Category: Amateur Radio

A 1:4 RF transformer for measurements – based on Noelec 1:9 balun assembly

The Noelec 1:9 balun (or perhaps Chinese knock off) is available quite cheaply on eBay (<$5) and provides a good hardware base for a 1:4 version.

Above is a modified device with the original transformer replaced with a Mini-circuits TC4-1TG2+ 1:4 transformer. The replacement is not exactly the same pads, but it is sufficiently compatible to install easily.

The loose parts are OSL calibration parts using 2.54mm pitch header pins with the middle pin removed. The resistor comprises two 100Ω 1% 1210 SMD resistors soldered back to back. (The 1% resistor code 1000 is for 100 * 10^0.)

The most notable departure from ideal of these small transformers is leakage inductance of 30nH give or take. Continue reading A 1:4 RF transformer for measurements – based on Noelec 1:9 balun assembly

Last update: 12th March, 2022, 11:39 PM

NanoVNA-H4 v4.3 – broken SMA connector

It has been my experience over more than 50 years that accurate measurements using SMA connectors requires that they be torqued to a consistent and adequate torque. Specifications for brass threads commonly runs around 0.6Nm, and I have a torque wrench calibrated for that torque which I use whenever the connectors are to be properly tightened.

In my NanoVNA-H v3.3, I reinforced the SMA connectors because of a sense that to tighten them to 0.6Nm (5.3inlb) caused the board to flex and over time might crack the tracks (Strength of reinforcement of nanoVNA-H connectors).

Joe Q Smith on NanoVNA SMA connectors

Joe Q Smith has an interesting video where he tests some Chinese SMA connectors to destruction, worth watching: NanoVNA Torquing SMA Connectors . In his tests, he needs upwards of 2Nm to damage the cheap Chinese end launch PCB connectors, more than three times the torque I use.

The new NanoVNA-H4

About 6 weeks ago, I took delivery of a new NanoVNA-H4, which as far as I can tell, is a ‘genuine’ Hugyen product.

I did not reinforce the connectors on my new NanoVNA-H4 because there was not the needed clearance for a similar adequate brass bar inside the case.

So, having used a torque wrench at 0.6Nm in the few weeks that I have owned the NanoVNA-H4, the connector on Port 1 failed.

Above, the failed connector has rotated, shearing two of the support pins and tearing the other two off the tracks. Continue reading NanoVNA-H4 v4.3 – broken SMA connector

Last update: 5th June, 2022, 6:29 AM

NanoVNA-H4 v4.3 – improving the SD card slot

One of the shortcomings of the NanoVNA-H4 v4.3 is that it is quite easy to drop the SD card inside the case when trying to insert it. Experience is that this is really easy to do in difficult field situations or poor lighting / visibility.

This could have been prevented by better design of the moulded case.

This article describes a simple modification to make it more difficult to miss the card slot.

Above, the modification is a small block of plastic that covers part of the aperture moulded into the case, it projects 1mm into the aperture. Continue reading NanoVNA-H4 v4.3 – improving the SD card slot

Last update: 12th March, 2022, 11:40 PM

Transformers and flux density

Lots of online discussions on ferrite cored transformer losses might make you think that the core material is in a path in series with the transferred power and that it acts to some extent like an attenuator.

That sort of thinking betrays a lack of understanding of how a transformer works.

If you take a good 50/60Hz 1:1 power transformer, assume no losses, no flux leakage, and ignoring distributed capacitance, you might ask: Continue reading Transformers and flux density

Last update: 10th March, 2022, 9:47 AM

Selfevidently

An online expert held forth on the design of ferrite chokes and transformers, and to quote one paragraph:

Equally selfevidently we don’t want ANY real part of the reactance in a transformer and, for a practical transformer, we want the self inductance on each side (primary and secondary) to be at least j10*R(Load or Source) and the coupling to be as close to 100% from primary to secondary. It is the real part that heats up transformers a LOT and, since ALL of the current is seen by the ferrite in a transformer, not just the part that got reflected back on the outside of the coax in a choke, losses are abos-posilutely-undubiously NOT desired and the u”R needs to be as close to zero as we can get at the designed frequency for minimum loss and minimum power dissipation.

Setting aside the hyperbole and the wooly thinking, let’s drill down on u”R needs to be as close to zero as we can get at the designed frequency for minimum loss and minimum power dissipation.

It is a pretty general statement without really specific quantities, needs to be as close to zero as we can get and minimum loss and minimum power dissipation does not give useful guidance of acceptable values of µ”, and may even impart the impression that the following chart is for material that is not suitable above perhaps 200kHz, if that.

Above, µ” is greater than 10 above about 200kHz, greater than 100 from about 2 to 100MHz. Is this what the quote condemns? Continue reading Selfevidently

Last update: 10th March, 2022, 9:45 AM

On testing two wire line loss with an analyser / VNA – part 6

Measuring velocity factor

This article discusses measuring velocity factor using the NanoVNA. The DUT is coax with N type connectors as it provides a better example to demonstrate and learn from. Having acquired competency, extension to two wire lines is just a matter of attending to the matters of a suitable transformer, and appropriate SOL calibration parts.

N type connectors

The ‘standard’ reference plane on N connectors is shown in the diagram above. For the purpose of this article, length measurements were made between the reference planes at both ends of the cable. Continue reading On testing two wire line loss with an analyser / VNA – part 6

Last update: 8th March, 2022, 10:23 AM

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)

Last update: 7th March, 2022, 7:33 AM

InsertionVSWR of Chinese 1-9 balun module – that load resistor

InsertionVSWR of Chinese 1-9 balun module showed a 450Ω load attached to the DUT, and I have been asked to explain further.

Above is the load resistor just visible to the left of the spring terminals on the module. The idea is that the leads need to be very short to avoid unintended / undesired impedance transformation. Continue reading InsertionVSWR of Chinese 1-9 balun module – that load resistor

Last update: 5th March, 2022, 7:32 AM

InsertionVSWR of Chinese 1-9 balun module

This article documents an InsertionVSWR test of a cheap Chinese 1-9 balun purchased for less than <$5 on eBay (shipped).

Above is a 1-9 balun, it would seem to be a clone of the Noelec 1-9 balun. The balun is a compensated voltage balun with the secondary centre tap grounded for these measurements. Continue reading InsertionVSWR of Chinese 1-9 balun module

Last update: 3rd March, 2022, 2:01 PM

On testing two wire line loss with an analyser / VNA – part 5

This article series shows how to measure matched line loss (MLL) of a section of two wire line using an analyser or VNA. The examples use the nanoVNA, a low end inexpensive VNA, but the technique is equally applicable to a good vector based antenna analyser of sufficient accuracy (and that can save s1p files).

Article On testing two wire line loss with an analyser / VNA – part 2 showed a 1:1 transformer for measuring two wire lines without encouraging significant common mode current.

Online experts suggest that the required transformer is one from 50Ω to Zo of the line being measured. It is often said that: Continue reading On testing two wire line loss with an analyser / VNA – part 5

Last update: 28th February, 2022, 12:40 AM