Inexpensive connectors - Page 1
Forum section: electronics / general

author: tkamiya
author_rank: Super Contributor
timestamp: January 11, 2018, 10:17:21 pm
content: I think this is a better place to ask this question than test gears forum....For example, on eBay, there are all sorts of inexpensive connectors and adapters from all over the world, mainly China. Then there is an Amphenol and Amp, well known makers. While some of their products may be made overseas, QA is superb.Most of what I do is less than 500Mhz, and my gears top out at 2Ghz. Most of my connectors are N and BNC. Given that, would it make appreciable difference between these connectors? I have "connector savors" in all of my gears so wear and tear isn't much of a concern, but repeatability and any meaningful effects on measurements are.Also, what is THIS connector?https://www.ebay.com/itm/BNC-Jumper-Patch-Cable-3-ft-Gold-Plated-Male-Male-50-Ohm/272957603014?epid=1065384399&hash=item3f8d8a7cc6:g:DE0AAOSwyP5aHFMxIf you look closely, it is missing the insulation around center pin and inner shield contact is awfully thick.There is a variation of this where insulation is there but the inner shield contact is bent inward and covers the face of insulation like this:https://www.amazon.com/Cable-Black-RG59-Male-Connectors/dp/B06XKKQYZN/ref=sr_1_1_sspa?ie=UTF8&qid=1515708941&sr=8-1-spons&keywords=bnc+50ohm+cable&psc=1Is this some type of 50/75 ohm hybrid? (as the notation in my last link suggests)It seems there are whole bunch of BNC cables where impedance of the cable and the connectors do not match. Why is this?Thank you!

author: edpalmer42
author_rank: Super Contributor
timestamp: January 11, 2018, 11:10:57 pm
content: N connectors are capable of handling much higher frequencies and power than BNC. They also provide a more stable connection due to their threaded coupling instead of BNC's bayonet coupling. A more stable connection will only be noticeable in unusually precise measurements. At a maximum frequency of 500 MHz, there won't usually be any difference.I wouldn't expect good RF performance from either of those cables.In general, a BNC connector that's missing the internal insulation is 75 ohms. This is typical for video cables which only have to handle a few MHz. Note that 75 ohm BNC connectors are capable of much higher frequency with no problem at all. Video cables don't really need much in the way of high performance. You often see 50 ohm connectors on 75 ohm cable and it seems to work okay.The first cable you linked to appears to be doing something odd to reduce the diameter of the outer connector. Will it measure out to 50 ohms rather than 75 ohms? Who knows?The second cable lists RG-59 cable, which is 75 ohms, yet claims the cable is 50 ohms. There might be a similar oddity in the diameter of the outer connector. The picture isn't very clear. The 75footlength is an unfortunate confusion factor.Neither gives good specs for the cable itself. Unfortunately, that's quite common for pre-made patch cables. The cable itself is often very poorly made with very poor shielding. Depending on your local RF environment, that can cause major problems.You might want to buy a chunk of good cable and either crimp or clamp connectors and make your own patch cables. That way, you know exactly what you've got. Nothing will drive you crazy faster than spending an hour troubleshooting a test and then find it was that damn, cheap cable!Ed

author: tkamiya
author_rank: Super Contributor
timestamp: January 12, 2018, 12:58:49 am
content: Well.... Good crimpers from Amphenol isn't cheap either! I ended up buying pre-made ones by Amphenol via Digi-key.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 12, 2018, 01:22:50 am
content: I have also used Cables on Demand. They seem to be well constructed and I plan to use them as tests cables for high ohm measurements. It is convenient having cables made to custom lengths that fit your needs.

author: tkamiya
author_rank: Super Contributor
timestamp: January 12, 2018, 01:56:16 am
content: I'm very intrigued about that 50/75 dual use connectors. I might just buy one to see what that really is..... Maybe because of CCTV market, there seem to be a huge number of questionable BNC connectors and cables.Thank you for the tip for Cables on Demand. Not very classy home page though.