Comparing toroidal inductors of different core dimensions

I often see comparisons of toroidal inductors of different core dimensions with all other characteristics (eg turns, core type, frequency) held the same.

There seems an implicit assumption by many that the bigger the core, the larger the inductance. There are several failure in that thinking.

The ‘inductance’ of a toroidal inductor is µ*n^2*a/l where:

• µ is complex permeability, µ0+µr;
• n is the number of turns;
• a is the cross section area; and
• l is the effective magnetic path length.

Note that at RF, permeability may be a complex frequency dependent value, and therefore ‘inductance’ will be a complex value.

Many online calculators incorrectly calculate l from core dimensions using a simplistic formula.

Many online calculators treat permeability as a real number that is not frequency dependent, they use initial permeability (µi). Continue reading Comparing toroidal inductors of different core dimensions

End fed half wave matching – voltage rating of compensation capacitors

The so-called End Fed Half Wave antenna system has become more popular, particularly in the form of a broadband matching transformer in combination with a wire operated harmonically over perhaps three octaves (eg 7, 14, 21, 28MHz).

The broadband transformer commonly uses a medium µ ferrite toroid core, and a turns ratio of around 8:1. Flux leakage results in less than the ideal n^2 impedance transformation, and a capacitor is often connected in parallel with the 50Ω winding to compensate the transformer response on the higher bands.

David, VK3IL posted EFHW matching unit in which he describes a ferrite cored transformer matching unit that is of a common / popular style.

Above is David’s pic of his implementation. It is a FT140-43 toroid with 3 and 24t windings and note the 150pF capacitor in shunt with the coax connector.

The article End fed matching – analysis of VK3IL’s measurements gives the following graph showing the effects of compensation for various resistive loads. Continue reading End fed half wave matching – voltage rating of compensation capacitors

Molasses derusting of steel

This article describes a setup for derusting small steel components, mainly machine tool accessories, using a Molasses solution.

A 10% Molasses solution can be an effective way to derust steel. Feed grade Molasses costs about \$2/kg at the local rural store.

The process is bacterial and activity depends on temperature. Experimentation suggests that optimal temperature is 30-35°, and derusting can be achieved in a few days at that temperature (subject to the degree of rust). At lower temperatures, the process may take many weeks. The nice thing compared to electrolytic derusting is that work is unlikely to be damaged by the process.

Above, the rust treatment system comprises:

• Bain marie stainless 1/3 module 200mm deep with lid;
• 1000W electric cooker;
• 230VAC thermostat with thermistor probe immersed in the process liquid;
• 230VAC dimmer to reduce the power of the cooker element.

Above is an internal view of the thermostat made from a Chinese 230VAC thermostat, a 3m extension cord and ABS plastic box. Continue reading Molasses derusting of steel

Review of inexpensive Chinese thermostat – MH1230A

This is a review of an inexpensive MH1230A Chinese bang-bang  thermostat that was purchased on eBay for around A\$15 complete with thermistor sensor and postage.

Above is the front view of the thermostat. There are many thermostats on the market with similar front panels, but they differ in internals and most importantly, performance and quality.

Above, the rating label is clear and informational, and it does give the sensor parameters.  Continue reading Review of inexpensive Chinese thermostat – MH1230A

On winding configuration of EFHW matching transformers

The net abounds with articles on broadband transformers (ie untuned) for matching End Fed Half Wave (EFHW) antennas to 50Ω. One of the aspects that is common to most designs is that the turns of the primary winding are wound ‘bifilar’ with the start of the secondary winding, indeed the twist pitch is often very short and articles often go into detail on how to make this magic thing.

The magic is that it is supposed to give closer to ideal behaviour of the transformers by way of minimising flux leakage.

The transformer above is styled on the common design, and it consists of a 2t primary and 16t secondary where the primary is wound bifilar, and a third 2t winding wound over the primary end of the transformer between the other turns. Continue reading On winding configuration of EFHW matching transformers

The sign of reactance – challenge reality check

The sign of reactance – a challenge posed a problem, a set of R,|X| data taken with an analyser of a quite simple network and asked readers to solve the sign of X over the range, ie to transform R,|X| to  R,X.

The sign of reactance – challenge solution gave a solution to the challenge, and The sign of reactance – challenge discussion provided some discussion about the problem and solution.

Some correspondents have asserted that the challenge (see above Smith chart) contains a response that is contrived for the purpose and not representative of real world antenna systems. Continue reading The sign of reactance – challenge reality check