owenduffy.net

An online expert discussing broadband RF transformers recently opined “… if you measure k, the correlation of k and performance is excellent” whatever “performance” means.

Presumably he means k as in the flux coupling coefficient of two flux coupled inductors, ie inductors with mutual inductance (meaning changing current in one inductor induces an EMF in the other inductor). k is the proportion of flux due to current in one inductor that cuts the turns of the other inductor, it is usually stated pu (per unit) but sometimes in % (per cent or per 100).

A common metric for the performance of a broadband transformer is its InsertionVSWR. Other factors might be considered, but InsertionVSWR is commonly most ranked. Note that to describe a transformer as 1:49 implicitly invokes InsertionVSWR as a measure of its performance.

One of the enemies of broadband performance is flux leakage, k less than unity. The equivalent leakage reactance is usually the main contirubutor to high frequency roll off (an increase in InsertionVSWR at high frequencies) in good designs.

Let’s explore the ‘magic’ using the calibrated model used at 1:49 EFHW transformer using a Jaycar LO1238 core – design workup.

Above is a chart from that model showing: Continue reading 1:49 EFHW transformer using a Jaycar LO1238 core – the magic k factor

This article presents measurements of an EFHW antenna system using the transformer design worked up at 1:49 EFHW transformer using a Jaycar LO1238 core – design workup and bench measurements at 1:49 EFHW transformer using a Jaycar LO1238 core – measurement of losses.

The antenna system

Let’s take a system view, component views including bench measurements as reference above are important in qualifying components (eg acceptable Loss), but at the end of the day, the system view is very important. Whilst this section gives a VSWR perspective, it does that in the context of qualified system components.

In this article the antenna system comprises 11m of RG58A/U cable, the transformer described above and 20m of ‘radiator’ wire. This configuration should have a fundamental resonance around 7MHz and support harmonic operation at around 14, 21, and 28MHz.

Note that these type of antenna systems exhibit some amount of inharmonicity, ie the higher modes are not exact integer multiples of the fundamental resonance, there are contributions from both the ‘radiator’ wire, ‘counterpoise’ system and transformer.

Above is the VSWR plot looking into 11m of RG58A/U cable. The VSWR at the transformer jack point will be marginally higher, but this plot is typical of what might be presented to a transceiver. Continue reading 1:49 EFHW transformer using a Jaycar LO1238 core – measurement with antenna

Introduction

This article presents a review of a EFHW transformer using a Jaycar LO1238 core, a pack of 2 for $8 at Jaycar stores (Australia). The LO1238 is a 35x21x13mm Toroid of L15 material (µi=1500). Boxed up, it is probably safely capable of about 5W continuous dissipation.

The design is described at 1:49 EFHW transformer using a Jaycar LO1238 core – design workup.

Implementation

Above is the internals of VK4MQ’s balun. I would not use the pink PTFE tape, the balun core is extremely low conductivity and it is doubtful the tape helps. Continue reading 1:49 EFHW transformer using a Jaycar LO1238 core – measurement of losses

Introduction

There are several articles on this site describing EFHW transformers using the Jaycar LO1238 toroid, two particularly relevant ones are:

• End Fed Half Wave matching transformer – 80-20m – LO1238 variant; and
• Another small efficient matching transformer for an EFHW – LO1238 – VK3PY, VK3TU build and measurement.

This article presents a design workup of a EFHW transformer using a Jaycar LO1238 core, a pack of 2 for $8 at Jaycar stores (Australia). The LO1238 is a 35x21x13mm Toroid of L15 material (µi=1500). Boxed up, it is probably safely capable of about 5W continuous dissipation.

I will use the meanings explained at On insertion loss.

The design was developed in a SimNEC model which models a EFHW transformer, and can be calibrated against measurements of implementations. This helps evolve the model and develop some experience for likely values for leakage inductance etc. Continue reading 1:49 EFHW transformer using a Jaycar LO1238 core – design workup