## 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, 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 potentially and often a cause of: Continue reading Sontheimer coupler – transformer issues – an alternative design – FT23-43

## Measuring modulation index of an amplitude modulated wave with an oscilloscope

A oscilloscope can be used to measure the modulation index of an amplitude modulated wave. Modulation index is a value from 0 to `1, but is often expressed as a percentage.

The discussion here assumes symmetric modulation, it does not apply to super modulation schemes or any other schemes that are asymmetric.

## Envelope method

If an oscilloscope is used to display the modulation envelope (as it is known), modulation index can be calculated from measured values of the peak voltage at the crest and valley of the envelope waveform.

Modulation index can be calculated as $$m=\frac{b-a}{b+a}$$. Continue reading Measuring modulation index of an amplitude modulated wave with an oscilloscope

## 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

## Sontheimer coupler – transformer issues

It is not uncommon that ham designs for Sontheimer coupers (aka Tandem coupler, Grebenkemper coupler) fall short in the design of the magnetic components resulting in one or both of:

• high InsertionVSWR; and
• high core loss.

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 potentially and often a cause of: Continue reading Sontheimer coupler – transformer issues

## Ferrite cored RF chokes in Class-E RF power amplifiers – core material issues

At Ferrite cored RF chokes in Class-E RF power amplifiers a design was offered for a choke using a Fair-rite 2843000202 core (commonly sold as a BN43-202), and the point was made that some products sold as BN43-202 might be significantly different.

Let’s look at the calibrated model estimates of choke impedance and core loss, side by side. Continue reading Ferrite cored RF chokes in Class-E RF power amplifiers – core material issues

## Ferrite cored RF chokes in Class-E RF power amplifiers

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

This article discusses a common issue with the design of the RF choke providing DC to the Class-E stage.

Above is a circuit above is from (Sokal 2001) which explains the amplifier and gives guidance on selection of components. One key recommendation is that the usual choice of XL1 being 30 or more times the unadjusted value of XC1. This spells out that L1’s role is essentially an RF choke, it is intended to pass DC but to largely prevent RF current, it needs a high impedance at RF, and low DC resistance. Continue reading Ferrite cored RF chokes in Class-E RF power amplifiers

## A desk study of the Swan 500CX PA

I recall the arrival of the Swan 500CX in Australia, it was regarded highly and talked up quite forcibly on air by the local agent.

At the time, I was still acquiring the knowledge and skills to analyse the PA design in the 500CX, but I recall lots of on air discussions that were disparaging, but were not convincing.

More recently, I have had occasion to perform a desk study of the 500CX PA.

The Swan 500CX used a pair of 6LQ6, low cost TV sweep valves. From the GE datasheet, the valves are rated at 30 W plate (anode) dissipation. No safe grid 1 current or dissipation is given, so the safe approach is to regard that they must be operated with zero grid current, Class AB1 in this case. Continue reading A desk study of the Swan 500CX PA

## MFJ-261 – review of review

A recent review of the MFJ-261 (Bogard 2021) was interesting.

From MFJ’s web site listing:

Connects directly to the transmitter with PL-259 connector. No patch cable used, reduces SWR. Finned aluminum, air-cooled heatsink. Handles 100 Watts peak, 15 Watts average. 50 Ohms. Covers DC to 500 MHz with less than 1.15:1 SWR. 1 ⅝” round by 3″ long.

That is pretty stunning for a device with a UHF connector, more on that later. Continue reading MFJ-261 – review of review

## A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer?

A reader of A common scheme for narrow band match of an end fed high Z antenna commented:

…if the coil is tapped at 1/3, surely then the coil is a 1:3^2 or 1:9 transformer and the capacitor simply ‘tunes out’ the coil reactance, what is the input impedance when it has a 450+j0Ω load?

That is very easy to calculate in the existing Simsmith model.

Above, with load of 450+j0Ω, the input impedance at 50MHz is 8.78+j34.36Ω (VSWR(50)=8.4), nothing like 50+j0Ω. Continue reading A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer?