nanoVNA-H – rework of v3.3 PCB to v3.4?

nanoVNA-H v3.4 is out, and I don’t yet see significant problem reports.

When I compare the circuit with v3.3 (which I have), apart from new battery charger IC etc, the changes are in three areas:

  1. decoupling power to the mixers;
  2. increasing the drive to the mixers; and
  3. higher attenuation of input on the rx port. Continue reading nanoVNA-H – rework of v3.3 PCB to v3.4?

A thinking exercise on Jacobi Maximum Power Transfer

At The system wide conjugate match stuff crashes out again I discussed the failure of Walt Maxwell’s teachings on system wide simultaneous conjugate match using an example drawn from an online expert’s posting.

The replicated scenario with matching with an L network where the inductor has a Q of 100, no other loss elements is shown below. (Quality real capacitor losses are very small, and the behavior will not change much, the inductor loss dominates.)

Above is a model in Simsmith where I have adjusted the lossy L network for a near perfect match. I have used a facility in Simsmith to calculate the impedance looking back from L1, often known as the source impedance at a node but in Simsmith speak the calculated L1_revZ on the form, (ie back into the L network)  from the equivalent load. Continue reading A thinking exercise on Jacobi Maximum Power Transfer

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

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

nanoVNA user post provides an interesting example for study #1

At 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

Antennas – disturbing the thing being measured – open wire lines #6

The article Antennas – disturbing the thing being measured – open wire lines #5 demonstrated an inconsistency between the notion of a balun Common Mode Rejection Ratio CMRR property and a complete NEC model for predicting common mode current behavior.

In that case, two scenarios were modelled with only a change in the feed line length, yet they showed quite different currents near the same balun.

A common metric bandied around is the Common Mode Rejection Ratio (CMRR) and the definition is a bit rubbery, but it tends to come down to the ratio of the magnitude of common mode current to the magnitude of differential current in a test scenario (usually a lab workbench… with intention that the metric is then applicable more generally). (Anaren 2005) gives a popular explanation.

It is worth noting that the conventional meaning of CMRR in relation to op amps is that it is the ratio of differential gain to common mode gain and large +ve dB values are goodness, and it makes sense. Common use in terms of baluns is the opposite, Anaren gives the expression CMRR=S1c/S1d which will give large -ve dB values as goodness, and which seems inconsistent with the descriptive name where a large ratio (more +ve dB value) would be goodness. The balun ‘crowd’s’ use of a -ve rejection ratio seems a bit tautological, as if they haven’t really thought this through, it is a bit like the hammy thing of talking about the attenuation of a length of coax as -xdB.

I don’t think CMRR is a useful property of baluns per se, certainly not as a component of practical antenna systems, so I have written this article to report common mode ratio (CMR) (being the ratio of common mode to differential mode current at the point of interest). CMR is not a property of the balun, it expresses the relationship between the magnitudes of the components of current at a point of interest.

Keeping in mind that the differential current and common mode current distributions are usually both standing waves in the general case (usually with different phase wavelength and therefore relative phase), another dimension of the antenna problem is to look at the current distribution on the feedline of the NEC model scenario used for this series of articles. The model used here is the 20m feed line height and current balun with Zcm=1130+j1657Ω.

Above is a plot of |Ic|, |Id| and CMR in the NEC-4.2 model scenario. Segments are numbered from the lower end to upper end of the 20m long feed line. Continue reading Antennas – disturbing the thing being measured – open wire lines #6