An article by K2PO in QST Feb 2020 entitled An SWR shifting T
illustrates the pitfalls in naive design and implementation of transmission line matching systems. I say naive because the article does not address the matter of loss, yet QST publishes it as an example. Continue reading Stub matching loss can bite you
Category: Transmission lines
nanoVNA-H – recovery
I often see reports that a nanoVNA has been ‘bricked’. The term seems to have become part of the vernacular of would be pros. The term ‘bricked’ certainly applies to electronics that can no longer be programmed through ‘ordinary means’ and is to all intents and purposes as useful as brick, but in most cases, the nanoVNA is recoverable.
The STM32F072 chip used on the original nanoVNA has some features that make the firmware update process simple and robust, and difficult to mess up.
The normal way of doing a firmware update is using the DFU protocol from a PC over the USB interface. To use this, the device has to be “put into DFU mode”, this means that the chip is reset and started executing the bootloader in permanent system memory.
The concept of DFU is that normal client programs used with the device can easily be extended to include the DFU function as just another menu function of the client software. I am not aware of any nanoVNA client that does this.
So, you need to use a programming client, and for Windows a good choice is ST’s DfuSeDemo. You may need to convert the distributed file format using Dfu file manager from the same distribution, not all developers distribute a .dfu file.
There is a pin on the board, BOOT0, that must be held high during reset to enter the on-chip bootloader. Later firmware versions also provide a menu option to enter the bootloader, but if an attempted upgrade messes up the menu, you may need to use the BOOT0 pin bridged to the adjacent VDD pin while you power cycle the nanovna.
Above is the rear view of the board, and a jumper using pogo pins to bridge BOOT0 to VDD. Continue reading nanoVNA-H – recovery
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:
- decoupling power to the mixers;
- increasing the drive to the mixers; and
- higher attenuation of input on the rx port. Continue reading nanoVNA-H – rework of v3.3 PCB to v3.4?
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 – measurement of two 920MHz LoRa antennas
The article IoT – exploration of LoRa – part 3 showed some components of a simple LoRa system.
This article reports measurements made on two antennas used in the prototype system.
Above is a view of the prototype system. Continue reading nanoVNA – measurement of two 920MHz LoRa antennas
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 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
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 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. 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
Antennas – disturbing the thing being measured – open wire lines #5
The articles Antennas – disturbing the thing being measured – open wire lines #3 Antennas – disturbing the thing being measured – open wire lines #4 demonstrated an inconsistency between a partial linear circuit model and a complete NEC model for predicting common mode current behaviour.
One of the oft proposed solutions to characterising a balun is to find the Common Mode Rejection Ratio (a term carried over from other applications, eg voltage driven operational amplifiers). (Anaren 2005) explains a method of finding balun CMRR. (Skelton 2010) goes so far as to say
The Common Mode Rejection Ratio (CMRR) of a balun is defined in professional literature as the ratio of wanted to un-wanted transmitted power. As rejection of common-mode transmission is the primary purpose of a balun, it follows that CMRR should be the key figure of merit.
Let us take the model scenario used in Antennas – disturbing the thing being measured – open wire lines #4 and lower the height of the dipole 10 10m, and compare the model ratio of Ic/Id.
Again, we will use Python to do the complex maths for the without and with scenarios at 20m height, and without and with at 10m height. Continue reading Antennas – disturbing the thing being measured – open wire lines #5
Antennas – disturbing the thing being measured – open wire lines #4
The article Antennas – disturbing the thing being measured – open wire lines #2 did a simple analysis of current flows in the model scenario using ideal voltage balun drive with a current balun.
It was mentioned that solution of the lumped values network is only a first approximation and not as good as the NEC solution which properly models the coupled conductors, and their mutual effect on the distribution of currents in the system.
This article reports the NEC current solution decomposed into differential mode and common mode components.
Ideal voltage balun drive
Above is a zoomed in view of the feed point with a balanced pair of voltage sources feeding the line against the ground electrode. Continue reading Antennas – disturbing the thing being measured – open wire lines #4