Strenght of Magnetic Field Produced by Coax Cable?

[cp123]

Hello, could someone tell me what the strength would be of the magnetic field produced by a coaxial cable that is supplying high-speed internet?

Could you put it into perspective, as in comparing it to the strength of a magnet field of something else, say a standard household 110v line?

Thank you very much for your help.

 

[marcusl]

Coax cable produces no magnetic field. That's one of the reasons it's used (despite its expense) in demanding applications.

 

[cp123]

Hi, thank you very much for your reply.

I was looking on Wikipedia and found this:

Signal leakage is the passage of electromagnetic fields through the shield of a cable and occurs in both directions. Ingress is the passage of an outside signal into the cable and can result in noise and disruption of the desired signal. Egress is the passage of signal intended to remain within the cable into the outside world and can result in a weaker signal at the end of the cable and radio frequency interference to nearby devices.

For example, in the United States, signal leakage from cable television systems is regulated by the FCC, since cable signals use the same frequencies as aeronautical and radionavigation bands. CATV operators may also choose to monitor their networks for leakage to prevent ingress. Outside signals entering the cable can cause unwanted noise and picture ghosting. Excessive noise can overwhelm the signal, making it useless.

An ideal shield would be a perfect conductor with no holes, gaps or bumps connected to a perfect ground. However, a smooth solid copper shield would be heavy, inflexible, and expensive. Practical cables must make compromises between shield efficacy, flexibility and cost, such as the corrugated surface of hardline, flexible braid, or foil shields. Since the shields are not perfect conductors, electric fields can exist inside the shield, thus allowing radiating electromagnetic fields to go through the shield.

Consider the skin effect. The magnitude of an alternating current in a conductor decays exponentially with distance beneath the surface, with the depth of penetration being proportional to the square root of the resistivity. This means that in a shield of finite thickness, some small amount of current will still be flowing on the opposite surface of the conductor. With a perfect conductor (i.e., zero resistivity), all of the current would flow at the surface, with no penetration into and through the conductor. Real cables have a shield made of an imperfect, although usually very good, conductor, so there will always be some leakage.

The gaps or holes, allow some of the electromagnetic field to penetrate to the other side. For example, braided shields have many small gaps. The gaps are smaller when using a foil (solid metal) shield, but there is still a seam running the length of the cable. Foil becomes increasingly rigid with increasing thickness, so a thin foil layer is often surrounded by a layer of braided metal, which offers greater flexibility for a given cross-section.

This type of leakage can also occur at locations of poor contact between connectors at either end of the cable.

Apparently, there is some type of electromagnetic field leakage from the cable. Are you saying it's so slight that it's immeasurable/undetectable?

Thanks for your insights.

 

[marcusl]

Yes, that description is accurate. Real cables do have some level of leakage through the shields, but it is generally very small. Shielding effectiveness values S_dB of 100 to 120 dB are typical of coax. To convert to a power ratio, take 10^(-S_dB/10). S_dB = 100 means that 10^-10, or only 0.1 billionths of the power incident on the cable, will leak through the shield. S is also frequency dependent. If you know the part number of your cable, you can get plots or tables of shielding effectiveness vs. frequency from the vendor. For most purposes you can consider the leakage to be negligible.

You asked specifically about magnetic fields, however, as opposed to electromagnetic radiation. Pure magnetic fields generally are not created outside of coaxial cables due to the concentric (coaxial) symmetry of the conductors. You would be hard pressed to measure a B field outside of coax.

 

[cp123]

Thank you very much for your in-depth explanation.

I'm sorry I failed to make a distinction between magnetic fields and magnetic radiation. I'm interested in magnetic radiation as well.

The magnetic radiation is the 0.1 billionth that you mentioned earlier, correct? So, would this be much smaller than the electromagnetic radiation given off by standard electrical wires running through the walls of a home or what is emitted by a cordless phone?

Thank you for putting this into perspective.

 

[marcusl]

Yes, coax is well shielded compared to open wires. What frequency range are you interested in? Your cordless phone works at 900 MHz (high UHF) while AC wires in your wall carry 60Hz. Behavior is so frequency dependent that you are comparing apples and oranges.

 

[cp123]

Well, you see I know a person who is sensitive to electomagnetic fields. Now, I know that this is a debatable issue, but let's assume that it's true.

Now, this man is interested in getting coax cable television and broadband internet. However, he is worried that this may create electromagnetic fields that may bother him.

Now from what you say, there aren't any electromagnetic fields that would emanate from the coax, just electromagnetic radiation at a rate of .1 billionths of the power incident on the cable.

Would this be equivalent to the "EMFs" that this person probably experiences just from the am/fm radio waves traveling through the air? Is it so slight that it's inconsequential?

Thank you for your help.

 

[marcusl]

The leakage from internet & TV coax is negligible. If this individual is so worried about EM emissions, he/she shouldn't own a TV or computer (they are big radiators compared to the cable). Or cell phone (huge). As for their claim to directly sense bodily effects of AM and FM radio signals, well, let's not go there.

 

[cp123]

Thank you for your reply. This person doesn't own a cell phone and doesn't have any effects from am/fm transmissions. That's why I used them as an example to show how equally negligible the emmissions are from a coax cable.

I've learned that the frequency cable/internet coax carries is in the 500-800Mhz range.

Would you agree that the leakage from coax is so small that it is probably immeasurable once you get only a few inches away?

I appreciate your knowledgeable reply.

 

[sophiecentaur]

Coax cable comes in many forms. The cheapest TV 'downlead' leaks like a seive, by according to some criteria whilst high quality transmission feeder may be a million times better (60dB). There are communications systems for use in mines and tunnels that rely on the leakiness of cheap TV feeder.

If the function of this screening is to reduce radiated fields to no more than what is careering about the place from all other (wireless) sources then you would need to scan the premises to find out what is actually there all the time. In an urban environment you could expect much higher level than in a rural area. Nearby TV and radio transmitters can lay down a very measurable field strength.
When you say this person is 'sensitive' to electromagnetic fields, what details do you have about the particular frequency bands and levels involved? To have any clue about a solution, the problem must first be defined. We are all 'sensitive' to electromagnetic radiation - light and infra red are detectable by most humans. Many people are affected adversely by their living environment and some will identify various causes, including 'fields'. There are two ways to approach this problem.
1. Snake Oil : Buy a gizmo in a box which is purchased from a fringe medicine organisation and which may work very well as a placebo. Possibly very good value.
2. Science : Measure the susceptibility of the subject to a range of em radiation sources and then reduce the local fields to an acceptable level.
Option 1 should be a lot cheaper but not one I would opt for, myself.


FCC regulations are published for permitted interference limits at various frequencies. The Mobile Phone question is still (afaik) unresolved but no one needs to use one if they don't want to. The broadband signals to your house are not carried at 800MHz because that frequency wouldn't get that far on cable that, in many cases, was designed to carry audio (telephone) signals. WiFi and Bluetooth, mobile and cordless phones involve those sort of frequencies and are pretty short wireless range. Modern cables for CATV and broadband are screened, in any case, so that they can be carried in large bundles without crosstalk. Telephony signals are conventionally carried on Twisted Pairs, which leak very little and a sheath on the outside makes things even better.

To sum up - Measure before you Spend. You could squander all your substance with any other approach.

 

[cp123]

Thanks for all of your replies and help.

Sorry to get back so late.