Look at this 50 ohm terminator. This isn't right is it?

[miguelvp]

I use the same T-connectors regardless if i'm using 75 Ohm cables and terminators, or 50 Ohm cables and terminators.
They don't make different T-connectors for different frequencies so they must be physically the same.

Any variation might be vendor specific.

[Richard Crowley]

Because of the absence of the white plastic dielectric mechanical support, the center contact of the female BNC connector is very fragile even when mating only with proper 75-ohm male BNC connectors.  I have had to use long-nose pliers to "reform" the contacts in many of my HD-SDI video gear.  75-ohm BNC connectors (of both genders) are significantly more fragile than the 50-ohm variety IME. 

I have three $300 lengths of special flexible RG-6 cable which I use between my cameras and the video switcher.  Alas, when we set up the mobile system last week, one of those expensive cables is dead.  Now I have to troubleshoot it to find out what went wrong.  The connectors are fragile to begin with, but any improper kinking of the cable will deform the eye pattern bad enough to kill HD-SDI digital signals dead.

I am ordering a 75-ohm version of the W2AEW pulse generator to use TDR to try to find the cable fault.
https://www.tindie.com/products/land_boards/cable-tester-fast-pulse-generator-50-ohm/

[miguelvp]

$300 cables? wow. And I complain when a coax is more than $1.50 per foot

[SL4P]

GEEK NOTE:
This is interesting to see a very minor problem that I had always been aware of - finally come into discussion.  It has always irked me that 'manufacturers' choose to use the wrong part - because it's cheaper or more convenient for them , and the dumb customers won't know any better... regardless of any chance of ''doing the job properly'' the first time. Finally !

This is often the reason first-world products are better made and more reliable than second- or third-world goods that are not specified or supervised by more experienced product teams.

The inferior products are not necessarily a bad idea or done wrongly - they are just made badly.

[SeanB]

If you want odd terminators look at waveguide terminators, which are basically a tapered waveguide filled with an epoxy and ferrite powder mix. They get nice and toasty in use with applied power.

[OilsFan]

OP here. Haven't been in the forum for a while but it seems some interesting discussion ensued. Just to clear up my case, that terminator in the photo I posted was 50ohm. It was stamped in the metal on the back and had a green label stating 50 ohms. I went and exchanged it for a new one and all seems fine now.

[macboy]

They are almost compatible.

The centre conductor pin diameters are different and from memory the 50 Ohm pin is likely to damage the 75 ohm female part of the connector and deform it beyond 75 ohm spec.

You may be thinking of N type RF connectors. Those are not mating compatible between 50 and 75 ohm versions.
BNC OTOH, differs only in the presence or absence of the dielectric in both the male and female ends. BNC was designed as 50 ohm and with the dielectric present, but it was discovered that leaving it out (using air instead) made it very nearly 75 ohms, so no other mechanical redesign such as pin diameter was necessary for the 75 ohm variant (unlike the N type).

Attached is a little picture I just took of a 75 ohm BNC coupler and cable with 75 ohm connector, plus 50 ohm variants for comparison. I have scores of 75 ohm BNC stuff because it tends to get dumpstered when Telcos redo their labs.

[IconicPCB]

While absence or presence of dielectric may indicate impedance level, there are slight differences between pins which do not preclude mating. Once mated ... well you know how it is.. pins may become distorted and structural return loss will be degraded meaning the connector is out of spec.

[edavid]

Because of the absence of the white plastic dielectric mechanical support, the center contact of the female BNC connector is very fragile even when mating only with proper 75-ohm male BNC connectors.  I have had to use long-nose pliers to "reform" the contacts in many of my HD-SDI video gear.  75-ohm BNC connectors (of both genders) are significantly more fragile than the 50-ohm variety IME. 

This is why it's better to use 50 ohm connectors for non-critical 75 ohm applications, like analog video.

While absence or presence of dielectric may indicate impedance level, there are slight differences between pins which do not preclude mating. Once mated ... well you know how it is.. pins may become distorted and structural return loss will be degraded meaning the connector is out of spec.

Do you have any evidence for this claim, like a manufacturer's statement?  As far as I know, it is not true.

[IconicPCB]

Personal experience.

I used to work in what was known as HP product type 01 ( analogue instrumentation maintenance and cal dc-40GHz) in early eighties .

Also Google listings point to this difference and use of different dielectric materials in 50 and 75 ohm fittings.

[macboy]

While absence or presence of dielectric may indicate impedance level, there are slight differences between pins which do not preclude mating. Once mated ... well you know how it is.. pins may become distorted and structural return loss will be degraded meaning the connector is out of spec.

There is no physical difference between the mating surfaces of 75 ohm and 50 ohm. The slightly pointed vs. rounded tip of the pin seen in the photo is of no consequence, just a matter of preference of the manufacturer. This datasheet from Amphenol/Connex details all mechanicals of their 50 and 75 BNCs. There is no difference noted for any metal surfaces between 50 and 75 ohm variants. And this manufacturer states that "It is implicit in IEC 169-8 that 75 ohm BNCs made to comply with that standard will mate in a non-destructive manner with the 50 ohm BNC connectors described in IEC 169-8.... In over 15 years and a many million BNC connectors we have no first hand experience of incompatibility between 50 ohm and 75 ohm types, other than extremely rare (and very obvious) manufacturing faults."

Again, N type connectors are definitely not physically compatible between 50 and 75 ohm variants, and this leads a lot of people to believe, erroneously, that the same is true of BNC.

[IconicPCB]

Non destructive mateable does not mean identical.

While the catalogue listing may make that claim it does not support it with a dimensioned drawing of coaxial parts of the connector.

I would need to to see the dimensioned drawing in order to accept identical mechanical structures are involved.

[SL4P]

OK - now we have that out of the way...  make sure your connectors and cable are matched.    Solid core vs stranded, foil vs braid, diameter etc...

[flolic]

Ever heard of 5 ohm terminator? 

Few days ago I received SMA 50 ohm terminator from *bay (item no. 380319202322) which behave very strangely during some RF tests. Quick check with DMM showed it is actually 5.1 ohms resistor inside 

Words of wisdom, do not buy chinese crap, or if you do, triple check it...

[Howardlong]

Ever heard of 5 ohm terminator? 

Few days ago I received SMA 50 ohm terminator from *bay (item no. 380319202322) which behave very strangely during some RF tests. Quick check with DMM showed it is actually 5.1 ohms resistor inside 

Words of wisdom, do not buy chinese crap, or if you do, triple check it...

LOL! Do you have a picture of the mini tear down by any chance?

[flolic]

I will dissect it and post pictures when I get home 

[tautech]

I will dissect it and post pictures when I get home 

Hacksaw? Grinder? Big hammer would be my recomendation. 

Get some decent 50 Ohm Feed-throughs and be done with it.
It doesn't take much of a demanding waveform to demonstrate how poor the performance of tee's and terminators is. 
Compare and see for yourself.

Got my pair of Tek 1x & 10x many years ago, think I paid US$30 for them both off eBay and it sure hurt at the time.
But quality lasts and knowing that, the price is forgotten in time.

Or a scope with 50 Ohm inputs. 

[Howardlong]

Not sure if you'd be intending to use an SMA terminator for use as a scope terminator, more likely it's for an RF application.

[flolic]

Hacksaw? Grinder? Big hammer would be my recomendation. 

Haha, I would like to use hammer, but I also want to see what's inside.

Get some decent 50 Ohm Feed-throughs and be done with it.

I have one of these (http://uk.rs-online.com/web/p/rf-terminators/5464695/), it's enough for my scope needs.

[flolic]

As promised, tear down images 










To be honest, I expected worse than this. Center pin is soldered to resistor and plastic insulator is made of PTFE. It does not melt when heated with soldering iron.
Resistor ground connection is by contact pressure.
There is only one "small" problem, wrong resistor value 

[Richard Crowley]

There is only one "small" problem, wrong resistor value 

50 ohms is NOT a standard resistor value, even in the E192 series

[c4757p]

Richard: he said 5.1, not 51!

Also, SMA is usually used at rather high frequencies - no way in hell that cheapo axial is still a resistor above 100 MHz or so...

[Richard Crowley]

Richard: he said 5.1, not 51!

Also, SMA is usually used at rather high frequencies - no way in hell that cheapo axial is still a resistor above 100 MHz or so...

Looks like Green - Brown - Black to me.  = 51 ohms.  Who said anything about 5.1?  That would be absurd.
I would have expected better "termination" of the ground side of the resistor, though.   

If that resistor is 5.1 ohms, then is is marked incorrectly.

[c4757p]

Green-brown-black-silver-brown, dude! 510 * 10^-2 ± 1%

[flolic]

Yap, that's 5.1 ohm resistor