75 ohm  RG-58? - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: January 08, 2014, 09:43:49 pm
content: A few months ago I bought a 6 ft test lead that had a BNC connector on one end attached to coax. Then after 5 ft, the coax transitions to a pair of separate black and red wires that end with banana plugs. Didn't pay much attention to the coax as I didn't intend to use it for RF work. Today I noticed the coax was marked RG58 75 ohms. The markings are very clear and are at regular intervals. I thought maybe someone messed-up with the print equipment at the factory for one batch. But no, go to alibaba.com and do a search, and you'll find lots of Chinese makers and sellers of 75 ohm RG58. Some of them give quite detailed specs like 75 +/- 3 ohms. I thought RG-58 had to be 50 (or 52) ohms. Apparently not in China at least. I think it would be best to avoid completely cable with conflicting markings. Which to believe? In any event, just because cable is labeled RG-58 these days, you can't automatically assume it's 50 ohms.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 08, 2014, 09:58:06 pm
content: sorry to hear that, maybe they forgot to change 8 to 9 during production. w2aew made a video on how to check your cable characteristic impedance, go check that one out.

author: w2aew
author_rank: Super Contributor
timestamp: January 08, 2014, 11:08:04 pm
content: If it's only 5 feet of coax, and terminated at one end with banana plugs, then it can't be used at any frequency where the difference between 50 ohms or 75 ohms will even matter.

author: ConKbot
author_rank: Super Contributor
timestamp: January 09, 2014, 01:19:03 pm
content: Quote from: w2aew on January 08, 2014, 11:08:04 pmBut you can put enough current though it where it will matter if its stranded/solid copper, or copper plated steelCheck the resistance of it before you use it as a general purpose test lead for power. Run 0.1 or 1A though it and measure the voltage dropped, if you don't have a DMM with very good low ohms resolution. If your meter has 0.01 ohm resolution and a relative mode, you might be able to get away with a 2-wire measurement. Otherwise use a power supply and millivolt mode on the meter.

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 09, 2014, 08:15:51 pm
content: Would be interesting to actually measure your cable's impedance to see what it actually is. Can be done with a pulse generator and a scope...

author: edpalmer42
author_rank: Super Contributor
timestamp: January 10, 2014, 02:55:46 am
content: Quote from: ConKbot on January 09, 2014, 01:19:03 pmSince there's a BNC connector on one end, I don't think it will be used for power. However, your point is a good one. I once had a situation where I had to get a satellite signal from the dish on the roof down through ten stories to the receiver. It wasn't working and the 'professional' installer said that there were other devices on the cable. The actual problem was that there was too much resistance in the cable due to the copper-coated steel center conductor and the light stranding on the shield. Replacing the cable with one that had a solid copper center and a heavily-stranded shield fixed the problem.Ed

author: edpalmer42
author_rank: Super Contributor
timestamp: January 10, 2014, 03:01:41 am
content: Quote from: calzap on January 08, 2014, 09:43:49 pmSince the cable has banana plugs on one end, there isn't really any point in thinking about impedance. It's more like shielded wire.Maybe the cable factory goofed on the labelling and the test lead company picked up the whole reel cheap since the cable factory couldn't sell it at regular price.Ed