Continually losing RX in 396-series scanners

[teufler]

I have read all the comments, si if you are going with the thought of too strong a received signal, check the 20 db attenuation box . Your antenna setup seems one that most of us would like but maybe its too good. Also check the band scope readout and see what is going on around you. This will take some time, not knowing if a strong signal is vhf or uhf.

 

[Ubbe]

. When the AC cables are carrying a heavy current, several volts can be induced in nearby wires.

Yes, but the nearby wires also goes in pair and if there are the same induced voltage in both wires they cancel each other out. But coax are shielded and are immune to magnetic fields. Metal does not let magnetic energy pass thru it.

There is a reason why network, thermostat, and other low-voltage / low power wiring is not allowed in the same conduit as AC power wiring, and running them parallel in close proximity is always a bad idea.

It's mostly a safety issue to minimize the risk of having high voltage come in contact with low level signalling systems in case of an accident or someone mixing the cables around by mistake.

/Ubbe

 

[jonwienke]

Yes, but the nearby wires also goes in pair and if there are the same induced voltage in both wires they cancel each other out. But coax are shielded and are immune to magnetic fields. Metal does not let magnetic energy pass thru it.
It's mostly a safety issue to minimize the risk of having high voltage come in contact with low level signalling systems in case of an accident or someone mixing the cables around by mistake.

/Ubbe

No. When the AC cables are carrying a heavy current, nearby wiring running parallel to them acts as the secondary of a transformer, and can pick up a surprisingly high induced voltage. I've seen Cat5 network runs pick up enough voltage to damage a network card because they ran too close to the feedline for a commercial air conditioner, even though there was no short between them.

Shielded or twisted-pair cables reduce the crosstalk problem, but don't eliminate it. They will still have significant common-mode voltage if run too close to AC power cables.

 

[Ubbe]

Lets anyhow agree that coax are non affected by this and are safe to have near any magnetic fields, as long as the shielding is sufficient the magnetic energy cannot penetrate the metal shield.

/Ubbe

 

[jonwienke]

Lets anyhow agree that coax are non affected by this and are safe to have near any magnetic fields, as long as the shielding is sufficient the magnetic energy cannot penetrate the metal shield.

No. You still have common-mode currents, even if the coax shielding is 100% effective. But the coax shielding is never 100% effective.

 

[Ubbe]

That small common mode current inductly transfered from the balanced mains wires are going to the outher skin of the coax and down to grounded chassi of the scanner making no harm.

/Ubbe

 

[jonwienke]

And yet, I've had to replace network cards that were fried because the cabling was run too close to AC power cables carrying heavy current, and the voltage induced when a large AC unit started up was more than the card could handle. Your theory assumes shielding is 100% effective, and it never is. There is always some imbalances in induced currents, due to the fact that copper is not a superconductor, and when cables are next to each other, more current is induced between the closest conductors than between the most distant.

 

[phask]

Totally different, yet some similarities. Had a ton of 10-base 2 & 10-base-5 system in a factory I worked at - back in the stone age. Had a lot of issues with the backbone. There were lots of electrical equipment - 440 - 3Ph machines, internal hi voltage runs of 13.6 ( or somewhere, this is from old memory). We blew every card, hub, repeater that there was. We finally went with a fiber backbone, and as availability increased down to local runs of fiber.

This was when fiber in a commercial installation was rare, we also were in a 150-year-old building a campus that was nearly a mile long.

- good ole' days -

And yet, I've had to replace network cards that were fried because the cabling was run too close to AC power cables carrying heavy current, and the voltage induced when a large AC unit started up was more than the card could handle. Your theory assumes shielding is 100% effective, and it never is. There is always some imbalances in induced currents, due to the fact that copper is not a superconductor, and when cables are next to each other, more current is induced between the closest conductors than between the most distant.

 

[Ubbe]

Totally different, yet some similarities.

Probably no similarities. The coaxial network systems where a nightmare. They had all network devices connected by the coax ground that produced ground loops and different devices could have different ground potentials which ment that when you disconnected and connected the BNC you could, and often did, blow up the interface. It's not so much related to the subject of transfered magnetic energy between cable types.

/Ubbe

 

[jonwienke]

It's not so much related to the subject of transfered magnetic energy between cable types.

No, because the failures occured while the cable was connected.

 

[tfhphoto]

UPDATE - I purchased and installed the limiter that was linked to in an early comment. Problem (knock on wood) solved. I'm marking this one up as due to mystery RF blasting the receiver. Thanks to all.