Someone wrote: I have a 50 ohm BNC coax connector on about 75 feet of RG6 75 ohm coax.

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Nulluser00
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Someone wrote: I have a 50 ohm BNC coax connector on about 75 feet of RG6 75 ohm coax.

Postby Nulluser00 » Thu May 16, 2019 12:05 pm

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Nulluser00
Posts: 31
Joined: Sat Jul 28, 2018 4:59 pm

Re: Someone wrote: I have a 50 ohm BNC coax connector on about 75 feet of RG6 75 ohm coax.

Postby Nulluser00 » Thu May 16, 2019 12:26 pm

Someone asked: "I have a 50 ohm BNC coax connector on about 75 feet of RG6 75 ohm coax. Is that RG-6 now 50 ohm coax?"

Nope, you've described the vast number of video interconnects for analog (NTSC) video....

I suspect there were at least a million 75 ohm analog video coaxial cable with 50 ohm connectors in the USA and Canada before digital TV came along.

Heck I'd almost be willing to bet there are a million 75 ohm analog video coaxial cables with 50 ohm connectors in the USA and Canada in use today with NTSCish security camera.

The technical specifications for NTSC were codified in federal law. I suspect very few analog security cameras, even those that are color, actually met the full NTSC specifications.

I know my three color analog security cameras don't even pretend to be NTSC. They will produce a very clear image on a traditional NTSC monitor....but they do not produce NTSC.

So NTSCish.

I happen to know a little about BNC connectors, when impedance matters and when it might not.

My high school was managed by feel good liberals who thought "mandatory voluntary community service" would be an help us "build our moral character."

[I'm not joking, my high school transcript actually says "completed "mandatory voluntary community service and received excellent reviews." I still have difficulty reconciling 'mandatory' and 'volunteer' shouldn't be used in the same week, much less in the same sentence....]

The suggestions were: painting dilapidated playground equipment, working at the community kitchen, picking up trash along narrow roads traveled by trucks loaded with logs---whose drivers had never heard of a speed limit, or, my least favorite of all, helping in a preschool/nursery.

I've got nothing against rugrats, I was one, my sister was one, and with luck, someday I'll father a few, but, given the litigious society we live in, I was unwilling to risk being accused of inappropriate contact.

So I cheated. I called the nearest PBS affiliate and volunteered to be an intern.
Washington PBS was still switching from analog TV to digital TV.

NTSC was the US analog broadcast TV format. It can be visualized as a near DC to 4MHz swath of RF.

Analog TV video was 75 ohm and putting even a short, 1M, segment of 50 ohm coax in a 100M run would mess up multiburst something horrible. Multiburst was a set of ever closer vertical lines that were used to quickly measure the video bandwidth.

I saw hundreds [really, at least 200 hundred] chassis mounted BNC female connectors on commercial equipment, equipment manufactured by everyone from Sony to RCA. [who invented NTSC]. Each and every BNC connector was 50 ohms.

I was tasked with disassembling an analog video control room. The equipment had many many coaxial cables. I can't claim to have viewed each and every 50 ohm female chassis mount BNCs, but I saw most of them.

The equipment was going to be sent to the scrap metal dealer and I removed at least 200 50 ohm BNC female connectors.

Hey they were free!

One might ask: "Well Mr. Smarty Pants how can you be so sure?"

It's really very simple.

50 ohm BNC connectors, both male and female have an plastic "tube" between the inner and outer conductors and 75 ohm BNC connectors don't.

BNC 50Ohm & 75Ohm.jpg
BNC 50Ohm & 75Ohm.jpg (88.83 KiB) Viewed 163 times


>All< of the older, analog video, which used 75 ohm video coaxial had 50 ohm connectors.

All.

Each and everyone.

The slight impedance discontinuity caused by the "wrong" BNC connector had no measurable effect on the video bandwidth.

Digital video might survive passing through a single cable with 50 ohm BNC connectors ... It might but I'd not want to put it to the test in any situation where it mattered.

Trust me, viewers and management get very upset over any down time. And no video for a TV facility is about as "down time" as it gets.

Digital video is unbelievable sensitive about every aspect of impedance.

Coax is coax is coax, except...for coax intended for digital video. The coaxial cable supplied by Belden for use with Digital video came in spools with individual measurement sheets that listed all the impedance discontinuities.

Each spool had a unique serial number with matching data sheets that, among other thing, had a printed spectral sweep it that cable.

It also cost about 5 times as much as "old style" 75 ohm video coax...

I was the gofer [go for this go for that] who helped a female engineer assemble ~50 100m 75 ohm coaxial cables for digital video.

The female engineer used 50 ohm BNC connectors.

The cables almost worked.

"Almost" meaning the video would be OK for a few seconds to minutes and then freeze or pixilate.

I asked the chief engineer: "What is the difference between 50 and 75 ohm BNC connectors," and he showed me.

It might help to remember I had just had my 15th birthday and knew next to nothing about BNC connectors.

I was extremely hesitant to mention that newest engineer had used the wrong BNC.

And yea all heck broke loose.

After the screaming was over, we cut off the 50 ohm BNC connectors and replaced them with 75 ohm BNC and the problems went away.

[The female engineer asked me why I didn't ask her. I was honest, she frightened me. Think Dykes on Bikes, tattoos, lots of visible body piercings, arm muscles bigger then my thighs. She really frightened me more then our high school quarterback who was a bit of a bully.]

Before screaming at your computer screen...

Consider RCA and GE used RCA connector for commercial 70cm radios to carry moderate power RF. I'm not certain, but, I doubt the RCA connector, which was designed to be the least expensive, yet serviceable, audio connector possible to use with phonograph turntables.

[correction, wv0h.com has in interesting technical article on the humble RCA phono connector and his data suggests the actual impedance is around 55 ohms. Who knew, I sure didn't. In class we were taught "The RCA phono connector is an exceptional cheap connector, it was designed for monophonic audio with a bandwidth of 30Hz to 15KHz. They are totally unsuitable for RF, video or serial data." Quote from my book on RF Connectors.]

Or consider the "UHF" connector.

The PL259 is the male and the SO259 is the female. They are and have been used in 50 ohm HF radios since WWII. They can have an impedance anywhere between 30 and 120 ohms, depending on the insulator, taper of the conductors and other variables.

We actually studied these in the EMI/EMC lab. Avoid $1 specials with phenolic dielectric. Forget VSWR bump, think extreme loss at 10MHz!

I wonder how many currently manufactured, high quality, amateur radio transceivers have SO259 RF output connectors?

Anyone care to guess?

My aunt/boss, who is a ham, said, "A better question might be how many full size HF amateur transceivers don't have a SO259 connector?"


Oh, and 50 and 75 ohm BNC connectors all use the same size male pin.
- - - - - -
Amphenol Connex 2001 states:
75 OHM SERIES

Within the internationally standardized BNC mating face dimensions, a perfect 75 ohm characteristic impedance cannot be realized. However, at frequencies up to 1000MHz, the small impedance deviation is negligible for practical applications. Connex true 75 ohm connectors with a typical VSWR reading of 1.06:1 at 2000MHz are identified by an asterisk following the part number.

All 75 ohm BNC connectors and 50 ohm BNC connectors are intermateable without restrictions.
- - - - - -

The cavity the coax center conductor fits into the male pin is >>!critical!<< for crimp connectors!

Please don't ask how I know, I have 5" of blue coax that defective connector installation framed in my radio room to remind me "Find out, ask before you do something."

If the male pin's cavity is too large, the crimp won't "grab" the wire, [BTDT] if the cavity is too small, the center wire will not fit. [BTDT also]

You >>!must!<< use the proper crimp tools with the proper dies to make BNC connectors!

Another thing, 75 ohm connectors are delicate. In the 16 weeks I was there, at least a dozen 75 ohm coax cables with 75 ohm BNC connectors were damaged and had to be replaced and several pieces of equipment with female 75 ohm BNC connectors had to be sent back to the manufacturer for repair.

Multiple layer PCBs are not easily "field repairable."

The plastic "tubes" of a 50 ohm BNC offer significant reinforcement.

One good thing about my volunteer experience was I ended up with gobs of "old" 75 ohm coax, 50 ohm BNC connectors, "Ts," "barrel," and, "right angle adapters", BNC to SO/PL259, BNC to TNC, N, "F", and "precision 75 ohm terminators" fabricated with 50 ohm BNC connectors.

In the old days of analog video, many devices had passive loop throughs which allowed daisy chaining video or waveform monitors, etc. The last device might have had a "termination switch" but more commonly required a 75 ohm termination to be placed on the loopthrough output. I'm guessing, because I never thought to ask, but I suspect the intrinsic input impedance of these devices was several thousand ohms. Consider the mismatch of 1000 ohms in parallel with 75 ohms.

Digital video terminal equipment, video or waveform monitors typically do not have loopthroughs, and when they do they are active. Which creates multiple failure points.

With physical loopthroughs it was easy to pinpoint problems, with active loopthroughs, every loopthrough is a potential failure point. We used digital video distribution amplifiers to avoid the need to use the active loopthroughs. YMMV


Digital video is finicky enough for a 75 ohm terminator in a 50 ohm BNC male to cause trouble. Trouble being: "No or intermittent video."

Anyone need a few dozen analog video 75 ohm in male 50 ohm BNC connector terminators?

My last few weeks there was spent chopping the 50 ohm BNC male connectors off cable that we couldn't pull.

The US NEC requires all unused cables to be removed. Ever try to pull long tangled coax cable from a rat's nest? The state fire marshal issued a waiver but insisted we removed as much as we could. That was hot, dirty [filthy] and exhausting work. And for all our work we were able to remove less then 10% of the coax under the floors or in the wall. Yea I know PVC burns and one would expect flammable cables to be routed sanely.

I also have more analog video 75 ohm coax then I'll ever need. It's double shielded, reasonably high quality and pretty useless to me.

One very nice gift were 3 Tromptor "video" patch bays. They all have 50 ohm BNCs, are rated for analog, NTSC, video, and work fine for RF up to at least 162.4MHz. I also have the hair pins, patch jack to BNC [50 ohm] female, more patch cables then I know what to with.

We switched from black 75 ohm coax to red for digital video, blue for digital audio, yellow for digital clock reference.

The station rented a mechanical coax stripper. Push the end of the coax in, and it is automagically stripped ready for insertion into the crimp BNC.

With the mechanical stripper, I could easily put two BNCs or three connector on per minute. We had one female engineer who could do 4 or 5 per minute. She wasn't the frightening female, she looked more like an elf then adult. [Hi Marie if you read this, thank you.]

Given we replaced every video cable in the studio and transmitter...that was a lot of cables and each cable required 2 BNC connectors...management, "The Suits," had a near meltdown over the "unanticipated" costs of upgrading.

Hint: If you ever have to plan any project, do your research, make your best estimate and then triple or quadruple it.

To be fair, the upgrade was never "let's take a moment and plan this out, it was ad hoc as management accepted "Oh that piece of equipment won't work?"

When I left the only analog video in the facility were the exterior security cameras and monitors. Except for the microphones, everything else was replaced.

They would have been money ahead to build a new studio, install everything, switch over at midnight, and demolish the old facility and haul everything to a landfill.

Hindsight, and not having any responsibility, makes that easy for me to say.

As part of my current daily work I have to review the figures generated by others for all sorts of projects. Upgrades are our biggest nightmare. I work with very smart, very experienced engineers and ever major upgrade has "unknowns."

And every unknown adds time delay and material expenses.

Crimped BNC connectors are extremely robust, old style BNC connectors are difficult and take a lot of time to assemble and, even if you exercise as much care as you can, fail way too often.


So back to the question that was asked: "I have a 50 ohm BNC coax connector on about 75 feet of RG6 75 ohm coax. Is that RG-6 now 50 ohm coax?"

Nope and it will not reliably pass digital video, or digital audio and probably not the digital clock signal.

However for any other practical use, including running off air ATSC [digital] UHF RF, the cable will function as well as a coaxial cable with the proper 75 ohm BNC connectors.

I verified this at work a few minutes ago but using the longest piece of "analog video cable" with a 50 ohm BNC male connector at each end to run RF from the three Lexington Kentucky commercial TV stations. I used a simple bowtie that has a "F" female connector.

Ant-"F" F>BNC BNC----BNC BNC>F F-TV.

It worked fine.

One last bit of inside info...
NTSC was reputed to stand for National Television Standards Committee but really stood for Never The Same Color. The Germans learned from our mistakes, NTSC has >many< design errors of fact, and went with PAL. They alternated the phase of color burst and corrected for multipath distortion. The French went their own way, with SECAM, which basically sent sequential red blue and green images, and let the human eye integrate the video.
Sort of like the failed CBS spinning color wheel system, that NASA used [?uses?] to reduce bandwidth.

Yea I learned a bit too much about TV. I learned several things interning at a PBS affiliate:
1)TV sucks
2) I will live under a bridge in a cardboard box before I work in any form of media.
3) And Satan will reconcile with God and turn hell into an ice skating rink before I give a single penny to CPB/PBS/NPR.

BTW, the technical people I worked with where great, they never treated me like the ignorant person I was, they always answered any and every question, when they didn't know they'd find out.

It isn't the technical staff who are the problem with TV. It's management and "talent." "Talent" on air "personalities" who have the egos of major movie stars and the talent of a 6th grade dunce.

One last point, if I were a SWL without the heirloom connectors I have, I'd be really tempted to use "F" connectors everywhere I could and fabricate a "F" male coax female RF connector my radio uses. Or, with the SDRplay series, I'd use "F" connectors for everything, SMA male to a thin, flexible coaxial cable to SMA male to SMA to "F".

"F" fittings offer "reasonable acceptable" performance up to 2GHz. They are inexpensive, easy to assemble and fairly robust if attention is given to their daily use.

And the higher quality, Ideal among others, are reasonably weather proof.

At the risk of being accused of being a shill, Ideal offers reasonably high quality F connector that are reasonably weather proof, coax stripping and crimp tools. In the USA, Lowe's and Home Depot have them.

Noel

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sdrom33
Posts: 79
Joined: Sun Dec 09, 2018 4:38 pm

Re: Someone wrote: I have a 50 ohm BNC coax connector on about 75 feet of RG6 75 ohm coax.

Postby sdrom33 » Wed May 22, 2019 8:52 pm

Hi Noel, I hope you will forgive me if i tell you that your posts are too long. You are probably very young and inexperienced, so it is a pity to see your long effort go most probably unread because one gets tired before reaching the end!

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