BNC Plug / Cable Question - Page 1
Forum section: electronics / rf-microwave

author: kaevee
author_rank: Regular Contributor
timestamp: September 12, 2020, 06:56:50 pm
content: I have TE 2-5329445-1 BNC Plugs. They are 50Ω connectors.https://www.te.com/global-en/product-2-5329445-1.htmlThe datasheet says these connectors are compatible with RG 124, RG 140, RG 210, RG 59, RG 59A, RG 59B, RG 62, RG 62A, RG 62B cables.I remember reading in the forum about using RG 59 with a 50Ω connector leads to impedance mismatch. How can a 50Ω connector compatible with RG 59 cable?

author: TimFox
author_rank: Super Contributor
timestamp: September 12, 2020, 07:40:37 pm
content: The characteristic impedance through the short length of the BNC connectors is 50 ohms, but the dimensions of this plug are mechanically compatible with those higher-impedance cables. Look at transmission line theory to evaluate the effect of a few cm of mismatched line at your frequency of interest. Note that there are actual 75 ohm BNC connectors that will mate with the more common 50 ohm BNCs, but 75 ohm N connectors will not mate with the 50 ohm connectors. If you want to use 93 ohm RG-62 cable, there are no true 93 ohm BNCs.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 12, 2020, 09:25:00 pm
content: Quote from: kaevee on September 12, 2020, 06:56:50 pmno, this is rightful in beginner section, since this is relating to RF this is also rightful for being here. General section when you want to talk bullsheet (politically or emotionally motivated subjective topics), imho.Quote from: kaevee on September 12, 2020, 06:56:50 pmthey are mechanically compatible, and also in some electrically low frequency application, they are perfectly usable. near DC, anything will be compatible to anything. at high enough frequency, even rated at specified impedance, if you assemble them wrong, too much solder or flux, few cuts here and there, they will show up as discontinuity or impedance mismatch, ymmv.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 14, 2020, 09:33:03 am
content: I seem to recall that lots of pro-cinema gear uses BNC for SDI (a standard protocol for high-resolution video data used in professional broadcast and cinema), and for some historical reason there is a mismatch there between connector and cable - I think they used 50 ohm BNCs on all the gear but use 75 ohm system (and cable) impedance, or at least used to.I think the reason was that they wanted to use the 75 ohm cable common in telco/broadcast/distribution, but historically only 50 ohm BNC connectors existed, and they just stuck with it because the impact is not to big anyways. (but don't quote me on that - this is just a hunch)

author: kaevee
author_rank: Regular Contributor
timestamp: September 14, 2020, 11:43:13 am
content: Quote from: Mechatrommer on September 12, 2020, 09:25:00 pmFor an uninitiated person, this can be quite confusing. I bought these connectors to make some custom 50Ω cables. It was too late by the time I realised they are compatible with 75Ω cables. The product had been already shipped.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 14, 2020, 12:58:08 pm
content: For the small cables, the main difference is the hole for the cable. You could probably get away with making an RG58 cable with them. Maybe heat shrink the jacket to match the diameter. This will probably not be super great for impedance matching either but who knows.You could make one and see how it performs.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2020, 01:19:55 pm
content: OP: what frequency range are you considering for these connections?

author: kaevee
author_rank: Regular Contributor
timestamp: September 14, 2020, 01:42:23 pm
content: Quote from: ChristofferB on September 14, 2020, 12:58:08 pmThanks. I will give a try.

author: kaevee
author_rank: Regular Contributor
timestamp: September 14, 2020, 01:56:50 pm
content: Quote from: TimFox on September 14, 2020, 01:19:55 pmI did not buy them for any specific purpose. One of these days while browsing for some other connectors, I found them on discount at attractive price. I thought of making some cables on my own and purchased a bunch of them. Later, I realized they are not compatible with RG58.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2020, 03:24:44 pm
content: For non-RF purposes (e.g. audio test equipment) low-loss versions of RG-59 can be useful for their lower capacitance per foot than 50 ohm cables.