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: Amateur Radio
A thinking exercise on Jacobi Maximum Power Transfer #4
The article A thinking exercise on Jacobi Maximum Power Transfer #3 discussed Kurokawa’s power reflection coefficient as in indicator of mismatch at a system node.
Above is a demonstration circuit in Simsmith, a linear source with Thevenin equivalent impedance of 50-j5Ω. The equivalent voltage is specified by useZo, which like much of Simsmith is counter intuitive (as you are not actually directly specifying generator impedance):
Vthev and Zthev are chosen so that ‘useZo’ will deliver 1 watt to a circuit impedance that equals the G.Zo. Zthev will be Zo*.
Continue reading A thinking exercise on Jacobi Maximum Power Transfer #4
The transmitter matching problem
In the article The system wide conjugate match stuff crashes out again I worked through an example proffered in an online discussion to show that Walter Maxwell’s teachings on system wide simultaneous conjugate match do not tend to occur in practical systems.
Why are hams so obsessed with conjugate matching?
The answer is on the face of it quite simple. Continue reading The transmitter matching problem
Strength of reinforcement of nanoVNA-H connectors
The nanoVNA-H connectors are end launch PCB connectors and they have a decidedly spongy feel as 1Nm torque is approached. This was due to flexing of the PCB and was likely to lead to track cracks in the longer term.
Specs for SMA connectors range from minimum of 0.2Nm torque to maximum of 1.7Nm, but 0.6Nm and 1.0Nm are common commercial practice.
Some nanoVNA sellers state:
As the SMA ports are made of cast copper, please not connect hard 50-7 / RG213 and other cables directly to the SMA ports through M-to-SMA connector to avoid damaging the SMA ports. You can use the included SMA cable to connect to the SMA port as shown in the picture below, and then use M to SMA connector.
Clearly Chinese Cheats, they will say anything to make a sale and anything to avoid commitment to quality. These connectors are very unlikely to be copper, but are likely to be a copper alloy: brass. What they also avoid in the above statement is claim for PCB damage due to flexure of the SMA connectors torqued to accepted industry torque for reliable connections and measurement.
Above is a pic of a modification to reinforce the connectors. This article sets out the analysis of just the solder joint within the cross section of the brass pieces.
A side effect is that this modification bonds the ground planes for the input and output parts of the nanoVNA via the brass bar where they have been kept isolated to some extent.
I should note that there has been much discussion online as to whether the noise floor of the nanoVNA is degraded by the shields fitted to the board, and various modifications to the shields. Some of this discussion proposes that the issue is mostly around the mixers and noise loops, and I note that in -H designs prior to v3.3, the mixer power supply was not adequately decoupled. It is possible that electrical connection of the SMA connectors in this way degrades noise performance at some frequencies. No significant change was observed in the noise floor of s11 or s21 channels from 1 to 300MHz (I don’t regard instrument performance to be good above 300MHz). I have not seen credible evidence of degradation of the nanoVNA-H v3.3 build.
If indeed bonding the two SMA connectors close to the instrument increases the noise floor or has other performance impacts as suggested, it questions whether the currents on the exterior of the coax influence measurement (which it should not) and it questions whether two port measurement fixtures or adapters should be attached close to the nanovna.
(See also Reinforcement of nanoVNA-H connectors – performance discussion.)
At first, the strength of the butt soldered joint might seem a simple case of beam analysis where the beam is of cast solder of the same cross section l x w as the soldered joint. Continue reading Strength of reinforcement of nanoVNA-H connectors
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
A thinking exercise on Jacobi Maximum Power Transfer #3
At A thinking exercise on Jacobi Maximum Power Transfer #2 I posed the question of a metric for the mismatch at the L2L1 junction in the following network where the calculated values L2L1_lZ is the load impedance at the L2L1 junction (looking left as Simsmith is unconventional), and L2L1_sZ is the source impedance at the L2L1 junction (looking right). The left three components are the fixed antenna representation.
Common practice is to speak of a “source VSWR” to mean the VSWR calculated or measured looking towards the source, and very commonly this is taken wrt 50+j0Ω which may be neither the source or load impedance but an arbitrary reference. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #3
A thinking exercise on Jacobi Maximum Power Transfer #2
At A thinking exercise on Jacobi Maximum Power Transfer I posed an unanswered Q2:
Keeping in mind that C2 and L2 are an adjustable matching network, usually adjusted for minimum VSWR as seen at the source G. So, the questions are:
-
Does the system take maximum available power from the source G when the load impedance seen by source G is equal to the conjugate of its Thevenin equivalent source impedance (ie C2.Z=G.Zo in Simsmith speak)?
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Does that ‘matched’ condition result in maximum power in the load L?
Above for reader’s convenience is the model conjugate matched at the GC2 interface. The calculated Po figure (lower right) is the power in the load L to high resolution. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #2
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?
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