DIY 50 ohm BNC cables on a budget - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2019, 11:16:03 am
content: Not a HAM, just tinkering out of curiosity with basic RF circuits, no Tx, less than 100MHz, low power.Bought some cheap, no name, RG58 cable + BNC connectors + crimp tool, in order to make a few BNC cables for the signal generator, scope, etc. So far, lousy results, that's why I'm asking.What will be the expected price for reasonable quality materials, amateur level, less than 1 GHz, low power- BNC price?- cable price?- crimp tool price?

author: tszaboo
author_rank: Super Contributor
timestamp: June 06, 2019, 11:41:39 am
content: The idea is, if you end up with a pricing larger than this:https://nl.farnell.com/amphenol/779829-58-2-0/lead-bnc-plg-plg-50r-2m/dp/1261918?st=plugSo a few EUR per cable, than it is probably cheaper to buy. And then you don't have dicky connectors.Check RF performance of the cable before you buy it though. It wont be on Farnell, but probably on the manuf. website.

author: hexreader
author_rank: Frequent Contributor
timestamp: June 06, 2019, 12:45:15 pm
content: In my experience - much cheaper to buy than make.In fact, I buy 10 metre BNC leads as being cheaper than the same length of RG58 cable, with two connectors ready-made as a bonus.Ready made cables are nearly always more reliable than home made ones.I do make my own cables, but only when no ready-made option is available, or if I don't want to wait for delivery.100MHz has never been a problem for me for hobby use - even using cheap RG58 and cheap crimp connectors. If you ever get to SMA connectors and/or thinner co-ax cable, then microphonic effects become an issue with cheaper cable.YMMV

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 07:51:51 pm
content: I have gotten excellent results with RG-400 (stranded) and RG-142 (solid) which are double shielded Teflon cables with the size of RG-58 using clamp connectors. I have also gotten good results with RG-316 which is RG-174 size but with a Teflon dielectric and crimp connectors.My RG-58 results have been mixed. RG-58 cable quality varies considerably. Forget RG-8X; it is more flexible and lower loss but easily damaged. RG-223 should be nice but why bother if you can use RG-400 instead?If you use crimp connectors, make sure to get a good crimper.

author: xavier60
author_rank: Super Contributor
timestamp: June 07, 2019, 02:07:15 am
content: There is a thread about loose BNC braid crimps being caused by thin ferrules,https://www.eevblog.com/forum/beginners/loose-crimp-sleeve-(outer-ferrule)-on-bnc-ferrule-after-crimping-(rg-58)/msg2203350/#msg2203350

author: radiolistener
author_rank: Super Contributor
timestamp: June 07, 2019, 04:22:55 am
content: I bought this one:https://www.aliexpress.com/item/Sale-100cm-RG58-Cable-BNC-Male-Plug-To-BNC-Male-Straight-Crimp-Coax-Pigtail-3ft-High/32596403751.htmlI'm using it for 0-80 MHz, works great. Also sometimes used it up to 500 MHz, also works ok, may be a little higher loss, but it doesn't matter for me.Also I tested it for impedance and it is very close to 50 Ohm.In comparison, other Chinese cables, such as LMR195, RG316 and other doesn't match well 50 Ohm.

author: hagster
author_rank: Frequent Contributor
timestamp: June 07, 2019, 04:55:54 am
content: Why do you think the results are lousy?Its hard to make a bad cable at less than 100MHz. Yes the quality of RG58 is very changable frim brand to brand. Some have very sparse coverage for the outer conductor. But for signals less than 100MHz you need really long runs to see much difference with more expebsive cable.

author: bob91343
author_rank: Super Contributor
timestamp: June 07, 2019, 06:06:29 am
content: I have had reasonable success with Chinese cables on ebay, maybe $2 shipped for a 3 foot long cable of RG-58 with BNC male on each end.For short runs, cable quality isn't too important.

author: radiolistener
author_rank: Super Contributor
timestamp: June 07, 2019, 12:18:14 pm
content: Quote from: hagster on June 07, 2019, 04:55:54 ambecause I have two 1 meter LMR195 Chinese cables, and they both have SWR about 1.5 on 27 MHz and significant loss on frequencies more than 5 MHz. They looks good, soldered central pin and crimped braid. I tried to investigate what is wrong with them and found that they have normal capacitance, but twice higher inductance. I don't have idea why...It is just 1 meter length and affects 7-30 MHz very significantly. Higher frequency higher SWR, it is completely unusable at frequencies above 30 MHz.

author: Kalvin
author_rank: Super Contributor
timestamp: June 07, 2019, 12:38:00 pm
content: Quote from: radiolistener on June 07, 2019, 12:18:14 pmIf the cable is 75 ohm, it will give SWR of 1.5.

author: CJay
author_rank: Super Contributor
timestamp: June 07, 2019, 01:41:49 pm
content: Quote from: radiolistener on June 07, 2019, 12:18:14 pmIf they're that bad they're useless so cut one up, strip back the insulation, outer and show the shielding, strip back the inner to show the core, take the plugs apart, post pics.

author: bob91343
author_rank: Super Contributor
timestamp: June 07, 2019, 04:03:57 pm
content: Yes some of the Chinese cables are not 50 Ohms. Again, for a short run it's not important but if you are working at frequencies where the electrical length is more than about one-tenth of a wavelength, it can be a problem.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 07, 2019, 07:02:00 pm
content: At this moment I think the purchase was not only bad, but anecdotally bad.Maybe I went too cheap, IDK, but those where the only parts they had for RG58 BNC crimping:- BNC male connector for crimping on RG58 type cable - about $0.47 / piece- RG58 type, 50 ohm cable, brand "ALIEN RG58 HIGH QUALITY COAXIAL CABLE 50 OHM" - total $7.06 for 30m (100 feet)- BNC crimp tool, brand "KEMOT", $17.88Went to the a brick and mortar shop, payed with cash and went home in a hurry to finish what I was tinkering. Yeah, I know this is not the way to shop for parts, but I was weak that day, and too eager to play with a DIY directional coupler.Looked at the w2aew's video of how to crimp a BNC, easy-peasy! Crimped my first BNC. Well, it turned out to be more complicated that I though it would be:- at first, the middle pin refused to crimp.No matter how hard I squeezing it, the pin was keep falling down under its own weight. Adjusted the ratcheting tool to crimp tighter, and tried again. This time was very hard to get to the last click.Still no luck.As a last resort, I crimped the middle pin by carefully squeezing it with a diagonal cutter...- moved on to the ferrule crimping.This time it crimped too tight, almost piercing the ferrule (ratcheting tool setting was to the max, left from the previous unsuccessful middle pin crimping).Anyways, even at normal settings the ferrule's hexagon shape is still snapped inside, like in the pics, just that it snapped less.At this moment I started to look closer at each itemThe middle pin had a too bigger hole, probably for stranded AWG20, IDK, while my cable core was solid and measured close to an AWG22. Note that the middle pin does not have a lateral hole, so most probably it was made for crimping, not for soldering.The ferrule looked thin, and indeed it doesn't keep the strand well. The crimped shield can be easily pulled back.So yeah. Too large and too thin ferrules (6mm/0.22mm aka 236mil/8.5mil or thou), or maybe the crimp tool hexagon is too small? Good question, what size is the crimping die?- looking closer to the crimping tool.On the back of the tool's blister it is a printed table with all the available crimping dies for my tool. Nice. Yet, my tool's die sizes doesn't match any combination from that table. What's the most scaring is the error specified at the top of the table: +/-0.1 !!! I just hope they were talking millimeters and not inches.On the die itself it is stamped ".256", ".318", ".068" and the most inner hole, ".218", supposedly for RG58. Yet, the .218 measures .203 to .205 depending on how hard are the jaws squeezedEven more, the clicking lever is bumping into the plastic handle by mistake, somewhere before the last 2-3 clicks. At first I thought that might be some kind of feature, but then I saw the dent in the green upper plastic handle. That dent in the following pic, near the clicking mechanism, proves it. Fixed that problem by tying the clicking lever with that brawny rubber band around the lower handle and the lever. Now the lever is still clicking but the rubber band is guiding it to the middle shaft, so it doesn't bump into the plastic upper handle any more.OK, at least the cable was nice and cheap, double shielded, looks like Copper.But no, it wasn't copper. All metal there, the core, the shield foil and the shield braid are all Al with Cu finish. Scratching the Cu finish shows it's all just aluminium, including the shielding, too.But wait, there is more!- measuring the AC and DC parameters of the cableR core wire = 10 ohm, for all 30m (100 feet)R shield = 3 ohmC = 1450 pF (48pF/m)To measure the AC parameters, let's improvise a TDR (Time Domain Reflectometer): a signal generator is outputting a square wave, the wave is traveling through the cable under test, it is reflected by the end of the cable, and travels back to the generator, where it is probed with an oscilloscope, like in the next pic.At the T connector is the cable under test. The other cable plugged in the generator is just to sync the oscilloscope.On the other side of the cable under test is a terminator with a variable resistor (100 ohm trimmer):When the terminator is left open, the wave will be 100% reflected, and the reflected voltage will add up with the generator's voltage at the generator side of the cable. When the terminator is a short circuit, the wave will be also 100% reflected, but this time it will subtract from the generator's voltage. If the trimmer is trimmed to the same value as the cable's impedance, then all the wave's energy will be transferred to the terminator's trimmer, and nothing will be reflected.OK, let's practice at first with another known as good cable, a 4m one (yellow trace is just for sync and reference, blue trace is the signal reflected back from the cable.First, with a proper 50 ohm terminator:Now, the same cable left open:Let's measure the delay for the wave traveling to the open end of the cable and back (so a distance of 8m in total), the delay is 38ns:Now, let's see how it looks when the cable is terminated in a short circuit:Now, that the method and the setup is tested and working, let's measure the unknown 30m (100ft) cable.Terminator open:Terminator open, delay 232 ns:Terminator short:Terminator 50 ohm, SURPRIZE:The bump is noticeable, definitely not a 50 ohm cable. Let's trim the resistance value of the terminator for the best flatness:Measuring the terminator with a DMM, 85 ohms. The cable impedance is 85 ohm, NOT 50At this point, it was "whatever", let's terminate the cable with 85 ohms and read the voltage on the terminal. The other end was fed this time with a sinusoidal signal, frequency swiped (linear sweep) from 1 Hz to 100 MHz:At the lower frequency the amplitude is visible higher, let's do another sweep from 1 Hz to 10 MHz:In conclusion, bad crimpers, bad connectors, fake 85 ohm cable marked as 50 ohm, advertised as Cu and made of Al.DIY BNC cable, 30 m, brand ALIEN 50 ohm, measured impedance ~ 85 ohmTDR terminator open, v = 60 m / 228 ns, velocity factor = 0.877R tot core = 10 ohmR tot shield = 3 ohmC tot = 1450 pFRG58 brand "ALIEN"================Core = AWG 22, Al with Cu finishShield = Al foil with Cu finish + Al braid with Cu finishR core = 0.33 ohm/mR shield = 0.10 ohm/mC = 48 pF/mZ = 85 ohmVF = 0.877

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 11, 2019, 10:59:13 am
content: I have to agree with others, for BNC, there is no point trying to do it yourself, unless you want to make them to an exact length (EG, back of bench setup for clock distribution or something). SMA is another matter, where acceptable SMA connectors can be had from reputable brands like Amphenol for a few bucks, that should work decent.I have ordered a Aliexpress crimping too myself, will see how it goes. I mainly want to use this (together with some leftover RG58 from belden that I got at work) to make some cables for clock distribution and low frequency signals at home. Most of those SMA connectors have a mating lifetime spec of 100 cycles, but should be just fine for low-frequency stuff.

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 11, 2019, 11:43:44 am
content: I use Canare BNC connectors for my cables and Japanese made RG56 coax.The crimper and die costs around $140 or so, so not low budget, but not ridiculous if you want to make a decent amount of cables.The connectors are top quality and stainless steel with the usual gold plated center pin.I've made a stack of BNC-BNC cables as well as BNC-alligator/test clip/etc leads so far and they all work great.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 29, 2019, 11:29:23 am
content: Quote from: xavier60 on June 07, 2019, 02:07:15 amThanks for the critical tip about the too thin ferrules.Went to the electronics brick shops street again (yes, there is a street here in Bucharest/Romania, full with about 10-20 small shops sprinkled at only feet away of each other, all with electronic parts, connectors, tools, etc. - it's like a red-light district, but with parts instead).This time took with me a strong magnifying glass and small diagonal cutters, so to identify coax cables that are not Cu, also took a sample of a bad (too thin) ferrule from the BNC connectors bought last time. Decided to buy only known brands.Bought this:$40 (35 Eur) - crimping tool Pro'sKit 6PK-230PA, 1 crimping die included, made in Taiwan, looks and feels like a good tool and the metal looks like quality carbon steel indeed -https://duckduckgo.com/?q=Pro%27sKit+6PK-230PA&iar=images&iax=images&ia=images$0.42 (0.37 Eur) per pcs. - RG58 BNC crimping connectors, made in China but at least they have a part number, WTY0474-1, and the ferrules are much thicker than the ones bought before$0.48 (0.42 Eur) / meter (3 feet) - 50 ohm RG58 cable brand 'Well', measured and it is 50 ohm indeed, Cu stranded core, Cu mesh + Al foil shieldVery happy with the match between cable/connectors and the crimping tool. Crimped BNC looks perfect from the first try, totalprice for a 1m (3feet) DYI BNC cable, $1.32 (1.16 Eur), and less than 50 cents for any extra meter (3 feet) long.It's easy to DIY BNC cables when using decent tool and parts, very rewarding.The good ferrule is 0.51mm (20 thou) thick, the bad one is 0.21mm (8 thou) thin.Left side: good RG58 BNC, outer ring with two deep enough dents, middle ring split, thick ferrule, center pin with lateral inspection/soldering hole, pin hole much larger thus crimp-able pin wallsRight side: ugly low quality BNC, middle ring solid, outer ring internal shafts not deep enough (the two lateral metal bumps from any mating pair does not fit well in those not deep enough shafts), ferrule too thin, center pin without lateral inspection hole, pin hole much smaller thus the pin wall is too thick to be crimped, overall quality unusableLeft side: thick ferrule crimps OK, with cable's shield and outer isolation properly anchored to the connector.Right side: thin ferrule almost snapped when crimped, cable shield slips away from connector if pulled back.Yay, finally, no BNC cable shortage!Thank you all for the help.

author: bob91343
author_rank: Super Contributor
timestamp: June 29, 2019, 05:26:51 pm
content: Good work, and good information! While it appears that, with enough effort, one can make good cables, I think I will stick with purchased ones. However I did that once and found the cable not to be 50 Ohms. I complained and got a refund. So I suggest to everyone who buys these to test each one upon receipt and demand correct operation.As indicated, TDR isn't black magic; a simple enough setup does the job. The main thing is to make sure the pulse rise time is short enough to allow time for the signal to make the trip to the end and back.Also, when adjusting the termination resistance, do so carefully so as to make the measurement accurate. A few Ohms off still looks fairly good so be critical. I have made repeated measurements that varied about 10 Ohms until I got the hang of it. And when you disconnect the pot to measure it, make sure you don't move the adjustment. It might be better to measure from the other end of the cable but then you are including cable resistance and mating part contacts. Whatever setup works for you is the one to use.Now to the inevitable (and perhaps insoluble) issue of how to store these cables. If I need a 2 meter cable, where do I look and how do I recognize it by its length? And some connectors are balky regarding mating and unmating. The females on my test gear all seem very good so the problem has to be with the male cable ends.

author: tkamiya
author_rank: Super Contributor
timestamp: June 29, 2019, 09:34:19 pm
content: I had a couple of BNC cables which outer part (part that you twist) was just a little too long. That made attaching it to female side very very difficult. That was the only problem I ever had with BNC. I don't even remember where it came from. Attaching and detaching BNC should be easy. If not, there is something wrong.About the length, you can label it if you must? I use zip ties with tab on them to label cables when necessary. I don't take any special care with BNC or BNC cables. They are pretty durable. SMA is entirely different story.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 30, 2019, 10:30:12 am
content: Pieces of drinking straws inserted near connectors works great to tile the cables. The advantages are many:- have about the right diameter- can be inserted while making the cable, or afterwards (cut the straw by a spiral line)- very cheap- can be handwritten with a marker- easy to replace/remove- usually they come in many colors in the same bag, so the colors can be used as a code for the cable's length, or simply to distinguish each cable goes where.

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2019, 09:04:36 pm
content: These days you can get shrink sleeve printers and kill 2 birds with one stone.Sleeve the ferrule like a commercial cableandlabel it.http://www.dymo.com/en-NZAnyone with an electrician or network HW specialist friend should be able to borrow their sleeve printer for a couple of cables.Some cables I get are properly characterized and come with Cal sheets and the cables are serial numbered with printed sleeves to match the Cal sheet #.