NanoVNA source mismatch error

One of the popular ideas online is that the correction process in the NanoVNA does not correct errors in mismatch at Port 1 and Port 2. This article deals with the first case ONLY, Port 1 mismatch.

An experiment with source VSWR nominally 2:1

A NanoVNA was configured with a SMA tee connected to Port 1 and a good 50Ω termination connected to the branch port, see the pic below. The left hand side of the tee becomes the new Port 1 interface, and by virtue of the additional 50Ω shunt termination, if the native Port 1 was indeed well represented by a Thevenin equivalent circuit with Zs close to 50+j0, the Thevenin source impedance is now closer to half that, Zs close to 25+j0.

Some would calculate this mismatch as causing a mismatch loss of 0.512dB that is additional loss in the s21 path.

Above is the test setup. The NanoVNA was SOLT calibrated with cal parts attached to the left hand side of the tee and the 200mm coax jumper from that point to Port 2. Continue reading NanoVNA source mismatch error

Implementation of G5RV inverted V using high strength aluminium MIG wire – 5 year review

This article continues on from Implementation of G5RV inverted V using high strength aluminium MIG wire documenting review after 5 years operation under a wide range of temperature, humidity and wind conditions.

Above is a view of the steel mast with the Inverted V G5RV rigged from the top of the 11m mast using a halyard though a purchase on a small gibbet to offset the antenna and feed line from the mast. There are lateral guys at 7m height, and the left hand one is non-conductive synthetic fibre rope. Atop the mast is a 2m/70cm vertical. Continue reading Implementation of G5RV inverted V using high strength aluminium MIG wire – 5 year review

NanoVNA-H – modification of v3.3 PCB to start the bootloader from the jog switch

Later NanoVNA-H* hardware allows the device to start in bootloader mode by holding the jog switch in whilst powering on. It is a very convenient facility for firmware update, much more convenient than taking the case apart to jumper BOOT0 to VDD. (Some later firmwares provide a menu option to start the bootloader… but of course that is only useful if the firmware is running properly and may not be useful in the event of a failed firmware update.)

This was a mod I devised prior to the v3.4 hardware change, it is not identical to that change as it preceded it, but it works fine on v3.3 hardware and may work on earlier versions.

Boot switch

The mod calls for replacing R5 with a 1k (1402) and running a short jumper from the T terminal of the jog switch to the un-grounded end of R6.

To use it, hold the jog switch in and turn the nanoVNA on.

Above a pic of the mod. It is a simple mod, but very fine soldering so it might not be within everyone’s capability.

NanoVNA-H* – a howto do firmware update using STM32CubeProgrammer

One of the many solutions for updating firmware on the NanoVNA-H* is using ST’s STM32CubeProgrammer.

It would seem that STM32CubeProgrammer deprecates the older DfuSe Demo utility… which remains available for download. Some online experts have inferred that the word Demo in the latter implies it is not the full quid… but they misunderstand the context.

Windows drivers

The two are kind of incompatible in that they use difference device drivers. If you set your machine up for one, it breaks the other until you switch the correct driver in.

STM32CubeProgrammer uses libusbk (or the like) whereas DfuSe Demo uses the STub30 driver.

Above is a dump of the driver properties in my working instance. Continue reading NanoVNA-H* – a howto do firmware update using STM32CubeProgrammer

NanoVNA-H4 – battery charge from discharged

This article documents the charge cycle of a NanoVNA-H4 from fully discharged to charged.

The DUT is probably a ‘standard’ H4, but with Chinese sourced produce, you never, never know.

The original battery fitted to the NanoVNA-H v4.3 is a 804050 (8.0x40x50mm) 2000mAh LiPo pouch cell (1S) with protection board.

The charger chip is a TP4056, and it would appear to be limited by Rprog to about 0.75A (which includes the current drawn by the working NanoVNA-H4) (though the circuit employed would appear to tweak that limit between VNA on and off conditions with R44). The TP4056 is simply a charger chip, it will not prevent over-discharge of the cell so it is wise to use a cell with protection board (as originally supplied on the DUT).

Above is a plot of the calibrated battery voltage reported by the NanoVNA-H4. Continue reading NanoVNA-H4 – battery charge from discharged

Center-Fed Dipole : elements length for a Z=200 +/- 0j ohms

A chap asked online for dimensions of a 50MHz dipole with a feed point of 200+j0 to suit 50Ω feed line and a 1:4 coax half wave balun. The “+/- 0j” is hammy Sammy talk from an ‘Extra’.

This type of balun, properly implemented, is a good voltage balun, and it is quite suited to a highly symmetric antenna.

A good voltage balun will deliver approximately equal voltages (wrt the input ground) with approximately opposite phase, irrespective of the load impedance (including symmetry).

Where the load is symmetric, we can say a good voltage balun will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance.

It is an interesting application, and contrary to the initial responses on social media, there is a simple solution.

One solution

Let’s take a half wave dipole and lengthen it a little so the feed point admittance becomes 1/200-jB (or 200 || jX). We will build an NEC model of the thing in free space.

Above is a sweep of the dipole which is 3.14m long (we will talk about how we came to that length later), and the Smith chart prime centre is 200+j0… the target impedance. Continue reading Center-Fed Dipole : elements length for a Z=200 +/- 0j ohms

U.FL connectors – hints

This article expands on discussion at nanoVNA – that demo board and its U.FL connectors.

Before looking at the specifics of the Hirose U.FL connector, clean connectors work better and last longer. That should not be a revelation.

A can of IPA cleaner and a good air puffer are invaluable for cleaning connectors. The air puffer show  has a valve in the right hand end, it doesn’t suck the dirt and solvent out of the connector and blow it back like most cheap Chinese puffers, this one was harder to find and expensive ($10!). Continue reading U.FL connectors – hints

4NEC2 plots of STL VSWR II (v5.9.3)

At 4NEC2 plots of STL VSWR  and 4NEC2 plots of STL VSWR II I explained a method of working around a limitation of 4NEC2 values for Zo that can be applied using the Settings menu.

I can advise that exactly the same change works in 4NEC2 v5.9.3

It appears that 4NEC2 enforces a requirement that Zo>=0.1, so having discovered that by trial and error, one wondered if it was possible to change that threshold by hacking the exe file. Continue reading 4NEC2 plots of STL VSWR II (v5.9.3)