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

NanoVNA-H – modification of v3.3 PCB to start the bootloader from the jog switch

Later NanoVNA-H* hardware allows the device to start in bootloader mode by holding the jog switch in whilst powering on. It is a very convenient facility for firmware update, much more convenient than taking the case apart to jumper BOOT0 to VDD. (Some later firmwares provide a menu option to start the bootloader… but of course that is only useful if the firmware is running properly and may not be useful in the event of a failed firmware update.)

This was a mod I devised prior to the v3.4 hardware change, it is not identical to that change as it preceded it, but it works fine on v3.3 hardware and may work on earlier versions.

Boot switch

The mod calls for replacing R5 with a 1k (1402) and running a short jumper from the T terminal of the jog switch to the un-grounded end of R6.

To use it, hold the jog switch in and turn the nanoVNA on.

Above a pic of the mod. It is a simple mod, but very fine soldering so it might not be within everyone’s capability.

NanoVNA-H* – a howto do firmware update using STM32CubeProgrammer

One of the many solutions for updating firmware on the NanoVNA-H* is using ST’s STM32CubeProgrammer.

It would seem that STM32CubeProgrammer deprecates the older DfuSe Demo utility… which remains available for download. Some online experts have inferred that the word Demo in the latter implies it is not the full quid… but they misunderstand the context.

Windows drivers

The two are kind of incompatible in that they use difference device drivers. If you set your machine up for one, it breaks the other until you switch the correct driver in.

STM32CubeProgrammer uses libusbk (or the like) whereas DfuSe Demo uses the STub30 driver.

Above is a dump of the driver properties in my working instance. Continue reading NanoVNA-H* – a howto do firmware update using STM32CubeProgrammer

Digital display for QRP labs 20W dummy load – part 1

The QRP labs 50-ohm 20W QRP HF Dummy Load is an inexpensive kit for a low power dummy load.

The load comprises 20 x 1W resistors, time will tell what its continuous power rating is actually.

This article explores a possible design for a digital display of power using the provided pads for a half wave detector.

Dimensional

Above, the supplied connector fails a gauge test (the female part sticks out 0.4mm+ (0.015+”) too much… I should have gauged it before assembling the thing.

RF performance

Above is a ReturnLoss plot from 1-100MHz, ReturnLoss is good below 60MHz, very good below 30MHz. Continue reading Digital display for QRP labs 20W dummy load – part 1

NanoVNA-H4 – battery charge from discharged

This article documents the charge cycle of a NanoVNA-H4 from fully discharged to charged.

The DUT is probably a ‘standard’ H4, but with Chinese sourced produce, you never, never know.

The original battery fitted to the NanoVNA-H v4.3 is a 804050 (8.0x40x50mm) 2000mAh LiPo pouch cell (1S) with protection board.

The charger chip is a TP4056, and it would appear to be limited by Rprog to about 0.75A (which includes the current drawn by the working NanoVNA-H4) (though the circuit employed would appear to tweak that limit between VNA on and off conditions with R44). The TP4056 is simply a charger chip, it will not prevent over-discharge of the cell so it is wise to use a cell with protection board (as originally supplied on the DUT).

Above is a plot of the calibrated battery voltage reported by the NanoVNA-H4. Continue reading NanoVNA-H4 – battery charge from discharged

RF Power Meter 2 (RFPM2) – 40dB external attenuator calibration and integration

RF Power Meter 2 is a logging RF power meter based on AD8307 and ESP8266.

This article describes its calibration and use with a 40dB 50Ω 20W attenuator to make a 20W or 43dBm RF power meter.

Above is a pic of the system under test on a nominal 5W transmitter, indicating 37.4dBm, equivalent to 5.5W. Continue reading RF Power Meter 2 (RFPM2) – 40dB external attenuator calibration and integration

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