50ohm Vs 75ohm bnc connectors

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50ohm Vs 75ohm bnc connectors

Postby Southwales » Thu Jan 25, 2018 1:51 pm

Hi, I never realised there was 50 and 75 ohm bnc connectors, I have just made my own antenna and used 75ohm cable, I used an f plug and then an adaptor to go to bnc, I think the adapter I purchased could be 50ohm, http://www.ebay.co.uk/itm/4-Pack-Of-BNC ... 2749.l2649 will this cause he signal loss? Or does this only effect digital signals?

I would not have questioned it but came across this article why trying to find out the difference between 50 and 75 ohm cable http://cablesondemandblog.com/wordpress ... ial-cable/

Finally, one last crucial point in regards to coaxial cables. The Impedance of the various devices being connected as well as the Coaxial Cable itself must match. So if you are, for instance, connecting a 75 Ohm video camera connection to a studio monitor, the coaxial cable must also be 75 Ohm AND the connectors on the coaxial cable (i.e. BNC connectors) must be 75 Ohm in Impedance. Every single time you have a mismatch in impedance, say between a 50 Ohm Coaxial Cable and a 75 Ohm Coaxial Connector (i.e. BNC), a standing wave develops.




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Re: 50ohm Vs 75ohm bnc connectors

Postby m0lsx » Thu Jan 25, 2018 3:24 pm

It's not a easy answer, as antenna science gets very complicated, very quickly, when you start to look at it's details.
The following is an approximation of what exists.
There will be a mis match when using 75 Ohm Coax on a radio that expects to see 50 Ohms. But the way that the impedance that a radio wants to see, is calculated, is not an easy or straight forward calculation. There are different ways of reaching that impedance.
So for simplicity, we just accept that radios want to see 50 Ohms. As that is the number that manufacturers give us. Not because it's an absolute & definitive figure.
Also if you are going to be using an antenna on a section of spectrum, not just a spot frequency, then there will always be a variation across that spectrum in what the impedance is at the feed point of the radiator.
Antennas such as Amateur Radio & CB, use loading coils which present as close to 50 Ohms as possible, but loading coils can often loose more energy than the mismatch it avoids.
In general on transmit we try to avoid SWR's of over 3:1. In simple terms, SWR is a measurement of the mismatch. A perfect match is 1:1.1, but normally anything below 1:1.5 across a whole band is seen as very good & 2:1 on the edges of the band, is nothing that most sane people worry about. And in the real world the difference between 75 Ohm & 50 Ohm's is only around 1:1.4 & a Dipole antenna (in free space) is 72 Ohms. But the ground & the real world effects this sum. So as you can see the deeper you look, the more complicated it gets. But in real terms, on a RX or TX 75 Ohm coax is fine.

Edited to add.
I hope I have not over complicated or over simplified things. I wanted to give more than a, it's fine answer, with no explantation of why. But also not confuse the issue either.
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Re: 50ohm Vs 75ohm bnc connectors

Postby Southwales » Thu Jan 25, 2018 6:58 pm

m0lsx wrote:It's not a easy answer, as antenna science gets very complicated, very quickly, when you start to look at it's details.
The following is an approximation of what exists.
There will be a mis match when using 75 Ohm Coax on a radio that expects to see 50 Ohms. But the way that the impedance that a radio wants to see, is calculated, is not an easy or straight forward calculation. There are different ways of reaching that impedance.
So for simplicity, we just accept that radios want to see 50 Ohms. As that is the number that manufacturers give us. Not because it's an absolute & definitive figure.
Also if you are going to be using an antenna on a section of spectrum, not just a spot frequency, then there will always be a variation across that spectrum in what the impedance is at the feed point of the radiator.
Antennas such as Amateur Radio & CB, use loading coils which present as close to 50 Ohms as possible, but loading coils can often loose more energy than the mismatch it avoids.
In general on transmit we try to avoid SWR's of over 3:1. In simple terms, SWR is a measurement of the mismatch. A perfect match is 1:1.1, but normally anything below 1:1.5 across a whole band is seen as very good & 2:1 on the edges of the band, is nothing that most sane people worry about. And in the real world the difference between 75 Ohm & 50 Ohm's is only around 1:1.4 & a Dipole antenna (in free space) is 72 Ohms. But the ground & the real world effects this sum. So as you can see the deeper you look, the more complicated it gets. But in real terms, on a RX or TX 75 Ohm coax is fine.

Edited to add.
I hope I have not over complicated or over simplified things. I wanted to give more than a, it's fine answer, with no explantation of why. But also not confuse the issue either.


So do you think I am okay to stick with my 50ohm adapter on my 75 ohm coax, what about the part of the article mentions the missmatch plug will cause a standing wave? Also I thought 75ohm was not suitable for transmission?

I am very impressed though with the performance of my homemade antenna even if there is a missmatch, I just need someone to look at the image of my coax connection to the electrical block to see if I have done it correct. viewtopic.php?f=2&t=6015&start=20
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Re: 50ohm Vs 75ohm bnc connectors

Postby G4RMT » Thu Jan 25, 2018 7:49 pm

In my job I have annoyingly to have to deal with both 50 and 75 Ohm cables and connectors.

For many years, people banged on about mechanical problems when 50 Ohm connectors were put into 75Ohm sockets. This was because the centre pin diameter of the 50Ohm types are thicker. The video versions have a much thinner pin, and the sockets have a 'hole' expecting the thinner version. People believed that the thicker pin would bend the wipe contacts and then plugging in a correct thinner connector would make poor or intermittent contact. I believed this for years until more research showed that this expected damage didn't happen.

Accidentally using a 3m video cable to go between a handheld and an antenna cable reduces power output because the VSWR rises quite steeply with the cable mismatch, and the radio lowers the output. The 3m cable drops the received signal a tiny bit - but it works well enough to not let me see the problem straight away.

The biggest problem with BNC, and TNC connectors (same thing but twist rather than bayonet) is their ability to remain reliable when the cable is the wrong type. The old style BNC connectors need very specific cable diameter - both on the centre conductor, but also on the braid. Some types, with the aluminium top hat that slides up under the outer braid works poorly with cables with a copper drain and much lighter coper braid. Satellite cable makes a rotten fit onto BNCs designed with conventional braided cable, and repeated inserts and disconnections soon cracks the copper foil, leaving the thin whisky braid the only connected part of the circuit and this then can't take the strain and soon fails. You can get plastic end protection which prevents the last few inches bending - an. d on the wrong cable, you can use these and glue the cable into them if they're a bit loose.

For scanners - or receive only, satellite cable offers low price and small diameter - but the stuff is installation cable, it is not designed to be patch cable. Cheap inter-series adaptors all add a bit of loss, and this is the only place BNC to BNC connections are less good. The tolerances of these connectors mean that even when connected they frequently are wobbly and prone to cause grief. If you make a long connection of these devices - maybe a dozen, and put them on a meter - the resistance changes visibly when you wiggle them. Do it with quality inter-series ones and you get a much more stable result.

Correct connector and correct cable for transmit and any connection where gain is critical. Bodges are fine for things where impedance, low loss and reliability don't matter so much.

Keep in mind that in commercial radio systems, even connectors that appear good, cost a lot but have plating - maybe nickel over aluminium are looked at as potentially dodgy - poor connectors can increase interference to other users on a tower. Many comms sites in their contracts ban these kinds of potentially destructive connectors!
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Re: 50ohm Vs 75ohm bnc connectors

Postby G4RMT » Thu Jan 25, 2018 7:49 pm

In my job I have annoyingly to have to deal with both 50 and 75 Ohm cables and connectors.

For many years, people banged on about mechanical problems when 50 Ohm connectors were put into 75Ohm sockets. This was because the centre pin diameter of the 50Ohm types are thicker. The video versions have a much thinner pin, and the sockets have a 'hole' expecting the thinner version. People believed that the thicker pin would bend the wipe contacts and then plugging in a correct thinner connector would make poor or intermittent contact. I believed this for years until more research showed that this expected damage didn't happen.

Accidentally using a 3m video cable to go between a handheld and an antenna cable reduces power output because the VSWR rises quite steeply with the cable mismatch, and the radio lowers the output. The 3m cable drops the received signal a tiny bit - but it works well enough to not let me see the problem straight away.

The biggest problem with BNC, and TNC connectors (same thing but twist rather than bayonet) is their ability to remain reliable when the cable is the wrong type. The old style BNC connectors need very specific cable diameter - both on the centre conductor, but also on the braid. Some types, with the aluminium top hat that slides up under the outer braid works poorly with cables with a copper drain and much lighter coper braid. Satellite cable makes a rotten fit onto BNCs designed with conventional braided cable, and repeated inserts and disconnections soon cracks the copper foil, leaving the thin whisky braid the only connected part of the circuit and this then can't take the strain and soon fails. You can get plastic end protection which prevents the last few inches bending - an. d on the wrong cable, you can use these and glue the cable into them if they're a bit loose.

For scanners - or receive only, satellite cable offers low price and small diameter - but the stuff is installation cable, it is not designed to be patch cable. Cheap inter-series adaptors all add a bit of loss, and this is the only place BNC to BNC connections are less good. The tolerances of these connectors mean that even when connected they frequently are wobbly and prone to cause grief. If you make a long connection of these devices - maybe a dozen, and put them on a meter - the resistance changes visibly when you wiggle them. Do it with quality inter-series ones and you get a much more stable result.

Correct connector and correct cable for transmit and any connection where gain is critical. Bodges are fine for things where impedance, low loss and reliability don't matter so much.

Keep in mind that in commercial radio systems, even connectors that appear good, cost a lot but have plating - maybe nickel over aluminium are looked at as potentially dodgy - poor connectors can increase interference to other users on a tower. Many comms sites in their contracts ban these kinds of potentially destructive connectors!
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Re: 50ohm Vs 75ohm bnc connectors

Postby Southwales » Thu Jan 25, 2018 9:05 pm

Hi, I certainly always felt that the play in a bnc connection could make it unreliable, I take it there are not many tx radios that use bnc, I would imagine a poor connection could cause damage to the transceiver?

Could I reliably connect this bnc cable to my pf100 coax as a stress relief, and if so what would be the best way? I Know it is thin but would there be much signal loss with it only being the last few inches? https://www.gearbest.com/ip-cameras/pp_690229.html

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Re: 50ohm Vs 75ohm bnc connectors

Postby G4RMT » Fri Jan 26, 2018 10:10 am

Solder and tape, or a bodge of connector strip and tape. For RX, it should be fine.

On the radios with BNCs - you are working ion the assumption that SMA connectors are old, they're not! ALL radios since the 1970s to quite recently had BNCS, or TNC. I've still got piles of my old Icom and Yaesu radios for Air Band, Marine Band, Commercial and Ham bands all with BNCs. They are hugely tougher than SMAs which were never intended for what we do with them nowadays. They snap. They have always snapped. They were never designed for mechanical strength just RF performance. BNC connectors were always stronger = but they're now considered too big. SMAs have trouble with cable sizes too.

Analogue video is of course dying out, with digits being the norm. Serial digital is now the norm, but with HD-SDI, the cable type and connector spec is getting more vital. With digits, reflections, similar to the reflections bad VSWR gives become very important. The design of the BNC becomes very important. Cheaper or non-spec BNCs plonked onto the end of decent cable shorten the maximum length it can be before the send and receive devices stop talking. HD at 1980x1080 is bad enough, but now we're going higher and higher and cable and connector impedance becomes critical if you want clean data out the other end. Proper HD-SDI cables are vital. 75Ohm 3G-HD-SDI BNCs can be purchased for two or three quid for the crimp on versions, while a crimp tool and die to put them on costs between 200 to 400 quid! Old fashioned solder on versions are so rare people like Canford don't do them. There are loads being sold because they have enhanced cable protection, or easy fit - but for HD video, solder is now just not good enough.
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