Cable quality - less is more?

[Pride Of Cucamonga]

I've read on several threads that for some high performance LNBs a long cable (some suggest up to 25 meters) is optimal to avoid a strong signal overwhelming the set top box. If true, could the same optimum be achieved with a lower quality, but shorter, cable?

Asking because I;m currently using a low quality 10m cable (from a Sky minidish pack on ebay) with an Inverto Ultra LNB and wondering if upgrading the cable would be positive or counter-productive.

thanks

 

[Lazarus]

I've never come across a real case of STB's of the current era being overwhelmed.

It sounds totally bonkers to fit a high-performance LNB then artificially create losses.

 

[Captain Jack]

Yes, Black Ultras can pump out a lot of signal on strong satellites. I had a 4m run from the dish once and it was overwhelming the receiver on some very strong signals - had to add an attenuator.

10m should provide enough attenuation not to overload the receiver but this is only relevant for very strong signals... Certainly upgrade the cable - "Sky" cable is only really good for convenience.

 

[Pride Of Cucamonga]

I've never come across a real case of STB's of the current era being overwhelmed.

It sounds totally bonkers to fit a high-performance LNB then artificially create losses.

Agree it sounds bonkers, but does higher quality cable actually make any difference over 10 metres, or is it just something we assume? I struggle to see how a weak satellite signal is relevant to the quality of cable needed when the LNB is fully powered. Over 10 metres surely every bit of information the LNB sends the receiver picks up, even when using the lowest quality cable?

 

[Captain Jack]

Well, you wouldn't want to send it over twisted pair!

Any cable of any length will introduce *some* loss. Lower quality cables have poorer shielding, so you are more likely to pick up external noise. And the longer the cable, the more noise it will pick up.

I always use the highest quality cable I can afford for the job required. I do use a WF65 twin cable supplied by Sky ... for Sky, simply because it doesn't matter there. For my main motorised dish where I deal with some very weak signals indeed, you should have as good as cable as you can get.

 

[Pride Of Cucamonga]

Well, you wouldn't want to send it over twisted pair!

Any cable of any length will introduce *some* loss. Lower quality cables have poorer shielding, so you are more likely to pick up external noise. And the longer the cable, the more noise it will pick up.

I always use the highest quality cable I can afford for the job required. I do use a WF65 twin cable supplied by Sky ... for Sky, simply because it doesn't matter there. For my main motorised dish where I deal with some very weak signals indeed, you should have as good as cable as you can get.

Ha, yes agree on the twisted pair!

I can understand your argument for a TV aerial cable, but the LNB is sending an amplified signal that can surely survive 10m of low quality coaxial?

 

[Captain Jack]

Again - depends on what you're trying to receive. Like I said, a strong signal won't suffer as much as a weak one - so if your signal is strong, you'll get away with pretty much any coax over that distance.

 

[PaulR]

I would always use decent co-ax. In an ideal environment it might work but in the real world you're subject to all sorts of interference over which you have no control. At least with a clean strong signal you can attenuate it. You can't strip intermittant interference from a signal after it's in.

 

[Pride Of Cucamonga]

I've never come across a real case of STB's of the current era being overwhelmed.

It sounds totally bonkers to fit a high-performance LNB then artificially create losses.

Just to clarify - I have read suggestions that the SNR as measured by the receiver can be higher with a longer cable - not that it's necessarily overwhelmed.

 

[Lazarus]

I entirely fail to see how a passive element like a cable can remove noise!

 

[smoggy07]

I entirely fail to see how a passive element like a cable can remove noise!

I always thought that high end cable didn't remove noise as that would be impossible as pointed out above, when its there its there, but...... High end coax stops (or limits) the noise entering the feed?

 

[Channel Hopper]

Cable is not a passive element when it is connected at each end to F plug connectors, carries a voltage differential between outer and inner conductors with additional pulsed DiSEqC data, and carries up to 1GHz of multiplexed data streams modulated onto multiple carriers at varying strengths.

 

[PaulR]

Sorry CH, must disagree with you on that.

An active device produces an output which is a product of what is put in. The whole point of a co-ax cable is to deliver what comes in at one end to the other without any changes.

 

[timo_w2s]

Just to add I've experience the short run problem with Black Ultra LNBs and about 4m of WF100 cable to my DrHD F15. I was getting must better results on fringe channels when I had another receiver temporarily looped through before the F15. In the end I got a variable attenuator and adjusted it for optimal signal and actually gained more channels...

I also tried the attenuator on a Sky box but it didn't seem to help so the Sky box is fed direct and seems to cope OK.

 

[Terryl]

Most modern receivers (satellite, cable box, AM FM radio or TV set) use what is called an AGC (Automatic Gain Control) circuit, it has an input range of about a -20 to -65 dBM (depending on the unit's tuner) the AGC reduces or amplifies the input signal to try and keep the signal going into the tuner at a constant level, if the signal is too high it will reduce the incoming signal, if too low it will amplify it, if the signal is out of it's tuning range (too high or too low) it can mute the signal to keep the tuner from being damaged.

On some multiple signals like a satellite with 20 to 30 transponders at different levels or multiple TV stations located at different distances from the receiver, the higher signals may cause the AGC to cut out the lower level ones.

Too high an input level can cause the AGC to go into an overload protection mode, this to protect the tuner.

So what you are experiencing may be AGC overload, adding attenuation helps lower the extremely high input signals to a level that the AGC can handle, and will not affect the receivers tuner.

 

[Channel Hopper]

Sorry CH, must disagree with you on that.

An active device produces an output which is a product of what is put in. The whole point of a co-ax cable is to deliver what comes in at one end to the other without any changes.

Not true

Coaxial cable differs from other shielded cable used for carrying lower-frequency signals, such as audio signals, in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line.

Coaxial cable also provides protection of the signal from external electromagnetic interference


Cable was never designed to carry out the demands of the - later - satellite equipment. Nodes aside, the reflections that might appear along the length of any run from LNB, (through a modern motor - forgot about those) , and the odd voltage fluctuation that is created by those 'quality' SMPS designs at the front end, it's no wonder things go tits up further 'down the line' on modern equipment.

 

[PaulR]

Not true

Coaxial cable differs from other shielded cable used for carrying lower-frequency signals, such as audio signals, in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line.

Coaxial cable also provides protection of the signal from external electromagnetic interference

Cable was never designed to carry out the demands of the - later - satellite equipment. Nodes aside, the reflections that might appear along the length of any run from LNB, (through a modern motor - forgot about those) , and the odd voltage fluctuation that is created by those 'quality' SMPS designs at the front end, it's no wonder things go tits up further 'down the line' on modern equipment

That doesn't make what I said untrue. ANY length of cable will be resonant at some frequency or other the trick is to make it irrelevant when you don't want it - which a coaxial cable will do when used as a feeder.

The constant spacing of dimensions ensures that the impedance remains the same along its length so as to avoid any mismatch - again not making what I said untrue.

It's only when you start making RF notches out of cut lengths of co-ax that you start to use the cable's resonant frequency properties.

Not sure what the Glen Canpbell video illustrates unless it's to show loss of signal over a cable.

 

[Steve Steve]

Just to add I've experience the short run problem with Black Ultra LNBs and about 4m of WF100 cable to my DrHD F15. I was getting must better results on fringe channels when I had another receiver temporarily looped through before the F15. In the end I got aariavariable ble attenuator and adjusted it for optimal signal and actually gained more channels...

I also tried the attenuator on a Sky box but it didn't seem to help so the Sky box is fed direct and seems to cope OK.

I got the same results as Timo......But only on a Panasonic Blu ray Freesat receiver .......I also have a Variable attenuator and it gains signal Quality by reducing signal strength the signal quality goes up......a long cable had the similar affect ....but the attenuator is easier to fine tune.........

The Panny is a twin tuner...Input 1 has the attenuator....Input 2 is direct.....Input 1..always keeps the signal longer....while at times input 2 has no signal..!!.....and yes.... i have tried switching the inputs..!!...result was ...input 2 was then the best,...

 

[Channel Hopper]

That doesn't make what I said untrue. ANY length of cable will be resonant at some frequency or other the trick is to make it irrelevant when you don't want it - which a coaxial cable will do when used as a feeder.

The constant spacing of dimensions ensures that the impedance remains the same along its length so as to avoid any mismatch - again not making what I said untrue.

It's only when you start making RF notches out of cut lengths of co-ax that you start to use the cable's resonant frequency properties.

The formulae for transmission line characteristics assumes that there is no mismatch, either in dimension or impedance. Modern satellite receiver design and cable connections appear to ignore both in spades.

I very much doubt the Asian designers of the tuner front end or the LNB really give two hoots about a true 75 ohm passage within the circuit board gubbins, much less the injection, noise suppression and/or band pass filtering of the voltage and tone generators for controlling what's outside.
As for the motor designs, anyone that has taken apart a modern motor will have seen the absolute quality of the internal electrics and shielding that the signal has to pass through on its way back down the line.

 

[PaulR]

None of which makes what I said any less true!