## Center-Fed Dipole : elements length for a Z=200 +/- 0j ohms

A chap asked online for dimensions of a 50MHz dipole with a feed point of 200+j0 to suit 50Ω feed line and a 1:4 coax half wave balun. The “+/- 0j” is hammy Sammy talk from an ‘Extra’.

This type of balun, properly implemented, is a good voltage balun, and it is quite suited to a highly symmetric antenna.

A good voltage balun will deliver approximately equal voltages (wrt the input ground) with approximately opposite phase, irrespective of the load impedance (including symmetry).

Where the load is symmetric, we can say a good voltage balun will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance.

It is an interesting application, and contrary to the initial responses on social media, there is a simple solution.

## One solution

Let’s take a half wave dipole and lengthen it a little so the feed point admittance becomes 1/200-jB (or 200 || jX). We will build an NEC model of the thing in free space.

Above is a sweep of the dipole which is 3.14m long (we will talk about how we came to that length later), and the Smith chart prime centre is 200+j0… the target impedance. Continue reading Center-Fed Dipole : elements length for a Z=200 +/- 0j ohms

## Sontheimer coupler – transformer issues – an alternative design – FT37-43

Sontheimer coupler – transformer issues discussed problems with the Sontheimer coupler in a recently published QRP transceiver ((tr)uSDX / trusdx), suggesting that the core loss in transformer T2 was excessive.

This article presents an alternative design for the transformer for a coupler for a 5W transmitter.

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. Let’s focus on transformer T2. It samples the though line RF voltage, and its magnetising impedance and transformed load appear in shunt with the through line. T2’s load is usually insignificant, but its magnetising impedance is significant and is often a cause of: Continue reading Sontheimer coupler – transformer issues – an alternative design – FT37-43

## End Fed Half Wave matching transformer – 80-20m – model and measurement

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

## Sontheimer coupler – transformer issues – an alternative design – FT23-43

Sontheimer coupler – transformer issues discussed problems with the Sontheimer coupler in a recently published QRP transceiver ((tr)uSDX / trusdx), suggesting that the core loss in transformer T2 was excessive.

This article presents an alternative design for the transformer for a coupler for a 5W transmitter.

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. Let’s focus on transformer T2. It samples the though line RF voltage, and its magnetising impedance and transformed load appear in shunt with the through line. T2’s load is usually insignificant, but its magnetising impedance is significant and is often a cause of: Continue reading Sontheimer coupler – transformer issues – an alternative design – FT23-43

## U.FL connectors – hints

This article expands on discussion at nanoVNA – that demo board and its U.FL connectors.

Before looking at the specifics of the Hirose U.FL connector, clean connectors work better and last longer. That should not be a revelation.

A can of IPA cleaner and a good air puffer are invaluable for cleaning connectors. The air puffer show  has a valve in the right hand end, it doesn’t suck the dirt and solvent out of the connector and blow it back like most cheap Chinese puffers, this one was harder to find and expensive (\$10!). Continue reading U.FL connectors – hints

## Switching times in Class-E RF power amplifiers

Class-E RF power amplifiers have become quite fashionable in ham radio in the last decade or two.

One of, if not the main contribution to efficiency in a Class-E RF amplifier is the operation of the active device in switching mode where it is either not conducting, or conducting hard (saturated, with very little voltage across it). Both of these are very low dissipation conditions, but in the transition between these states there is significant current and voltage present, the product of which gives significant instantaneous power… so the idea is to make this transition very fast so that the average power is low.

Above is a circuit above is from (Sokal 2001) which explains the amplifier and gives guidance on selection of components. Continue reading Switching times in Class-E RF power amplifiers

## NanoVNA-H4 – inductor challenge – part 7

One method described online on YouTube and in social media is the 90° method as I will call it.

The reason why people make measurements at +/- 90 degrees on the smith chart is because the measurement accuracy using the shunt configuration when trying to measure the nominal value of an inductor or capacitor is highest at 0+j50 ohms (or 0-j50 ohms… OD).

To be clear, this is the phase of s11 or Γ being + or – 90° as applicable.

Is there something optimal when phase of s11 is + or – 90°?

Does the software / firmware / hardware give significantly more accurate response under such a termination?

Above is a diagram from a HP publication, slightly altered to suit the discussion. Continue reading NanoVNA-H4 – inductor challenge – part 7

## FNIRSI FNB48 USB meter – cable resistance measurement

The FNIRSI FNB48 USB meter is a flexible USB power meter that incorporates a facility to measure the resistance of a USB cable. The tested unit uses firmware v2.50.

The operation also needs a constant current load of 0.5 – 1A (for most cables).

## NanoVNA – measuring Q of an inductor using s21 – fails?

There is a fashion of seeing s21 measurements as the answer to all things, and amongst the revelations is an explanation of measuring inductor Q using s21 shunt through configuration.

Let’s explore the use of s21 shunt through to directly find the half power bandwidth of a series tuned circuit and calculate the Q from that and the resonant frequency (as demonstrated by online posters).

To eliminate most of the uncertainties of measurement, let’s simulate it in Simsmith.

The simulation has a series tuned circuit resonated at 3400kHz, and the source and plot are set to calculate |s21| in dB. Though the model specifies Q independent of frequency, the D block adjusts Q for a constant equivalent series resistance (ESR) which simplifies discussion of resonance and Q. Continue reading NanoVNA – measuring Q of an inductor using s21 – fails?