Broadcast - 15 March 2026
Just like Star Wars, todays item is actually a prequel to the clubs last technical night’s presentation by Colin VK7ZCF and Lionel VK7ZLB on Vector Network Analyser “through testing.” Before the evening we tested the laptop software and its ability to interface correctly with the VNA, everything worked perfectly and the measurements looked as expected and the large wall monitor display was sharp and clear. The vast improvement in visibility offered by the large wall monitor being the reason for the laptop and software.
However….. As often happens with live demonstrations, not all the equipment was available until “show time.” Colin’s Halibut Electronics Common Mode Current Choke test jig wasn’t part of the earlier testing. On the night, once the full setup was connected, we began observing inconsistent variations in the VNA’s displayed graphical output. The measurements were being significantly affected by physical movement, in particularly movement of the USB interface cable between the laptop and the VNA.
Ironically, the topic of the night was Common Mode Currents, and it turned out the USB cable was the source of the problem. Adding a ferrite toroid core and looping the USB cable through it half a dozen times completely resolved the issue. Most likely minimising spurious radiation from the switch mode or display driver, importantly the evening continued.
Another contributing factor was the test jig’s default configuration. All the interface connectors for the S1 port are tied together, and likewise for the S2 port. That’s a significant amount of exposed copper that can radiate, receive, and couple unwanted signals. Fortunately, the designer included the option to isolate and use only a single connector type by removing specific solder links on the PCB, which greatly reduces the amount of unnecessary copper.
This is a good reminder to carefully consider your test setup, the range of your instrument, how you are connecting equipment and what might influence the results. If you can, always have some calibrated or “known” high tolerance components to check if the instrument did indeed calibrate correctly. Use the relevant formulae and see if the result is close to what the instrument displays.
All this aside, we should have realised that if something can go wrong, it probably will.
As usual pictures will be available on the NTARC Web site under “Blogs” for this broadcast. NTARC Blogs
UPCOMING EVENTS
On Air Test and Technical Net session - Every Wednesday night, Test-Net and CW course on 3.580 MHz from 7 pm, then a Technical Net on 3.567 MHz from 7.30 pm till 8.30 pm. Your host for the evening is Nic, VK7WW.
Club Room Technical night - The next session will be this Wednesday the 18th March and will commence at the usual time of 6.30 pm at the Club Room Archer Street, Rocherlea.
Coffee Morning - Held every Friday in the NTARC Club rooms. Time is from 10 am to noon.
Finally - If you have any items of news please email them to the Secretary at the following address news@ntarc.net all items to be received no later than 5 pm on the Friday prior to the Broadcast.
That’s all folks,
73, Stefan VK7ZSB.
