Category: Measurement

I have reported issue with the USB-C plug / socket arrangement on the nanoVNA-H.

It is very sensitive to any jiggling of the cable or connector, causing a reset of the nanoVNA which almost always means lost work.

Having tried a number of different cables that have worked reliably on other devices, I initially thought there was little difference.

I did have a good response to jetting plug and jack with IPA, but the effects are shortlived.

This brings me to consider whether the connector is degrading making debris that makes for unreliable contact, or whether this is too little spring pressure in the plug.

Above is a view into the supplied USB-C plug. The pic has been taken with care to line up the die parting marks at back and front of the connector, so the view is in line with the connector axes. Continue reading nanoVNA-H – continuing USB-C woes

At Measuring coaxial cable loss by reflection with a VNA I discussed measuring terminated coax cable loss by reflection with an VNA, and you might ask the question can it be done with a scalar network analyser, return loss bridge, or directional wattmeter, all of which provide a measure of the amplitude of reflection wrt some reference impedance.

This article explores using a Bird 43 directional wattmeter to measure line loss in a similar scenario. We will use 6m of Belden 8359 (RG58A/U) @ 3.6MHz.

Expectation

A short digression, what is the specification Matched Line Loss (MLL) at 3.6MHz? Using TLLC we get 0.171dB, that is our expectation.

Return Loss of SC section

(Bird 2004) gives the following advice.

Line loss using open circuit calibration: The high directivity of elements can be exploited in line loss measurements, because of the equality of forward and reflected power with the load connector open or short circuited. In this state the forward and reflected waves have equal power, so that φ = 100% and ρ = ∞.
Open circuit testing is preferred to short circuit, because a high quality open circuit is easier to create than a high quality short. To measure insertion loss, use a high quality open circuit to check forward and reverse power equality, then connect an open-circuited, unknown line to the wattmeter. The measured φ is the attenuation for two passes along the line (down and back). The attenuation can then be compared with published data for line type and length (remember to halve Ndb or double the line length to account for the measurement technique).

This also contains the hoary old chestnut that a good OC termination is hard to achieve, but this author’s experience of measurement with modern VNAs is not consistent with Bird’s assertion.

So lets do a theoretical simulation of the Bird 43 applied to this problem.

Lets say we connect a source to the line section with a short circuit (SC) termination, and that the Bird 43 reads Pfwd=90W, and we read Pref=78W, we can calculate return loss \(RL=10 \cdot log_{10}\frac{P_{fwd}}{P_{ref}}=0.65dB\), so RL/2=0.65/2=0.325dB.

Continue reading Measuring coaxial cable loss by reflection with a directional wattmeter

At Measuring coaxial cable loss with a voltmeter I discussed measuring terminated coax cable loss with an RF voltmeter, and it had some real practical limitations.

This article explores using a nanoVNA to measure line loss in a similar scenario. We will use 6m of Belden 8359 (RG58A/U) @ 3.6MHz.

The same technique could be used with a quality antenna analyser.

Expectation

A short digression, what is the specification Matched Line Loss (MLL) at 3.6MHz? Using TLLC we get 0.171dB, that is our expectation.

Return Loss of SC section

A common method proposed is to measure Return Loss (RL) of a section with load end RL=0dB and halve it. Many experts advise that the section should be terminated in a short circuit (S) because short circuits are more reliable than open circuits. So let’s get cracking.

Above is measured |s11| using a nanoVNA with recent OSL calibration from 1-30MHz. |s11| @ 3.6MHz is by eye -0.651dB, RL=-|S11|, so RL/2=0.651/2=0.325dB. Continue reading Measuring coaxial cable loss by reflection with a VNA

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