and indeed possibly better overall than direct measurement.

This article discusses some issues that may arise in referring measurements from one place to another (eg near transmitter to antenna feed point).

Characteristics of transmission line categories

Let’s consider two categories of transmission lines in terms of characteristic impedance Z0 and propagation constant γ:

Lossless line; and
practical line.

A lot of theoretical analysis uses lossless line for simple explanations, and whilst for a lot of purposes, approximation of practical line as lossless line serves well, at other times the error may be significant.

Lossless Line

A Lossless Line has imaginary part of Z0 equal to zero and the real part of γ equal to zero.

Practical line

A practical line has non-zero imaginary part of Z0 and non-zero real part of γ, and these are frequency dependent.

Under standing waves, attenuation along a practical line is not uniform, in most practical applications conductor loss/m is higher than dielectric loss so loss is higher near current maxima than near current minima.

For the purpose of this article, it is the frequency dependence of Z0, particularly the non-zero imaginary part that is significant.

A model

A model of a load similar to a 7MHz half wave dipole fed with 10m of RG58A/U was created in Simsmith to provide a basis for discussion. Whilst the model is subject to some errors computation, it is much less than comparing two field measurements at both ends of a transmission line.

VSWR at each end of the transmission line

Let’s look at the ACTUAL VSWR. Actual means that if you were to observe the standing waves on the line (eg with a voltage probe), this is the VSWR you would expect to observe.

Firstly, observe that the source end VSWR (orange) is a little lower than the load end VSWR. This is by virtue of the attenuation on the line. The difference between the two can be calculated, but it is moderately complicated. Continue reading Where is the best place to measure feed point VSWR – error in Z0

Last update: 13th August, 2023, 6:52 PM

Another small efficient matching transformer for an EFHW – 2643251002 – #5 – improved Simsmith model (v1.03)

This article applies the improved model: An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03) to the design at Another small efficient matching transformer for an EFHW – 2643251002 – #1 – design workup.

Above is the prototype transformer wound with 14t of 0.71mm ECW tapped at 2t. The mm rule gives some scale. The turns are close wound, touching on the inner diameter of the core. Continue reading Another small efficient matching transformer for an EFHW – 2643251002 – #5 – improved Simsmith model (v1.03)

Last update: 29th January, 2023, 7:38 PM

A Simsmith model of a Ruthroff 1:4 voltage balun – 2843009902 (BN43-7051)

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

Another small efficient matching transformer for an EFHW – 2 x 5943000601 (FT82-43) – VK4JJ build and measurement

The transformer is an autotransformer of 3+21 turns single layer close wound on a ‘stack’ of two Fair-rite 5943000601 cores (FT82-43?). Note that these were genuine Fair-rite stock, other #43 mix products in the market place may be significantly different (see Ferrite cored RF chokes in Class-E RF power amplifiers – core material issues for more discussion).

There are a plethora of designs using FT82-43 published on the ‘net, most of them have appalling loss.

Above is a Simsmith model and measurement of the transformer for reconciliation. The blue VSWR curve is the measurement and the magenta curve is the calibrated model, they agree well considering the tolerance of ferrite materials. Continue reading Another small efficient matching transformer for an EFHW – 2 x 5943000601 (FT82-43) – VK4JJ build and measurement

Last update: 17th November, 2022, 10:37 AM

Another small efficient matching transformer for an EFHW – LO1238 – VK3PY, VK3TU build and measurement

This article documents the process of design, prototyping, measurement and final build of a 1:49 impedance ratio (1:7 turns ratio) EFHW transformer, exploring some alternative designs along the way, a collaboration between VK3PY and VK3TU with a little guidance.

The transformer is wound on a Jaycar LO1238 35x21x13mm toroid of L15 material (L15 appears to be a NiZn ferrite based on its very high resistivity), they sell at $7 for a pack of two.

2:14 winding

The first test was of a 2:14 turn winding terminated in a 2450Ω load. The transformer is an autotransformer of 2+12t with 91pF compensation capacitor installed in shunt with the 2t winding.