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genie high-frequency connectors


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46 replies to this topic

#41 OFFLINE   slice1900

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Posted 13 April 2014 - 07:18 PM

Yes, SWM systems actually use LOWER frequencies than traditional DirecTV systems....

 

SWM splitters have "2 - 2150 MHz" printed on them, and SWM only receivers like the H25 and Genie use SoCs capable of tuning 950 - 2150 MHz. So while that is and always has been true for SWM, it isn't a safe bet that it always will be.


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#42 ONLINE   veryoldschool

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Posted 13 April 2014 - 08:08 PM

It is not so much a signal loss issue per se, as it is an impedance matching or VSWR issue. There is virtually no resistance to attenuate signal in any connector, but signal can be attenuated by bad impedance matching. And this is exactly why some connectors, once cut onto a cable, will exhibit anywhere from .1 to 1 dB of loss.

Still, it is difficult to simulate all of the aspects of coax through a connector...

We all did have to stop using crimp connectors because they showed reflections above about 1.8 GHz or so......

Care to elaborate on the "VSWR issue" as to the source or cause?

P Smith brought up the dielectric air gap, but that only has a VSWR of 1.2:1 or so, which is well within the specs of these systems.

Since a F connector is merely an outer shell, if the dielectric isn't damaged and the foil is in tact, what's left to cause a problem?

Seems to be only the connection to the shield and a good tech should know how and be able to install a connector "correctly".

Not that I'm supporting the use of crimp connectors, but following the same line of reasoning, what is it that causes "reflections above 1.8 GHz"?

This stuff isn't magic, so there should be a reason or cause which can be traced.

While you were connectorizing over 25,000 RF and video cables, I was working in an R&D lab measuring these things, so "I know" there has to be a reason.


A.K.A VOS

#43 OFFLINE   hasan

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Posted 15 April 2014 - 06:44 PM

I don't know about frequency dependencies, but a poorly installed crimped connector can narrow the distance between the shield and the center conductor and that by definition would cause an impedance bump. Could that be what he was referring to?


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#44 ONLINE   veryoldschool

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Posted 15 April 2014 - 06:52 PM

I don't know about frequency dependencies, but a poorly installed crimped connector can narrow the distance between the shield and the center conductor and that by definition would cause an impedance bump. Could that be what he was referring to?

That would seem like the only way, but someone who claims to have "connectorized 25,000 cables" should know how to install them correctly.


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#45 OFFLINE   dishinitout

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Posted 15 April 2014 - 07:01 PM

That would seem like the only way, but someone who claims to have "connectorized 25,000 cables"


Funny thing about that is it seems like a ton of experience but really may not be. Take a conservative 20 "conectorizations" a day working 250 days a year and that's only 5 years experience. More likely, a busy tech can do that in 2-3 years. Makes me wonder how many I've done over the years since many days I've gone well over a bag of 50 on a job.

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#46 OFFLINE   TomCat

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Posted 18 August 2014 - 08:51 PM

Care to elaborate on the "VSWR issue" as to the source or cause?

...Since a F connector is merely an outer shell, if the dielectric isn't damaged and the foil is in tact, what's left to cause a problem?

Seems to be only the connection to the shield and a good tech should know how and be able to install a connector "correctly".

... following the same line of reasoning, what is it that causes "reflections above 1.8 GHz"?

This stuff isn't magic, so there should be a reason or cause which can be traced.

While you were connectorizing over 25,000 RF and video cables, I was working in an R&D lab measuring these things, so "I know" there has to be a reason.

 

Sorry, VOS; I somehow never returned to this thread; it only came up today in a search for something else. My apologies.

 

As you must well know, coax is similar to waveguide technology, meaning that to a RF signal, a properly-matched impedance-wise cable or connector appears to that signal in a perfect world as an infinite length of cable, in this case, a pure 75 ohms. But that is only the ideal. It is nearly impossible to manufacture a connector or even a cable where the impedance does not vary somewhat, especially over a wide frequency range. That is why we sweep cable reels before we put them on the truck, not to see if it is perfect or expecting it to be perfect, because it never is; to see if it is close ENOUGH to the ideal.

 

And as I'm also sure you know, the second it varies, it no longer appears as an infinite length of cable, and reflections can develop, typically in relationship to how much it varies. This is why every connector, no matter how well it is manufactured or how skillfully it is cut on, will see a loss in signal. The loss in signal is not important; what is important is where that signal went, which is back up the cable in the other direction. Cable has to abide by the same rules. Flatten it just a little on one side, or if the center-conductor isn't precisely in the center of the cable coming off the production line, and getting a signal from one end to the other at all without degradation becomes a real problem. Installers also stopped using staples and cable clips. Why? They compressed the cable and caused, you guessed it, reflections.

 

Impedance is controlled by center-conductor size, dielectric size, and the distance between them, primarily. How could we ever expect a connector that by its nature squeezes the cable during installation (and in the case of a crimp connector unequally at six different places around the circumference), to not create an "impedance bump" as hasan calls it?

 

I was as surprised as anyone to hear that crimps had a flaw such as this at higher frequencies, but that was probably because I was never dealing with digital signals riding on frequencies above about 550 MHz. But I don't need to put my green visor on and take it into an "R&D lab" to get to the bottom of anything, either, I am perfectly comfortable taking the advice of those who claim that crimps have this flaw. And as much as I hate compression connectors, I have embraced them, because I have been told that they are better for that particular reason, and I am willing to accept that.

 

The crimp connector itself is where the flaw lies that I am speaking about. I have long approved of crimp connectors over compression connectors, because I think that if done correctly, they are less likely to fail. But the compression connector has two advantages, which are that it is easy to install compared to the learning curve with a crimp connector (a high percentage of these end up being done incorrectly), and that while fine for cable TV when we had 75 channels and the signals were analog, the crimp connector inherently is flawed over a higher frequency range, especially for digital signals.

 

Reflections such as these are pretty benign for analog, and sometimes even act as artificial image enhancement ironically making an analog SD picture look artificially sharper. But reflections in digital are destructive to decoding, very much in the same way that inband multipath can thwart ATSC reception.

 

You are correct; a good tech should indeed know how to install a connector properly, just like a well-made connector and cable should have no reflection. But the real world reveals that none of that is all that perfect. I probably did not really install crimps properly for the first few months because it takes time to develop the technique (and as we both know, I'm as perfect as it gets!), and typically instruction from local mentors is pretty bad as well.

 

And quite often, the foil does not remain intact. The proper way to install a crimp connector is to force the connector over the dielectric and foil backwards for just a moment, so that the foil gets formed properly back tightly around the dielectric. Gilbert AHS-USA connectors even have a bevel that allows the installer to do this without the connector grabbing the foil and pushing it down or bunching it up. But my guess is that few if any field installers use this technique; many of them just tear the foil off every time.

 

A connector that has a high VSWR, for whatever reason, will not just ripple the frequency response curve up and down the cable in a constructive/destructive interference pattern; it can cause actual in-band interference that closes the eye pattern at a shorter distance for digital signals and leads to digital artfacts and unreliable reception. It is not a matter of signal attenuation due to reflection and resultant interference nodes, it is the packet to packet interference itself that becomes problematic.

 

So because the compression connector has a lower VSWR over a wider frequency range and is hard to screw up, it has gained favor with DBS and cable. Long gone are the days that you can solve every distribution problem by attacking it in response to how much signal loss there is. In the world of digital and higher frequencies, reflections become problems that do not show on a SLM, problems that can accumulate over a cable run and cause issues that are hard to track down and solve.

 

So there is a definite cause, and it is the fragility of that formula that determines the impedance of coaxial cable and how imperfect connectors all are, how imperfect cable is, and mostly how imperfect installation of connectors is, all imperfect at maintaining characteristic impedance and avoiding reflections. Compression connectors solve much of this. They are better, but also not perfect. And no, there is no magic involved, it is simple physics.

 

And you don't have to have ever worked in R&D to have a full understanding of that. The conditions of the real world are a much more reliable teacher, if one is willing to simply pay attention.


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#47 ONLINE   veryoldschool

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Posted 19 August 2014 - 04:12 AM

Sorry, VOS; I somehow never returned to this thread; it only came up today in a search for something else. My apologies.

 

As you must well know.....,

And you don't have to have ever worked in R&D to have a full understanding of that. The conditions of the real world are a much more reliable teacher, if one is willing to simply pay attention.

"On the whole" your post was good.

The change to digital basically requires a higher degree of attention to detail and care.

What you "used to" be able to get away with can cause problems today.

Perhaps the difference we have is I've worked mostly with Mil-spec cables/transmission lines in the DC-40+ GHz range.

"The problems" you're describing are quite significant the higher the frequency.

 

Impedance is controlled by center-conductor size, dielectric size, and the distance between them, primarily.

 

The missing component here is the dielectric constant of the dielectric material. This determines the distance to the shield for a given impedance.

Impedance always has a reactive component, so it varies with frequency.

Mil-spec is expensive, where RG6 and F type connectors are cheap.

"Cheap works" because the systems and components are spec'd to a VSWR range of 2:1 - 1.3:1 (at best).

All "connections" have a mismatch, but if it causes a VSWR of 1.22:1, this is lost in the higher reflections of the rest of the system.

 

Hex crimped connectors are still used in the manufacture of cable for use in the 2 GHz range because "they meet spec", since they don't distort the coax. "The crimp" is outside the foil and between the sleeve of the connector and the braiding of the coax.

"If installed correctly" the six points of contact to the braiding will be the same as 100% contact.

 

Compression connectors and the tools to install them are just easier to "get right", and why have become "preferred".

They're not "foolproof" as I've seen many with the dielectric "short" of the mating plane of the connector.

If I had 100 connectors to install, compression connectors would be my choice.

If I had to repair a cable and I only had crimp connectors, I would take the time to install them correctly and have the same electrical performance.

And you don't have to have ever worked in R&D to have a full understanding of that. The conditions of the real world are a much more reliable teacher, if one is willing to simply pay attention.

It helps to have both, as you can measure "the real world" and see why and where the "lesson" comes from.


A.K.A VOS




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