FT37-43 for a 49:1 EFHW transformer enquiry

A correspondent asked whether Sontheimer coupler – transformer issues – an alternative design – FT37-43 could be used to inform design of a 49:1 EFHW transformer based on the same core, but with a 2 or 3t primary.

In the case of the Sontheimer coupler the winding with the higher number of turns appears in shunt with the nominal 50Ω load, and its effect on InsertionVSWR and the core loss can be predicted reasonably well and confirmed by measurements as in the referenced article.

In that instance, a 7t winding in shunt with the nominal 50Ω load causes excessive core heating, a 3t winding will be worse, and 2t worse again.

The case of an EFHW transformer is somewhat similar, the difference is now that the winding with less turns in approximately in shunt with the nominal 50Ω primary referred load. The same Simsmith model can be used to predict likely InsertionVSWR due to primary magnetising admittance, and the core loss.

Let’s try the 3t case first, with the experience of the referenced article we can expect it will have insufficient turns for good performance.

Above is the Simsmith model of a Fair-rite 5943000201 core (equivalent dimensions to FT37-43) with a 3t winding. Note this does not apply to Amidon #43 as their material is significantly different in characteristic. Continue reading FT37-43 for a 49:1 EFHW transformer enquiry

Digital display for DIY 25W dummy load – part 2

Digital display for DIY 25W dummy load – part 1 described VK4MQ’s build of a DIY 25W dummy load / digital wattmeter with very good performance. As part of the project, Bruce made an exhaustive set of measurements of Prf vs Vdc from 0.001W to 25W. A second order curve fit was calculated and is used in the instrument to transform measured Vdc to Prf for display.

That project was an elaboration of a design worked up at Digital display for QRP labs 20W dummy load – part 1 and following articles. That workup included an LTSPICE model of the half wave detector with BAT46 diode, 0.1µF capacitor and 56k+1k voltage divider. A second order curve fit was calculated and is used to transform measured Vdc to Prf for display.

This article compares the LTSPICE model data set, its curve fit, the measurements of Bruce’s implementation, and its curve fit. Continue reading Digital display for DIY 25W dummy load – part 2

Digital display for DIY 25W dummy load – part 1

Digital display for QRP labs 20W dummy load – part 1 and following articles laid out a initial study into the feasibility of an approach of a similar project. This project uses the same display solution for a DIY 25W dummy load / digital wattmeter with very good performance.

This article describes Bruce, VK4MQ’s, build.

Implementation

Bruce built the dummy load wattmeter into a small die cast box.

Above, the front panel view, the OLED display shows power in watts and dBm, and a bar chart display. The unit is battery powered, and has a on/off switch on the front panel. Continue reading Digital display for DIY 25W dummy load – part 1

NanoVNA source mismatch error

One of the popular ideas online is that the correction process in the NanoVNA does not correct errors in mismatch at Port 1 and Port 2. This article deals with the first case ONLY, Port 1 mismatch.

An experiment with source VSWR nominally 2:1

A NanoVNA was configured with a SMA tee connected to Port 1 and a good 50Ω termination connected to the branch port, see the pic below. The left hand side of the tee becomes the new Port 1 interface, and by virtue of the additional 50Ω shunt termination, if the native Port 1 was indeed well represented by a Thevenin equivalent circuit with Zs close to 50+j0, the Thevenin source impedance is now closer to half that, Zs close to 25+j0.

Some would calculate this mismatch as causing a mismatch loss of 0.512dB that is additional loss in the s21 path.

Above is the test setup. The NanoVNA was SOLT calibrated with cal parts attached to the left hand side of the tee and the 200mm coax jumper from that point to Port 2. Continue reading NanoVNA source mismatch error

Digital display for half wave detector with cubic spline interpolation – part 4

Digital display for half wave detector with cubic spline interpolation – part 1 and following articles laid out the design concepts of a power meter display.

Digital display for half wave detector with cubic spline interpolation – part 2 described a simple cubic spline interpolation model.

This article examines the problem a little deeper to arrive at an improved solution.

The chart above compares the response of an ideal peak detector (cyan) with an LTSPICE simulation of a BAT46 with 57kΩ load (orange x). Whilst the simulation approaches the ideal at peak RF voltage Vp greater than 3V, it departs greatly at very low Vp. Continue reading Digital display for half wave detector with cubic spline interpolation – part 4

Becen 25W type N dummy load

 

I purchased an inexpensive 50Ω dummy load rated at 25W from Becen on Aliexpress (~$20 inc shipping), the intended application is HF.

Above, the dummy load.

Above, the centre pin passes a gauge test, doesn’t satisfy the Precision N criteria, but well within tolerance of non-precision N connectors. Continue reading Becen 25W type N dummy load

Digital display for half wave detector with cubic spline interpolation – part 3

Digital display for half wave detector with cubic spline interpolation – part 1  and following articles laid out the design concepts of a power meter display.

Whilst the preferred target was an Arduino Zero (SAMD21G) for its 32bit architecture, speed, and 12bit ADC, the code was developed to run on a Zero or a Arduino Nano (ATmega328P).

This article explores a trial of an external ADC module using an ADS1115 chip, 4 channels 16bit +/- ADC.

Above is the prototype, Arduino Nano on the left,  the ADS1115 module is to the right of the display. Continue reading Digital display for half wave detector with cubic spline interpolation – part 3

Digital display for half wave detector with cubic spline interpolation – part 2

Digital display for half wave detector with cubic spline interpolation – part 1 laid out the design concepts of a power meter display.

Whilst the preferred target was an Arduino Zero (SAMD21G) for its 32bit architecture, speed, and 12bit ADC, the code was developed to run on a Zero or a Arduino Nano (ATmega328P). Initially, my preferred approach of storing device calibration parameters in EEPROM was shelved because the SAMD21G does not have EEPROM, and it’s NVM alternative is not nearly as convenient.

Notwithstanding that, EEPROM support has been plumbed in and tested. For devices supporting Arduino EEPROM library, calibration coefficients can be supplied in EEPROM, or inline in source code.

The interpolation table is calculated separately in Excel (using a custom VBA function library), but could be done in any suitable tool.

Above is a screen shot  extract of the spreadsheet, the column on the right is C array initialisation code for pasting into the project source code. The same values are loaded into the EEPROM data structure if used. Continue reading Digital display for half wave detector with cubic spline interpolation – part 2

Digital display for half wave detector with cubic spline interpolation – part 1

Digital display for QRP labs 20W dummy load – part 1 and the following articles discussed an approach to compensating the non-linear response of the half wave detector by finding a polynomial curve fit over a desired range. Unfortunately, the range for a good fit can be smaller than one desires.

This article discussed an alternative using cubic spline interpolation and might be applicable to extend the range or for responses that aren’t well approximated by a simple curve fit.

Introduction

Essentially, this technique applies a piecewise polynomial to fit the data points, and a relatively small number of data points may provide a very good approximation.

The graph above shows: Continue reading Digital display for half wave detector with cubic spline interpolation – part 1

Digital display for QRP labs 20W dummy load – part 2

Digital display for QRP labs 20W dummy load – part 1 laid out a initial study into the feasibility of an approach to the project.

A prototype has been built based on an Ardunio Nano (ATmega328P 5V 16MHz). The ‘328P is loaded with a custom build of Optiboot 8 supporting reading and writing EEPROM.

Above, another prototype using a 0.96″ 128×64 OLED display, an end to end test of a BAT46 prototype for function testing using an Arduino Nano and OLED display. This prototype is well within 5% accurate based solely on the LTSPICE model, assuming no error in the voltage divider, tracking well from 1W to 20W. When calibrated for the voltage divider and ADC Vref error, power displayed was within 2% of a proven power meter at several spots from 0.8W to 25W, on a spot check it is within 10% (0.4dB) at 1mW. Continue reading Digital display for QRP labs 20W dummy load – part 2