neutral on CATV sheaths

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karl riley

Senior Member
Re: neutral on CATV sheaths

Hal, I went to Radio Shack today and got two 75 to 300 ohm transformers. One of them has screw connectors for the 300 ohm flats from the other one, so it's an easy connect. The sheath tested discontinuous and should block low voltage neutral current.

This would be installed on the street side of the ground block and so it will not break the gounding connection of the receiver inside the house, which will still be grounded to the service ground (both its EGC and its cable sheath).

Since the sheath of the incoming cable is grounded at the pole or transformer, and the other side of this double balum is grounded to the building service ground, a nearby lightning strike could cause a voltage difference inside the balums and destroy the balums (cost, $7.48)as it blasts through to the grounding electrode.

Seems like a good solution to the objectionable neutral current on the cable sheath, no? Anyone want to share their opinion tonight? I'm going to the job tomorrow.

Karl
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Re: neutral on CATV sheaths

Three problems I can see.

First is the #150-1253 is not weather resistant, not meant for outdoor use and only has a push-on connector. It's what people used to stick on the back of a TV with a 75 ohm input to convert it to twinlead (God forbid!) from an antenna.

Second is that the twin lead on the #150-1140 is not shielded which could lead to ingress/radiation problems showing up as interference. This was used to convert an old TV with a 300 ohm input to coax.

Third is that I'm not sure the setup would pass the full CATV bandwidth of today properly. I also think that if the cable company saw that on their line it wouldn't stay for sure. It's jury rig at it's best.

If you are going to do this I would suggest something like the isolator Hurk shows.

-Hal
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
Re: neutral on CATV sheaths

IMPORTANT!!!
Karl and anyone else reading:
Cable signals use most of the radio frequency spectrum to broadcast their programing. These frequency's also include very vital frequency's that are in use by Police, Fire, Ambulance, Aircraft, Etc... When this signal is shielded inside of the cable it causes no interference. But when doing cable work or installing cable that might be used on a cable network it is of most importance to make sure that all shields are properly terminated and blocking caps are used on any open terminals that will be connected to a live cable line. If this is not done and you cause a leakage to the air it could interfere with very important communications that could prevent a emergency people from saving a life or an aircraft from calling in a distress call!!! The FCC can levy heavy fines and penalty's if they can find you at fault and have done so in this area. We are in the landing pattern for O'Hare and when this happened the cable leakage was blocking the reception from the tower to the planes. This was caused by an electrician that was cutting off the shield when using a twist on F connector. The shield must be left on and folded back over the outer covering to make connection to the shield in the wall jack. Without this shield the short jumper that went to the cable box acted like an antenna and re-radiated all the signals back out.

Karl when I used the balun's I covered them with foil and made a connection to shield on only one side (toward the street) The one I showed in the photo is only $10.00 I have gotten them for as low as $4 or $5 bucks also from a supply house.

[ March 16, 2005, 02:52 AM: Message edited by: hurk27 ]
 
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karl riley

Senior Member
Re: neutral on CATV sheaths

Thanks Hurk and Hal. Will substitute the isolator Hurk pictured. So much for improvising. I knew about using tin foil, but this sounds too serious to fool around with.

As for weather, the baluns were inside a weather proof box.

Karl
 
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mc5w

Senior Member
Re: neutral on CATV sheaths

Radio Shack carries some ferrite inductance cores that can be clamped around telephone cords and CATV to block common mode curents. They also have a laminated inductance core in the same drawer that can be threaded over a cable but you have to disconnect the cable.

The clamp on ferrite cores when applied to telephone handset cords helps reduce noise pickup from fluorescent lights.

This is a standard technique that is in the ARRL Radio Frequency Interference Handbook.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
Re: neutral on CATV sheaths

Mc5w:
Radio Shack carries some ferrite inductance cores that can be clamped around telephone cords and CATV to block common mode currents.
Click to expand...
Ferrite cores are only good in the RF range, They will not stop current @ 60hz from flowing on the cable shield. The most common use is to stop Rf energy from getting into circuits where it will affect the intended purpose of the circuit Ie. audio circuits that are part of the modulating amplification input to the drive in a transmitter which includes the Mic. This helps prevent RF feed back. also digital circuits where RF energy could affect the performance of the circuit.
 
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