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FWIW a bit of TDR porn on "finger tight" vs torqued. I think it should be noted that in this example, the ringing is at over 5 GHz, so it really doesn't matter for the current version of the nanoVNA.



This is a little more extreme than most cases, but I often am watching the 11801 while I tighten the connections and see similar results as I switch from the initial finger tight to torqued. Most of the problem is because the SMA hex is so hard to get a grip on. I commonly see such effects disappear while tightening N connections.with my fingers.

I replaced both of the SMA connectors on my nanoVNA with the bulkhead type, so with the connector clamped to a case, the PCB connections won't be strained.

I think a knurled round wrench about the diameter of an N-M would be entirely adequate until you got well above 10 GHz. Perhaps some enterprising merchant will offer some on ebay. Simple to make. Knurl a piece of 2 cm bar, part off, and mill a slot to fit the flats. About $1200 US in tooling and with practice probably 5 minutes to make. I'd buy a well made pair for $10-12 delivered in a heartbeat just for the convenience of not making them myself. So a person in the US should be able to make over $30/hr making and selling them. Pretty good business for someone who is retired and is only looking for play money to buy more toys. I've got the machine tools, so I'll make a pair eventually if no one decides to make them for sale.

Have Fun!
Reg

On Thu, Sep 19, 2019 at 06:53 PM, Reginald Beardsley wrote:

Most of the problem is because the SMA hex is so hard to get a grip on.

Reg, for this reason I picked up some "SMA Nuts" on eBay after I discovered I had the same issue with my NanoVNA. Now each of my three SMA standards wears a collar.

Below are a couple of eBay links to sellers in the States.





If they interest you, maybe there's someone in the UK selling the same? Try searching for "drone SMA nuts"

So far, they are great! (I used the first vendor, above). As for durability, we will see...

- Jeff, k6jca

Hello Reg,
you can also print them by your own 3D printer.
See

I printed them with PLA in 102% size to fit tight.

73, Rudi DL5Fa

I've laser-cut some from 3mm acrylic sheet. Black for 0 ohms, green for 50,
clear for open.

John

I found some finger "wrenches" here:


and a torque limiting "wrench" here:

I tested different SMA connectors with hand connection with no wrench and don't find any noticeable difference.
Probably because NanoVNA works with less than 1 GHz.

Does somebody have any confirmation that using wrenches improves measurement with NanoVNA?

I think a torque wrench on a nanoVNA is overkill. The ringing I observed was well above the maximum frequency range.

The issue is getting *consistent* connections. Any time you change the sweep parameters you need to do a cal. I think that some means of ensuring more consistent connection torque is needed. But a "finger wrench" should be adequate.

There would be no way to answer your question except by repeating the same measurement many times.

I plan to knurl and slot a small disk of steel or brass rather than put plastic disks on all my SMA connectors. That's just personal preference. A small square or triangle of steel with an 8 mm hole drilled in the center and a slot 8 mm wide cut to the hole should be entirely satisfactory. A 4 mm lever arm is rather short and lots of SMA-M nuts are very smooth.

Have Fun!
Reg

If you get the "thin sma spanner tool" at like 3-4 GBP from sdr-kits, and
you have a dynamometer capable of measuring 6N accurately, you can actually
get a fairly accurate .3 Nm pulling at the tiny hole with the spanner
attached to an SMA collar nut, the spanner as vertical as possible and
pulling the dynamometer horizontally.