BNC / RG-59 for low frequency applications - Page 1
Forum section: electronics / beginners

author: girishv
author_rank: Regular Contributor
timestamp: November 13, 2020, 01:55:27 am
content: Hi,Can I use BNC connector with RG-59 cable for low frequency / dc applications? For example, connecting 50MHz or less op-amp connections between boards?I have lots of good quality RG-59 BNC connectors and would like to put them for good use.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 13, 2020, 10:25:26 am
content: Yes.But it may be not very convenient, because a cable may be rigid (stiff), and may break somewhere at the soldering point.

author: jmelson
author_rank: Super Contributor
timestamp: November 13, 2020, 06:15:16 pm
content: Quote from: girishv on November 13, 2020, 01:55:27 amThere is a version of RG-59 that has a solid center conductor. You probably do NOT want that type for lab interconnect cables. Otherwise, you can certainly use RG-59 or RG-58 for lots of lab hook-up applications, connecting signal generators, scopes and such together.Jon

author: janoc
author_rank: Super Contributor
timestamp: November 13, 2020, 08:19:33 pm
content: Well, careful - RG-59 is 75cable. If you use this to connect DC it doesn't matter, it doesn't matter if you use 75BNCs and only use 75impedances everywhere. Just be aware that common lab equipment (generators, oscilloscopes, spectrum analyzers ...) all uses 50standard and if you mix the two, you are going to get reflections.RG-59 is meant for video applications.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 13, 2020, 08:28:03 pm
content: I wouldn't use RG-59 cables or the associated BNC connectors for general lab use because they are 75 ohm. At low frequency the impedance doesn't matter but the connectors won't mechanically mate properly to test equipment that almost universally uses 50 ohm BNC jacks. 50 MHz is also not really low frequency unless you are only using short cables. The wavelength in the cable is about 4 meter, so a 1 meter cable is lambda/4.

author: TimFox
author_rank: Super Contributor
timestamp: November 13, 2020, 08:44:48 pm
content: True-75-ohm BNC connectors are mechanically compatible with 50 ohm BNC connectors. This is not true for type-N connectors, where the 50 and 75 ohm versions are not compatible.Most BNC connectors for RG-59/U 75 ohm coaxial cable are actually 50 ohm BNCs, but with cable-interface dimensions to work with RG-59/U. The short length of mismatched line through the connector is only important at relatively high frequencies. The cable length in a mismatched system may be important, since it is much longer.For applications at frequencies where the cable length is short compared with the wavelength, the foam-dielectric "low-loss" RG-59 variants have less capacitance per unit length than the solid-dielectric cables, which can be useful. High-voltage coaxial connectors of similar size to BNC (e.g., MHV and SHV) usually use RG-59/U for a few kilovolts of DC. The foam-dielectric cables are not appropriate (I learned the hard way) for that application. In grad school, we replaced the MHV panel connectors on photomultiplier power supplies with SHV, since a determined student could mate a BNC plug with the MHV panel connector, with possible damage and insufficient voltage rating.

author: TimNJ
author_rank: Super Contributor
timestamp: November 13, 2020, 08:45:28 pm
content: Regarding Vovk_z's comment, if flexibility is a concern, maybe RG-58C is what you want. "C" means stranded conductor. I've tried a variety of RG-58C cables, from different manufacturers. Some are more stiff than others.I've found this one from Cal Test (CT2942-100) to be soft and flexible, almost doesn't feel like PVC, but it is:https://www.digikey.com/en/products/detail/cal-test-electronics/CT2942-100/6005475That said, a more flexible cable means it can be bent more severely with a tighter bend radius, so probably not as durable long-term. So far, so good, though.

author: girishv
author_rank: Regular Contributor
timestamp: November 14, 2020, 12:05:37 am
content: Quote from: ejeffrey on November 13, 2020, 08:28:03 pmI have 50\$\Omega\$ BNC crimp connectors compatible with RG-59 cables. So, mechanical connections should not be a problem. I do know a bit about losses in RG-59 cables, but not enough.

author: girishv
author_rank: Regular Contributor
timestamp: November 14, 2020, 12:12:40 am
content: Quote from: TimNJ on November 13, 2020, 08:45:28 pmThanks for the information. Can I use RC-58C cable with a connector designed for RG-59 cables?

author: girishv
author_rank: Regular Contributor
timestamp: November 14, 2020, 12:25:05 am
content: Quote from: TimFox on November 13, 2020, 08:44:48 pmI will use this calculator for calculating losseshttps://www.qsl.net/co8tw/Coax_Calculator.htm

author: TimFox
author_rank: Super Contributor
timestamp: November 14, 2020, 03:03:26 am
content: Quote from: girishv on November 14, 2020, 12:12:40 amRG-58 cables have a considerably smaller dielectric diameter than RG-59 cables. You can put an RG-59 BNC on RG-58C/U cable, but the fit will be loose.