Re: QCX mini: RF output BNC or SMA #poll-notice

Shane Justice


The life cycle mating on SMA is for the full frequncy range, plus mechanical wear aspects. At HF frequencies, the wear cycles should be greater than for the microwave frequencies. 

That said, SMA is not the most rugged connector for field use, even though there are many used in HTs. If you look how the sma base antennas are constructed for HTs, they have a wider base of plastic that physically.mates with the body of the radio to reduce torque that the SMA will experience if the radio is dropped and lands on the antenna, or experience other torques from the radio being grabbed by the antenna, for example. Motorola even made some of their radios with the SMA in reverse, so the recepticle was mounted within the chassis, and the antenna had the protruding  threaded SMA as part of the antenna. If the SMA were to fail, it would be easier to replace the antenna. Think about what cable you might use with the SMA- will you have PL259s on the cable? Do you plan to use a threaded adapter? You may lose a few dB with that adapter.

BNCs are subject to wear and toque issues as well- I've seen more than a few failures in rugged environments. Again, what size cable will you use? RG8x? 

TNCs are probably much more rugged than BNCs or SMAs, but I saw one soldier radio where that rugged connector had the outer rim of the jack side crimped inward, with tailtale signs of jaw marks from heavy duty pliers across the folded-in part of the connector. It was clear that some gorilla thought he'd prove that connected could be damaged. This was a brand new Soldier Radio, on its initial field trial. First day with the troops.

If we assume that we hams will treat our equipment much more carefully, then we can proceed with a slightly different tack- reasonable use, and attempt to accommodate some of the more likely accidents that can occur; people tripping over the cable between antenna and operating position, wind gusts causing the tree to "yank" on the antenna and a taut coaxial cable, an antenna/tower falling and launching the radio trebuchet - style, as examples.

In these scenarios, whatever torques experienced by the connector should be stopped at the case and not permitted to transfer the load to the PCB. In other words, it should be bulkhead mounted.

Now, let's say the connector is damaged. Which connector, in bulkhead form, is most likely to be available? 

I have used services that provide 1 week turnaround after receipt of order for connectorized SMA-TNC cables of any length under 12 inches for $6-7, including shipping. You could order spares to your specification ahead of time and carry one and the tools necessary to replace it in your pack, but that is adding weight.

If you want the most rugged connector, the TNC is really hard to beat. I am using them for my telemetry link to my high power rockets, where an approximately one foot long spring steel (measuring tape) radial is being buffeted by 110 feet per second winds on descent (thats a little over 60MPH if my mental math holds) for at least one minute, as the 35 pound rocket descends from apogee to the main parachute opening height of 1300 feet. The TNC is a bulkhead mounted connector and seems no worse for wear. All that said, I have only launched this rocket once, as there are logistics issues with getting to a suitable launch location with FAA clearance, so no long history of many flights to stand upon here.

The fact that the TNC is the treaded cousin to the BNC makes it a more rugged connector  that will stay tight at finger torque levels and has a purchase you won't get with either an SMA or BNC plug end, making it easier to connect/disconnect with cold or gloved hands in the Winter, or sweaty hands in the Summer. It's slightly larger than a BNC, but smaller than a SO239.

Speaking of SO239s, while they are big, have you ever heard of someone needing to replace one? I certainly don't recall one. They are ubiquitous, and almost any coaxial cable made has a PL259 that can accommodate it. They are heavier than the other connectors considered.

How much weight is too.much to carry on a SOTA expedition? I am unlikely to attempt a SOTA due to my age and medical issues, so I leave this to those who will be making this adventures.

In conclusion, whatever connector is ultimately used, it should be ridgedly mounted to the metal case, not to the PCB, to prevent damage to the PCB, and prevent mission impact.


On Aug 6, 2020 at 06:34, Hans Summers <hans.summers@...> wrote:

Hi Russ

I have to admit to a preference for BNC. 

What kind of issues have they seen? 

73 Hans G0UPL

On Thu, Aug 6, 2020 at 4:28 PM Russ@va3rr via <> wrote:
The NanoVNA group has been seeing some issues with SMA connectors.

It's too bad TNC connectors aren't more prevalent in amateur equipment.  I have some LMR cables with TNC connectors and they really are quite robust...

Join to automatically receive all group messages.