RFPM2 – current probe – #3

RFPM2 – current probe described a current probe for use with a power meter calibrated in dBm (eg RFPM1 and RFPM2).

RFPM2 – current probe – #2 exposed some of the build details.

This article reports the completed article.

Above the current probe with RFPM2, the display does not show well at this camera angle… it is actually a lot clearer when viewed from a higher angle.

The instrument noise floor is around -76dBA or 0.16mA. When coupled to a conductor the background noise level will raise that by some site dependent amount, at my home coupled to an antenna feed line it bounces between -75 and -65dBA. Continue reading RFPM2 – current probe – #3

nanoVNA-H – v3.3 USB problems

At nanoVNA – measurement of two 920MHz LoRa antennas I mentioned my growing frustration with the USB interface on the nanovna, particularly the tendency to reset the nanoVNA with the slightest wiggle and the frustration in trying to use the resulting mess.

I have previously cleaned both plug and socket a couple of times, the last time was after some board modifications and flux residue was washed from the board keeping the USB socket dry, then the USB socket was flushed with clean solvent and blow dry.

The USB problems have become apparent only recently and rapidly got worse. Continue reading nanoVNA-H – v3.3 USB problems

A mount for the coax relay driver development prototype

I was recently revising the code for the Coax Relay Driver to use a PIC16F1827 chip, and thought a good improvement would be a board that held the prototype electronics and the pulse latching relay together as an assembly.

Above is the design from Fusion360 to be cut from 3mm clear PVC sheet on the CNC router.

Above is the cut piece with four M3 heat melt threaded inserts for the electronics.

Above is the completed prototype assembly. The white cable to the left is an ICSP connection for programming / in circuit debugging of the code.

Whilst I used a CNC router do cut the board out, it could easily be done with a jigsaw and drill.


nanoVNA – that demo board and its U.FL connectors

One of the many nanoVNA cloners makes an interesting little inexpensive demo board with a selection of components, filters etc to develop familiarity with the nanovna.

Above is a pic of the demo board and the supplied jumper cables. The demo board may not include information relevant to using the cables and connectors supplied. Continue reading nanoVNA – that demo board and its U.FL connectors

Coaxial Collinear – dielectric loading the outer conductor

Recent discussion with a correspondent about the design issue of the so-called Co-Co collinears, vertical collinears made with alternating sections of common coax ranged onto the conflict between the phase velocity of the wave on the inside of the coax and the wave on the outside of the coax, and the difficulty in aligning both the outside standing wave pattern for optimal pattern, and the internal phasing to feed those sections with optimal phase. Continue reading Coaxial Collinear – dielectric loading the outer conductor

nanoVNA user post provides an interesting example for study #1

At https://groups.io/g/nanovna-users/message/9185 a user posted a measurement made with his nanoVNA of a length of coax with termination.

Above is his initial reported measurement of an approximate 350′ length of coax with a known good dummy load on the opposite end. 350′ is 106.7m. Whilst this chart is less value than a Smith chart rendering, understanding the nature of things allows us to infer the Smith chart. Continue reading nanoVNA user post provides an interesting example for study #1

Normalised RMS voltage of a full wave phase controlled power waveform

The recent article Soldering iron – temperature control failure gave a plot of V’rms vs conduction angle for a simple full wave phase controlled AC waveform, and I have been asked to explain the derivation.

The phase controlled switch turns on at some delayed time from the zero crossing of the AC waveform, and conducts until the next zero crossing.

With the simplest circuits, there is a practical limit to the achievable stable range of conduction angle, and a minimum of about 50° to a maximum of about 160° is typical.

The RMS voltage is the square root of the mean of the square of the instantaneous voltage. We can write an expression for the normalised RMS voltage as a function of conduction angle θ. Continue reading Normalised RMS voltage of a full wave phase controlled power waveform

nanoVNA-H – measure 144MHz Yagi gain – planning / feasibility

This article documents a feasibility study of using the modified nanoVNA-H to measure the gain of a 4 element 144MHz Yagi, the DUT.

The intended configuration is the DUT will be connected to the tx port (Port 1 or CH0 in nanoVNA speak), and a known ‘sense’ antenna connected to its rx port (Port 2 or CH1 in nanoVNA speak).

nanoVNA |s21| noise floor

To make useful measurements of the received signal, the rx signal level must be a reasonable amount higher than the noise floor, 10dB should be sufficient.

Above is a plot of the |s21| noise floor around 146MHz. Continue reading nanoVNA-H – measure 144MHz Yagi gain – planning / feasibility